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1
Winter, David A., Aftab Patla e Francois Prince. "Stiffness control of balance during quiet standing". Gait & Posture 5, n. 2 (aprile 1997): 154–55. http://dx.doi.org/10.1016/s0966-6362(97)83378-4.
Testo completo
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Winter, David A., Aftab E. Patla, Francois Prince, Milad Ishac e Krystyna Gielo-Perczak. "Stiffness Control of Balance in Quiet Standing". Journal of Neurophysiology 80, n. 3 (1 settembre 1998): 1211–21. http://dx.doi.org/10.1152/jn.1998.80.3.1211.
Testo completoAbstract (sommario):
Winter, David A., Aftab E. Patla, Francois Prince, Milad Ishac, and Krystyna Gielo-Perczak. Stiffness control of balance in quiet standing. J. Neurophysiol. 80: 1211–1221, 1998. Our goal was to provide some insights into how the CNS controls and maintains an upright standing posture, which is an integral part of activities of daily living. Although researchers have used simple performance measures of maintenance of this posture quite effectively in clinical decision making, the mechanisms and control principles involved have not been clear. We propose a relatively simple control scheme for regulation of upright posture that provides almost instantaneous corrective response and reduces the operating demands on the CNS. The analytic model is derived and experimentally validated. A stiffness model was developed for quiet standing. The model assumes that muscles act as springs to cause the center-of-pressure (COP) to move in phase with the center-of-mass (COM) as the body sways about some desired position. In the sagittal plane this stiffness control exists at the ankle plantarflexors, in the frontal plane by the hip abductors/adductors. On the basis of observations that the COP-COM error signal continuously oscillates, it is evident that the inverted pendulum model is severely underdamped, approaching the undamped condition. The spectrum of this error signal is seen to match that of a tuned mass, spring, damper system, and a curve fit of this “tuned circuit” yields ωn the undamped natural frequency of the system. The effective stiffness of the system, K e , is then estimated from K e = Iω2 n, and the damping B is estimated from B = BW × I, where BW is the bandwidth of the tuned response (in rad/s), and I is the moment of inertia of the body about the ankle joint. Ten adult subjects were assessed while standing quietly at three stance widths: 50% hip-to-hip distance, 100 and 150%. Subjects stood for 2 min in each position with eyes open; the 100% stance width was repeated with eyes closed. In all trials and in both planes, the COP oscillated virtually in phase (within 6 ms) with COM, which was predicted by a simple 0th order spring model. Sway amplitude decreased as stance width increased, and K e increased with stance width. A stiffness model would predict sway to vary as K −0.5 e . The experimental results were close to this prediction: sway was proportional to K −0.55 e . Reactive control of balance was not evident for several reasons. The visual system does not appear to contribute because no significant difference between eyes open and eyes closed results was found at 100% stance width. Vestibular (otolith) and joint proprioceptive reactive control were discounted because the necessary head accelerations, joint displacements, and velocities were well below reported thresholds. Besides, any reactive control would predict that COP would considerably lag (150–250 ms) behind the COM. Because the average COP was only 4 ms delayed behind the COM, reactive control was not evident; this small delay was accounted for by the damping in the tuned mechanical system.
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Bakshi, Avijit, Paul DiZio e James R. Lackner. "Multiple roles of active stiffness in upright balance and multidirectional sway". Journal of Neurophysiology 124, n. 6 (1 dicembre 2020): 1995–2011. http://dx.doi.org/10.1152/jn.00612.2019.
Testo completoAbstract (sommario):
Our previously published engaged leg model shows how stiffness plays complex multicausal roles in balance. In one role, it is crucial to stability, with task contingent influences over balance. In another, it overcomes viscous drag. Task-dependent stiffness alone does not explain stable balance; geometrical, invariant aspects of body biomechanics also matter. Our model is fully applicable to clinical balance pathologies involving asymmetries in movement and balance control.
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Winter, David A., Aftab E. Patla, Shirley Rietdyk e Milad G. Ishac. "Ankle Muscle Stiffness in the Control of Balance During Quiet Standing". Journal of Neurophysiology 85, n. 6 (1 giugno 2001): 2630–33. http://dx.doi.org/10.1152/jn.2001.85.6.2630.
Testo completoAbstract (sommario):
This research presents new data and reanalyzed information to refute the criticisms of our model of stiffness control during quiet standing. A re-review of their references to biomechanical research on muscle ankle stiffness confirmed muscle stiffness estimates of the ankle series elastic elements that agreed closely with our estimates. A new technique is presented that directly estimates the muscle stiffness from the ankle moment (N · m) and sway angle (deg). The linear regression of 10 subjects standing quietly for 10 s estimated the stiffness (N · m/deg) to be safely above the gravitational spring. The R 2 scores for this linear regression averaged 0.92, confirming how closely the model approached a perfect spring that would have an R 2 = 1. These results confirm our model of a simple muscle stiffness control and refutes the criticisms.
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Hua, Gao, Shuai Changgeng e Xu Guomin. "Study on Mechanical Model of Balance and Stiffness Characteristics of Fiber Reinforced Bellows Type Rubber Hose". Science of Advanced Materials 12, n. 7 (1 luglio 2020): 981–93. http://dx.doi.org/10.1166/sam.2020.3735.
Testo completoAbstract (sommario):
The mechanical model related to the balance and stiffness characteristics of the bellows type rubber hose under internal pressure was studied. Based on the thin shell theory without considering bending moments and shear force, the equilibrium equation of the bellows type hose was established to obtain the mechanical equilibrium angle under different mechanical environments. Considering the deformation characteristics of the rope structure and the mechanical equilibrium angle of the hose, the deformation of the bellows type rubber hose was divided into two stages, including winding angle deflection and tensile deformation of fiber. Then the constitutive model of anisotropic material was introduced, and the physical equation of the bellows type hose was established to obtain the mechanical model of the balance and stiffness characteristics. According to the mechanical model, the influence of initial fiber winding angle, fiber layer thickness, the radius at the two ends of the hose, the length of hose, the curvature radius and internal pressure of hose on the balance and stiffness characteristics of hose was studied. Eventually, the structure of the hose was designed based on the mechanical model, to optimize the balance and stiffness characteristics of hose. The balance and stiffness characteristics of the optimized hose were verified by experiments. The theoretical and experimental results indicated that, the mechanical model of the balance and stiffness characteristics of the hose can be the theoretical basis for the optimization of structural parameters.
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Morasso, Pietro G., e Vittorio Sanguineti. "Ankle Muscle Stiffness Alone Cannot Stabilize Balance During Quiet Standing". Journal of Neurophysiology 88, n. 4 (1 ottobre 2002): 2157–62. http://dx.doi.org/10.1152/jn.2002.88.4.2157.
Testo completoAbstract (sommario):
This communication addresses again the hypothesis that the stabilization of balance during quiet standing is achieved by the stiffness of ankle muscles without anticipatory active control. It is shown that a recently proposed method of estimating ankle stiffness directly from the analysis of the posturographic data is incorrect because it ignores the modulation of motoneuronal activity and grossly overestimates the real range of values in relation with the critical value of stiffness. Moreover, a new simulation study with a realistic model of ankle muscles demonstrates the mechanical instability of the system when there is no anticipatory control input. However, the simulations also suggest that in normal subjects the active stiffness mechanisms of stabilization have similar weights in determining the restoring forces that are necessary for preventing the body from falling.
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McIlvain, Grace, James B. Tracy, Charlotte A. Chaze, Drew A. Petersen, Gabrielle M. Villermaux, Henry G. Wright, Freeman Miller, Jeremy R. Crenshaw e Curtis L. Johnson. "Brain Stiffness Relates to Dynamic Balance Reactions in Children With Cerebral Palsy". Journal of Child Neurology 35, n. 7 (23 marzo 2020): 463–71. http://dx.doi.org/10.1177/0883073820909274.
Testo completoAbstract (sommario):
Cerebral palsy is a neurodevelopmental movement disorder that affects coordination and balance. Therapeutic treatments for balance deficiencies in this population primarily focus on the musculoskeletal system, whereas the neural basis of balance impairment is often overlooked. Magnetic resonance elastography (MRE) is an emerging technique that has the ability to sensitively assess microstructural brain health through in vivo measurements of neural tissue stiffness. Using magnetic resonance elastography, we have previously measured significantly softer grey matter in children with cerebral palsy as compared with typically developing children. To further allow magnetic resonance elastography to be a clinically useful tool in rehabilitation, we aim to understand how brain stiffness in children with cerebral palsy is related to dynamic balance reaction performance as measured through anterior and posterior single-stepping thresholds, defined as the standing perturbation magnitudes that elicit anterior or posterior recovery steps. We found that global brain stiffness is significantly correlated with posterior stepping thresholds ( P = .024) such that higher brain stiffness was related to better balance recovery. We further identified specific regions of the brain where stiffness was correlated with stepping thresholds, including the precentral and postcentral gyri, the precuneus and cuneus, and the superior temporal gyrus. Identifying brain regions affected in cerebral palsy and related to balance impairment can help inform rehabilitation strategies targeting neuroplasticity to improve motor function.
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Louessard, Aude, Xavier Bonnet, Anita Catapano e Helene Pillet. "Quantification of the Influence of Prosthetic Ankle Stiffness on Static Balance Using Lower Limb Prosthetic Simulators". Prosthesis 4, n. 4 (8 novembre 2022): 636–47. http://dx.doi.org/10.3390/prosthesis4040051.
Testo completoAbstract (sommario):
After a transtibial amputation, the prosthetic foot aims at replacing the missing ankle joint. Due to alteration of proprioception and mobility, the static balance of amputees is challenging. The stiffness of most of the usual prosthetic feet cannot adapt according to the situation. Thus, the control of the user’s balance is closely related to the ankle stiffness value. The aim of this study is to evaluate both the impact of the ankle stiffness and the visual system on static balance. In order to avoid bias relative to different levels of residual proprioception among individuals, the study has been carried out on healthy subjects wearing lower limb prosthetic simulators under each foot. This configuration could be considered as a relevant model to isolate the effect of the stiffness. Eleven subjects wearing prosthetic feet with different modules were asked to remain as static as possible both with open eyes (OE) and closed eyes (CE). The center of pressure (COP) displacements and the joint angles range of motion (ROM) were experimentally assessed. The length of the major axis of the COP 95% confidence ellipse was projected on the antero-posterior direction (AP range). Linear regression models of the AP range and joint angles ROM as a function of the situation (OE and CE) and of the normalized ankle stiffness were created. A one-way analysis of variance test was performed on the model of the AP range. Linear regression coefficients and 95% confidence intervals (CI) were calculated between the AP range and the normalized ankle stiffness and between the joint angles ROM and the normalized ankle stiffness both in OE and CE. This study confirmed that static balance decreases when ankle stiffness decreases. The results also showed that a visual system alteration amplifies more significantly the decrease of static balance of people wearing prosthetic feet and has no significant influence on non-amputated subjects. The slope of the linear regression for the AP range according to the normalized ankle stiffness was equal to −9.86 (CI: −16.03, −3.69) with CE and −2.39 (CI: −4.94, 0.17) with OE. Both the normalized ankle stiffness and the visual system had a significant impact on the AP range (pvalue<0.05). The ankle stiffness is an interesting parameter as it has a high impact on the gait and on the static balance of the users and it must be controlled to properly design prosthetic feet.
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Wang, Pingjun, Gangyan Li e Xueping Li. "Vibration Characteristics Analysis of O-Shaped Damping Ring to Balance Damping Gear Transmission System for Three-Cylinder Engine". Processes 10, n. 9 (25 agosto 2022): 1685. http://dx.doi.org/10.3390/pr10091685.
Testo completoAbstract (sommario):
Balance shafts are often used to improve the engine vibration characteristics of three-cylinder engines. The balance damping gear with a damping ring is an important part connecting the crankshaft and the balance shaft transmission. The stiffness characteristics of the damping ring and the unbalance of the gear have an important influence on its vibration suppression performance, but the coupled influence of the stiffness characteristics of the damping ring and the unbalanced characteristics of the vibration damping gear is unknown. In this paper, a multi-body dynamic bending–torsional coupling model of the transmission system of a three-cylinder engine with a balance damping gear is constructed considering the equivalent stiffness of the balance shaft support. Based on the fourth-order Runge–Kutta method, the influence laws of different rotational speeds, load torques, gear unbalance, radial stiffness and torsional stiffness of the damping ring on the vibration characteristics of the transmission system are obtained. The results show that the vibration amplitude increases linearly with the increase in the rotational speed and the amount of unbalance. As the load torque increases, the noise radiation of the system increases. The change in the equivalent torsional stiffness of the damping ring has little effect on the radial vibration suppression effect of the gear. As the equivalent radial stiffness of the damping ring increases, the vibration suppression rate decreases linearly. Combined with the calculation formula of damping ring stiffness, when the inner and outer diameters of the damping ring are relatively large, the vibration suppression performance decreases sharply with the increase in the thickness of the damping ring. Therefore, in order to achieve a better vibration attenuation effect, the inner to outer diameter ratio of the damping ring should be given priority in the design of the damping gear. Thus, the thickness of the design can meet the requirements of the vibration attenuation performance and a vibration attenuation of more than 90% of the radial vibration can be achieved. The model of the damping ring size and the vibration suppression effect established based on the method presented in this paper can be used to guide the design of balance damping gears.
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Kal, Elmar, Han Houdijk, John van der Kamp, Manon Verhoef, Rens Prosée, Erny Groet, Marinus Winters, Coen van Bennekom e Erik Scherder. "Are the effects of internal focus instructions different from external focus instructions given during balance training in stroke patients? A double-blind randomized controlled trial". Clinical Rehabilitation 33, n. 2 (31 agosto 2018): 207–21. http://dx.doi.org/10.1177/0269215518795243.
Testo completoAbstract (sommario):
Objective: This study aimed to assess if external focus instructions result in greater improvements in motor skill and automaticity compared to internal focus instructions in stroke patients. Design: Double-blind randomized controlled trial. Setting: Inpatient stroke rehabilitation unit. Subjects: A total of 63 stroke patients (Meanage = 59.6 ± 10.7 years; Meandays since stroke = 28.5 ± 16.6; MedianFunctional Ambulation Categories = 4). Interventions: Patients were randomly assigned to an internal ( N = 31) or external ( N = 32) focus instruction group. Both groups practiced a balance board stabilization task, three times per week, for three weeks. Balance performance was assessed at baseline, and after one and three weeks of practice. Main measures: Primary outcome was the threshold stiffness (Nm/rad) at which patients could stay balanced. Secondary outcomes were patients’ sway (root-mean-square error in degrees) at the baseline threshold stiffness under single- and dual-task conditions, and their performance on the Timed Up and Go Test and Utrecht Scale for Evaluation of Rehabilitation. Results: Both groups achieved similar improvements in threshold stiffness (∆= 27.1 ± 21.1 Nm/rad), and single- (∆= 1.8 ± 2.3° root-mean-square error) and dual-task sway (∆= 1.7 ± 2.1° root-mean-square error) after three weeks of practice. No differences were found in improvements in clinical tests of balance and mobility. Patients with comparatively good balance and sensory function, and low attention capacity showed greatest improvements with external focus instructions. Conclusion: External focus instructions did not result in greater improvement in balance skill in stroke patients compared to internal focus instructions. Results suggest that tailoring instructions to the individual stroke patient may result in optimal improvements in motor skill.
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Azaman, Aizreena, e Shin Ichiroh Yamamoto. "Ankle Joint Stiffness and Damping Pattern under Different Frequency of Translation Perturbation". Applied Mechanics and Materials 393 (settembre 2013): 703–8. http://dx.doi.org/10.4028/www.scientific.net/amm.393.703.
Testo completoAbstract (sommario):
The change of effective stiffness and damping characteristic of ankle joint are able to indicate degeneration of balance ability due to ageing effect. This paper will discuss the ankle joint stiffness and damping pattern along repeated translation perturbation. Six young healthy subjects were exposed to five trials of five different frequencies of perturbation (quiet standing, 0.2 Hz, 0.4 Hz, 0.6 Hz and 0.8 Hz). The result showed that the mean of effective stiffness was reduced with the increase of frequency applied; meanwhile the mean of damping value increased with increasing frequency. Additionally, a cubic polynomial curve (u-shape) was estimated to represent stiffness pattern when using curve fitting method with correlation R2>0.5. These estimations also suggested that ankle joint does not oscillate like spring-damper system which is based on inverted pendulum model; however, it applied a different strategy to maintain balance, in particular during initiation, middle and termination of perturbation. These also indicate the influence of sensory processing and adaptation to maintain balance under a long period of disturbance. On the other hand, damping pattern seems to be similar over different frequencies and under repeated perturbation. Besides, the change of stiffness pattern at higher frequency of perturbation (0.8 Hz) recommends the change in posture strategy from ankle to hip strategy. These findings indicated that stiffness and damping are able to describe adaptation of human posture strategy to keep balance and motor learning under repeated perturbation.
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Shahid, Shafuq, Zeest Hashmi, Hira Rehman, Saima Riaz, Muhammad Sulman e Rehan Ramzan Khan. "Association of Hand Stiffness with Balance Impairment in Chronic Stroke Patients". Pakistan Journal of Medical and Health Sciences 16, n. 4 (30 aprile 2022): 612–13. http://dx.doi.org/10.53350/pjmhs22164612.
Testo completoAbstract (sommario):
Objective: To determine the association of hand stiffness with balance impairment in chronic stroke patients. Methodology: A cross sectional study was conducted on eighteen chronic stroke patients with age range between 40 to 70 years. Patients were taken from Riphah Rehab Center and Govt. Kot Khawaja Saeed Teaching Hospital KEMU Lahore. The balance of patients was assessed by using Berg Balance Scale (BBS) and Timed Up and Go Test (TUG) while motor function of the hand was assessed by using Fugl Meyer Assessment-Upper Extremity (FMA-UE). Results: The analysis of data done by using SPSS version25. The mean value of age of the patients was 54.9 + 9.09 years. FMA-UE tool was used to assess motor activity of hand. Postural balance was measured by using BBS and TUG. The results showed a positive correlation of FMA-UE with BBS (rs = 0.704, p =<0 .001) and a negative correlation with TUG (rs = -.705, p = .001). Conclusion: This study concluded that there is association of hand stiffness with balance impairment in chronic stroke patients Key Words: Hand stiffness, Balance impairment, Chronic stroke, Postural instability, Proprioception.
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Lau, S. L., e W. S. Zhang. "Nonlinear Vibrations of Piecewise-Linear Systems by Incremental Harmonic Balance Method". Journal of Applied Mechanics 59, n. 1 (1 marzo 1992): 153–60. http://dx.doi.org/10.1115/1.2899421.
Testo completoAbstract (sommario):
The incremental harmonic balance (IHB) method is extended to analyze the periodic vibrations of systems with a general form of piecewise-linear stiffness characteristics. An explicit formulation has been worked out. This development is of significance as many structural and mechanical systems of practical interest possess a piecewise-linear stiffness. Typical examples show that the IHB method is very effective for analyzing this kind of systems under steady-state vibrations.
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Liu, Qiang, Kedong Zhou, Chao Shen e Lei He. "Effect of an Internal Impact Balance Mechanism on the Perceptible Recoil of Machine Gun". E3S Web of Conferences 231 (2021): 03001. http://dx.doi.org/10.1051/e3sconf/202123103001.
Testo completoAbstract (sommario):
In order to reasonably reduce the perceptible recoil of firearms, and design a recoil reduction device that can be commonly used in automatic rifles, semi-automatic rifles, and machine guns, a balancing mechanism for recoil reduction technology was proposed in this paper. With the perceptible recoil generated by the impact from automat to the balance mechanism as the objective function, the parametric analysis of the mass of the balance weight and the spring stiffness of the balance mechanism was conducted. The analysis showed that the influence of the mass of the balance weight on the reduction of the perceptible recoil of the balance mechanism is obviously greater than that of the spring stiffness of the balance mechanism within the allowable ranges of the two factors. The research results provide a reference for gun recoil reduction technology.
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Liu, Heng, Jie Hong, Shilun Ruan, Zheng Li e Gengdong Cheng. "A Model accounting for Stiffness Weakening of Curvic Couplings under Various Loading Conditions". Mathematical Problems in Engineering 2020 (11 febbraio 2020): 1–17. http://dx.doi.org/10.1155/2020/1042375.
Testo completoAbstract (sommario):
Curvic couplings are frequently used in aeroengine rotors. The stiffness of the curvic couplings is of guiding significance to the engineering design of aeroengine rotors as it is significantly different from that of continuous structures. In this paper, definitions and relations of the structure parameters for a curvic coupling are firstly introduced. Based on this proposed mechanical framework, a novel mechanical model accounting for the stiffness weakening under shearing, compression, bending, and torsion is developed for curvic couplings. In this model, a three-spring system, which consists of two types of springs, is adopted to describe the equivalent stiffness of a pair of meshing teeth of curvic couplings. The spring stiffness is obtained by employing the plane strain analysis of a discretized tooth with trapezoid pieces. Subsequently, the stiffness matrix of curvic couplings is deduced based on the deformation compatibility of each tooth and the force balance of the whole structure. A series of analyses of curvic couplings with various structure types are performed to demonstrate the mechanism behind the proposed model, and the results are verified against those obtained from finite element analyses. It is shown in this study that the pressure angle is the major factor affecting the stiffness of curvic couplings, while the compression stiffness and bending stiffness are more sensitive than other stiffnesses. Furthermore, the stiffness of curvic couplings is considerably smaller compared to that of continuous structures, indicating the importance of appropriate modelling of stiffness weakening in the design of aeroengine rotors.
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Jo, Sun-Ha, Hyuk-Jae Choi, Hyeon-Seok Cho, Jin-Hwan Yoon e Won-Young Lee. "Effect of Core Balance Training on Muscle Tone and Balance Ability in Adult Men and Women". International Journal of Environmental Research and Public Health 19, n. 19 (26 settembre 2022): 12190. http://dx.doi.org/10.3390/ijerph191912190.
Testo completoAbstract (sommario):
(1) Background: The amount of physical activity most adults perform is less than the recommended amount, and the resulting decrease in physical strength makes them vulnerable to various diseases. A decrease in muscle size and strength due to damage caused by disease or aging negatively affects functional strength. Muscle evaluation in adults can yield results that are predictive indicators of aging and unexpected disability. In addition, balance ability is essential to prevent falls and injuries in daily life and maintain functional activities. It is important to develop and strengthen balance in the lower extremities and core muscles to maintain and enhance overall body balance. This study aimed to analyze the effects of core balance training on muscle tone and balance ability in adults. (2) Methods: The participants of this study were 32 adult male and female university students (male: mean age = 21.3 ± 1.9 years, weight = 74.2 ± 12.6 kg, BMI = 23.4 + 2.5, n = 14; female: mean age = 21.0 ± 1.4 years, weight = 64.6 + 1.2 kg, BMI = 22.4 ± 2.4, n =18). Thirty-two adults (training group: 16, control group: 16; male: 16, female: 16) participated in the Myoton PRO (gastrocnemius lateral/medial, tibialis anterior), Pedalo balance system, and Y-balance test. (3) Results: The following results were obtained for muscle elasticity, stiffness, and dynamic/static balance ability after 10 weeks of core balance training. 1. There was no significant difference in muscle elasticity (gastrocnemius lateral/medial, tibialis anterior) (p < 0.05). 2. Muscle stiffness (gastrocnemius lateral/medial, tibialis anterior) significantly increased (p < 0.05). 3. Dynamic/static balance ability significantly increased (p < 0.05). (4) Conclusions: In future, data for the age and sex of various participants, should be accumulated by recruiting participants to study muscle characteristics, such as muscle elasticity and stiffness. Estimating the appropriate injury range and optimal exercise capacity is possible through follow-up studies. The findings can then be used as a basis for predicting injuries or determining and confirming the best time to resume exercise.
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Box, Finn, Doireann O’Kiely, Ousmane Kodio, Maxime Inizan, Alfonso A. Castrejón-Pita e Dominic Vella. "Dynamics of wrinkling in ultrathin elastic sheets". Proceedings of the National Academy of Sciences 116, n. 42 (30 settembre 2019): 20875–80. http://dx.doi.org/10.1073/pnas.1905755116.
Testo completoAbstract (sommario):
The wrinkling of thin elastic objects provides a means of generating regular patterning at small scales in applications ranging from photovoltaics to microfluidic devices. Static wrinkle patterns are known to be governed by an energetic balance between the object’s bending stiffness and an effective substrate stiffness, which may originate from a true substrate stiffness or from tension and curvature along the wrinkles. Here, we investigate dynamic wrinkling induced by the impact of a solid sphere onto an ultrathin polymer sheet floating on water. The vertical deflection of the sheet’s center induced by impact draws material radially inward, resulting in an azimuthal compression that is relieved by the wrinkling of the entire sheet. We show that this wrinkling is truly dynamic, exhibiting features that are qualitatively different to those seen in quasistatic wrinkling experiments. Moreover, we show that the wrinkles coarsen dynamically because of the inhibiting effect of the fluid inertia. This dynamic coarsening can be understood heuristically as the result of a dynamic stiffness, which dominates the static stiffnesses reported thus far, and allows control of wrinkle wavelength.
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Gong, Fei, Rupeng Zhu e Qibo Wang. "Dynamics Modeling and Load-Sharing Performance Optimization of Concentric Face Gear Split-Torque Transmission Systems". Applied Sciences 13, n. 7 (29 marzo 2023): 4352. http://dx.doi.org/10.3390/app13074352.
Testo completoAbstract (sommario):
The concentric face gear split-torque transmission system (CFGSTTS) has the advantages of a large reduction ratio and high power density. The CFGSTTS has considerable potential to be applied in helicopter main reducers. As such, in this study, we analyzed the load distribution characteristics of a dual input–dual output concentric face gear split-torque transmission system. A load-dependent time-varying meshing stiffness surrogate model was designed based on a feedforward neural network. The difference in the meshing stiffness between the pinion driving and face gear driving was analyzed. The coupled lumped parameter dynamic model of the bending–torsion–axis–pendulum was developed through Newton’s second law, and the influences of the time-varying meshing stiffness, backlash, comprehensive transmission error, support stiffness, and damping were considered. Finally, the impact of the support stiffness on the load-sharing coefficient was analyzed. An optimization model was constructed with the objective function of minimizing the sum of the load-sharing coefficients and was solved by the marine predator algorithm. In addition, the validity of the optimization results was verified with a finite element model. The results indicate that (1) smaller support stiffnesses of input gears benefit the corresponding load balance; (2) the support stiffnesses of the face gears have different laws of influence on the load-sharing coefficient at the input gear and idler, and the support stiffnesses of the other gears need to be comprehensively considered; (3) the larger supporting stiffnesses of the idler gears and tail gear are beneficial for decreasing the load-sharing coefficient at the input gear; and (4) the optimized load-sharing coefficients at Input Gears 1 and 2 and the idler gear decrease by 23.7%, 24.2%, and 4.6%, respectively.
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Kashani, H., e A. S. Nobari. "Structural Nonlinearity Identification Using Perturbed Eigen Problem and ITD Modal Analysis Method". Applied Mechanics and Materials 232 (novembre 2012): 949–54. http://dx.doi.org/10.4028/www.scientific.net/amm.232.949.
Testo completoAbstract (sommario):
Identification of nonlinear behavior in structural dynamics has been considered here, in this paper. Time domain output data of system are directly used to identify system through Ibrahim Time Domain (ITD) modal analysis method and perturbed eigen problem. Cubic stiffness and Jenkins element, as case studies, are employed to qualify the identification method. Results are compared with Harmonic Balance (HB) estimation of nonlinear dynamic stiffness. Results of ITD based identification are in good agreement with the HB estimation, for stiffness parts of nonlinear dynamic stiffness but for damping parts of nonlinear dynamic stiffness, method needs some additional improvements which are under investigation.
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Xu, Bin, Shien Yu e Jianzhong Zhang. "Design Method of Three-Component Optic Fiber Balance Based on Fabry–Perot Displacement Sensor". Sensors 23, n. 17 (29 agosto 2023): 7492. http://dx.doi.org/10.3390/s23177492.
Testo completoAbstract (sommario):
This article proposes a new type of three-component optic fiber balance based on Fabry–Perot displacement measurement technology based on the structure of the pulse wind tunnel balance. This paper systematically introduces the force measurement principle and design process of a three-component optic fiber balance and conducts relevant simulation analysis and experimental verification. The simulation results show that the Fabry–Perot sensor can achieve significant sensitivity to cavity length changes, and when used in existing balance structures, sensitivity gains can be achieved by changing the probe height without the need to modify the original structure of the balance. Finally, the feasibility of the design method was verified through calibration experiments: the optic fiber balance has high sensitivity and good linearity compared to simulation sensitivity, the error is less than 6%, and the calibration accuracy of each component is better than 0.13%, which is better than the existing traditional strain balance (0.37%). The pulse wind tunnel force measurement test has a short test time and a large model mass, and the balance needs to have a large stiffness to meet the short-term force measurement requirements. The introduction of more sensitive optic fiber balance force measurement technology is expected to solve the contradiction between the stiffness and sensitivity of force measurement systems.
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Guo, Wei, Shiyin Qiu, Fusheng Zha, Jing Deng, Xin Wang e Fei Chen. "A novel active balance assistive control strategy based on virtual stiffness model of XCoM". Assembly Automation 40, n. 1 (23 maggio 2019): 132–42. http://dx.doi.org/10.1108/aa-10-2018-0159.
Testo completoAbstract (sommario):
PurposeThis paper aims to propose a novel balance-assistive control strategy for hip exoskeleton robot.Design/methodology/approachA hierarchical balance assistive controller based on the virtual stiffness model of extrapolated center of mass (XCoM) is proposed and tested by exoskeleton balance assistive control experiments.FindingsExperiment results show that the proposed controller can accelerate the swing foot chasing XCoM and enlarge the margin of stability.Originality/valueAs a proof of concept, this paper shows the potential for exoskeleton to actively assist human regain balance in sagittal plane when human suffers from a forward or backward disturbing force.
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Engelhart, D., J. H. Pasma, A. C. Schouten, R. G. K. M. Aarts, C. G. M. Meskers, A. B. Maier e H. van der Kooij. "Adaptation of multijoint coordination during standing balance in healthy young and healthy old individuals". Journal of Neurophysiology 115, n. 3 (1 marzo 2016): 1422–35. http://dx.doi.org/10.1152/jn.00030.2015.
Testo completoAbstract (sommario):
Standing balance requires multijoint coordination between the ankles and hips. We investigated how humans adapt their multijoint coordination to adjust to various conditions and whether the adaptation differed between healthy young participants and healthy elderly. Balance was disturbed by push/pull rods, applying two continuous and independent force disturbances at the level of the hip and between the shoulder blades. In addition, external force fields were applied, represented by an external stiffness at the hip, either stabilizing or destabilizing the participants' balance. Multivariate closed-loop system-identification techniques were used to describe the neuromuscular control mechanisms by quantifying the corrective joint torques as a response to body sway, represented by frequency response functions (FRFs). Model fits on the FRFs resulted in an estimation of time delays, intrinsic stiffness, reflexive stiffness, and reflexive damping of both the ankle and hip joint. The elderly generated similar corrective joint torques but had reduced body sway compared with the young participants, corresponding to the increased FRF magnitude with age. When a stabilizing or destabilizing external force field was applied at the hip, both young and elderly participants adapted their multijoint coordination by lowering or respectively increasing their neuromuscular control actions around the ankles, expressed in a change of FRF magnitude. However, the elderly adapted less compared with the young participants. Model fits on the FRFs showed that elderly had higher intrinsic and reflexive stiffness of the ankle, together with higher time delays of the hip. Furthermore, the elderly adapted their reflexive stiffness around the ankle joint less compared with young participants. These results imply that elderly were stiffer and were less able to adapt to external force fields.
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Pasma, J. H., D. Engelhart, A. B. Maier, A. C. Schouten, H. van der Kooij e C. G. M. Meskers. "Changes in sensory reweighting of proprioceptive information during standing balance with age and disease". Journal of Neurophysiology 114, n. 6 (1 dicembre 2015): 3220–33. http://dx.doi.org/10.1152/jn.00414.2015.
Testo completoAbstract (sommario):
With sensory reweighting, reliable sensory information is selected over unreliable information during balance by dynamically combining this information. We used system identification techniques to show the weight and the adaptive process of weight change of proprioceptive information during standing balance with age and specific diseases. Ten healthy young subjects (aged between 20 and 30 yr) and 44 elderly subjects (aged above 65 yr) encompassing 10 healthy elderly, 10 with cataract, 10 with polyneuropathy, and 14 with impaired balance, participated in the study. During stance, proprioceptive information of the ankles was disturbed by rotation of the support surface with specific frequency content where disturbance amplitude increased over trials. Body sway and reactive ankle torque were measured to determine sensitivity functions of these responses to the disturbance amplitude. Model fits resulted in a proprioceptive weight (changing over trials), time delay, force feedback, reflexive stiffness, and damping. The proprioceptive weight was higher in healthy elderly compared with young subjects and higher in elderly subjects with cataract and with impaired balance compared with healthy elderly subjects. Proprioceptive weight decreased with increasing disturbance amplitude; decrease was similar in all groups. In all groups, the time delay was higher and the reflexive stiffness was lower compared with young or healthy elderly subjects. In conclusion, proprioceptive information is weighted more with age and in patients with cataract and impaired balance. With age and specific diseases the time delay was higher and reflexive stiffness was lower. These results illustrate the opportunity to detect the underlying cause of impaired balance in the elderly with system identification.
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Hirano, Atsushi. "Study on Wheel Stiffness Considering Balance between Driving Stability and Weight". SAE International Journal of Commercial Vehicles 8, n. 1 (14 aprile 2015): 205–12. http://dx.doi.org/10.4271/2015-01-1755.
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Bak, Christian, Nicholas Gaudern, Frederik Zahle e Tomas Vronsky. "Airfoil design: Finding the balance between design lift and structural stiffness". Journal of Physics: Conference Series 524 (16 giugno 2014): 012017. http://dx.doi.org/10.1088/1742-6596/524/1/012017.
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Le Mouel, Charlotte, e Romain Brette. "Anticipatory coadaptation of ankle stiffness and sensorimotor gain for standing balance". PLOS Computational Biology 15, n. 11 (22 novembre 2019): e1007463. http://dx.doi.org/10.1371/journal.pcbi.1007463.
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Kang, Hyun Gu, e Lewis A. Lipsitz. "Stiffness Control of Balance During Quiet Standing and Dual Task in Older Adults: The MOBILIZE Boston Study". Journal of Neurophysiology 104, n. 6 (dicembre 2010): 3510–17. http://dx.doi.org/10.1152/jn.00820.2009.
Testo completoAbstract (sommario):
Distractions affect postural control, but this mechanism is not well understood. Diversion of resources during cognitive stress may lead to decreased motor drive and postural muscle tone. This may appear as decreased postural stiffness and increased postural sway amplitude. We hypothesized that dual tasking leads to decreased stiffness and increased sway amplitude. Postural sway (center of pressure; COP) data were used from 724 participants aged 77.9 ± 5.3 yr, a representative sample of community-dwelling older adults, the MOBILIZE Boston Study cohort. Subjects stood barefoot with eyes open for 30 s per trial on a force plate. Five trials were performed each with and without a serial subtractions-by-3 task. Sway data were fit to a damped oscillator inverted pendulum model. Amplitudes (COP and center of mass), mechanical stiffness, and damping of the sway behavior were determined. Sway amplitudes and damping increased with the dual task ( P < 0.001); stiffness decreased only mediolaterally ( P < 0.001). Those with difficulty doing the dual task exhibited larger sway and less damping mediolaterally ( P ≤ 0.001) and an increased stiffness with dual task anteroposteriorly (interaction P = 0.004). Dual task could still independently explain increases in sway ( P < 0.001) after accounting for stiffness changes. Thus the hypothesis was supported only in mediolateral sway. The simple model helped to explain the dual task related increase of sway only mediolaterally. It also elucidated the differential influence of cognitive function on the mechanics of anteroposterior and mediolateral sway behaviors. Dual task may divert the resources necessary for mediolateral postural control, thus leading to falls.
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Yan, Peng, Shaoran Kang e Lebo Ma. "The Influence of Glass Fiber/Glass Fiber Powder with β-Nucleating Agent on the Properties of Polypropylene". International Journal of Polymer Science 2023 (27 marzo 2023): 1–16. http://dx.doi.org/10.1155/2023/1240792.
Testo completoAbstract (sommario):
Glass fiber-reinforced polypropylene (PP/GF) has been widely used due to its high stiffness, but for some applications that need low-module characteristics, PP/GF will have limitations due to its lower toughness, so it is necessary to develop glass fiber-modified polypropylene with good stiffness–toughness balance performance. In this study, two average length glass fibers (4.5 mm and 12 mm) and glass fiber powder, combined with β-nucleating agent, were used to investigate the effects on the crystallization and mechanical properties of polypropylene. The results show that compared with glass fiber, glass fiber powder cooperates with β-nucleating agent reinforced polypropylene composite showed good stiffness–toughness balance performance, and β-crystals were found in the composite measured by Differential Scanning Calorimetry (DSC), the presence of β-crystals can improve the toughness of the composite.
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Qiao, Jinwei, e Na Liu. "Numerical Simulation of Bending Stiffness Analysis for Spring Linkage Applied to In-Pipe Robot". Mathematical Problems in Engineering 2020 (27 agosto 2020): 1–11. http://dx.doi.org/10.1155/2020/6701696.
Testo completoAbstract (sommario):
Spring linkage can be applied to in-pipe robots for connecting different modules together and can make it pass through elbows more easily. However, its stiffness cannot be set to be too hard or too soft. This paper tries to make a balance between the compressive stiffness and the bending stiffness of the spring. After a brief introduction to the construction mechanism and some assumptions, the mathematical representation of the spring bending stiffness was deduced based on the Kirchhoff theory which describes the spatial curve with displacement rather than time. Then, some simulations aiming at verifying the correctness of the deduced bending stiffness expression were carried out. Finally, the relationship between the two rigidities was found out, which helps to find a way to decrease the bending stiffness of spring while keeping its compressive stiffness strong enough.
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Zhang, Yuanyuan, Huaiju Liu, Caichao Zhu, Chaosheng Song e Zufeng Li. "Influence of lubrication starvation and surface waviness on the oil film stiffness of elastohydrodynamic lubrication line contact". Journal of Vibration and Control 24, n. 5 (8 agosto 2016): 924–36. http://dx.doi.org/10.1177/1077546316655024.
Testo completoAbstract (sommario):
Stiffness properties of interfacial engineering surfaces are of great importance to the dynamic performance of relevant mechanical systems. Normal contact stiffness and oil film stiffness of line contact problems are studied in this work analytically and numerically. The Hertzian contact theory and the Yang–Sun method are applied to predict the contact stiffness, while the empirical elastohydrodynamic lubrication (EHL) film thickness method and the complete numerical EHL model are used to predict the oil film stiffness. The numerical model mainly consists of the Reynolds equation; the film thickness equation, in which the regular surface roughness is taken into consideration; the force balance equation; and the viscosity-pressure equation. The effects of the normal load, rolling speed, regular surface waviness, and starved lubrication level on the oil film stiffness are investigated.
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Jiang, Bing. "DO WE NEED VERY STIFF FILLERS?" Rubber Chemistry and Technology 90, n. 4 (1 ottobre 2017): 743–50. http://dx.doi.org/10.5254/rct.18.82673.
Testo completoAbstract (sommario):
ABSTRACT The influence of reinforcing fillers on the stretching of a rubber matrix is analyzed. It is shown that a filler stiffness higher than a critical stiffness does not further enhance the stiffness of the reinforced elastomer. The stiffer filler induces a higher stress–strain concentration and causes filler–rubber dissociation or chain scission at a lower macroscopic strain. Reducing filler stiffness can reduce the stress–strain concentration and therefore delay rubber chain scission or dissociation from the filler surface. Accordingly, the toughness of the reinforced elastomer could be improved. A simple material model is developed to predict the maximum macroscopic strain without bond scission in a reinforced elastomer. It is shown that reduced filler stiffness is beneficial for cases with (i) reduced bond strength, (ii) increased rubber matrix stiffness, and (iii) increased application strain of the reinforced elastomer. The model can be used to design the appropriate filler stiffness to balance trade-offs of stiffness and toughness of reinforced elastomers.
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Guo, Yang, Shulin Sun e Huixuan Zhang. "Modification of the core–shell ratio to prepare PB-g-(MMA-co-St-co-GMA) particle-toughened poly(butylene terephthalate) and polycarbonate blends with balanced stiffness and toughness". RSC Adv. 4, n. 102 (2014): 58880–87. http://dx.doi.org/10.1039/c4ra08646e.
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Li, Chang He, Wei Ping Mao e Yu Cheng Ding. "Numerical Investigation into Spindle System Stiffness of High-Speed Grinder". Key Engineering Materials 487 (luglio 2011): 490–94. http://dx.doi.org/10.4028/www.scientific.net/kem.487.490.
Testo completoAbstract (sommario):
This study was focused on the theoretical modeling and numerical investigation about the dynamic and static stiffness of spindle system of high speed grinder. The moment balance and the transition matrix, the state vector, field matrix of spindle system of high speed grinder were analyzed and deduced. The theoretical models about dynamic and static stiffness were established using the transfer matrix method. The numerical results showed that increas of the preload could result in the improvement of static stiffness of spindle end within the range of its working speed; the reduction of front overhang length would improve the stiffness of spindle end, as well as the dynamic stiffness of spindle at the working speed; the stiffness of spindle end decreased with the increase of speed with different bearing spans within the range of working speed of spindle.
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Khan, Fayaz, Mohamed Faisal Chevidikunnan, Raghad Ahmad Almalki, Mawada Khaled Mirdad, Khadeeja Abdulaziz Nimatallah, Shahad Al-Zahrani e Aysha Abdulmalek Alshareef. "Stiff-Person Syndrome Outpatient Rehabilitation: Case Report". Journal of Neurosciences in Rural Practice 11, n. 04 (20 agosto 2020): 651–53. http://dx.doi.org/10.1055/s-0040-1715081.
Testo completoAbstract (sommario):
AbstractStiff-person syndrome (SPS) is a rare neurological disorder that causes muscle rigidity and stiffness of the trunk and proximal limb muscles, leading to movement difficulties and impaired function. Due to the rarity of the disease, studies on the benefit of rehabilitation for this disorder are quite limited. A 46-year-old female patient diagnosed with SPS complained of imbalance and movement difficulty. We prescribed therapeutic exercises aimed to reduce the stiffness of the trunk and proximal limbs and improve her function. Baseline measurement of the patient's range of motion, muscle power and tone, balance and functional abilities were taken pre- and post-program. Outcome measures showed a general improvement in the patient's muscle flexibility, balance, and functionality.
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Wang, Shuo, Meng Cao, Hongqian Xue, Fanglin Cong, Xiaodong Li, Changbao Zhao e Weiguo Su. "Nano-silica reinforced epoxy resin/nano-rubber composite material with a balance of stiffness and toughness". High Performance Polymers 33, n. 6 (20 gennaio 2021): 685–94. http://dx.doi.org/10.1177/0954008320988752.
Testo completoAbstract (sommario):
In the electronics and aerospace industries, epoxy resins are generally regarded as economical and efficient adhesives and have a high status. However, epoxy resins are highly crosslinked polymers and are very brittle adhesives where they are prone to fast crack propagation under dynamic loads. Therefore, it is very necessary to enhance the toughness of epoxy resin adhesives. Nano-rubber has been proved to be an important toughening agent for epoxy resin, which can significantly improve the fracture toughness of epoxy resin. However, increasing the toughness of epoxy resin by adding nanomaterials is often accompanied by decreasing the strength and stiffness of resin. Therefore, in this work, rigid nano-silica particles were added to improve the rigidity and tensile strength reduction caused by the addition of rubber particles. And further increase the toughness of the epoxy resin to obtain an epoxy adhesive with balanced stiffness-toughness. As a result, it can be found that the addition of silica particles can significantly improve the decrease in stiffness caused by the addition of rubber particles. For example, Young’s modulus and tensile strength are increased by 28%, and 23%, respectively, with 4% silica is added based on rubber particles. Through the single lap shear experiment, it is found that the shear strength of the epoxy/RnP/silica composite adhesive has increased, which further proves that the addition of nano-silica particles can increase the stiffness of the epoxy composite. The dynamic mechanical analysis experiment found that after adding nano-silica particles, the storage modulus of epoxy composites increased, which also shows that adding nano-silica particles can improve the stiffness of epoxy composites. Scanning electron microscopy analysis was performed to study the reinforcement mechanism of epoxy/RnP/silica composite materials. The thermal stability of epoxy composites was characterized by Dynamic mechanical analysis and thermogravimetric analysis.
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Liu, Gang, e Robert G. Parker. "Nonlinear, parametrically excited dynamics of two-stage spur gear trains with mesh stiffness fluctuation". Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science 226, n. 8 (17 maggio 2012): 1939–57. http://dx.doi.org/10.1177/0954406212447509.
Testo completoAbstract (sommario):
This work studies the nonlinear, parametrically excited dynamics of two spur gear pairs in a two-stage counter-shaft configuration. The dynamic model includes parametric excitations and contact loss of both tooth meshes with two different mesh frequencies. The time-varying mesh stiffnesses and nonlinear tooth separation functions are reformulated into forms suitable for perturbation analysis. The periodic steady-state solutions are obtained by the method of multiple scales and compared against a semi-analytical harmonic balance method as well as numerical integration for fundamental and subharmonic resonances for ranges of system parameters. The interaction of the two meshes is found to depend strongly on the relation of the two mesh periods. The dynamic influences of design parameters, such as shaft stiffness, mesh stiffness variations, contact ratios, and mesh phasing, are discussed. The closed-form solutions provide design guidelines in terms of the system parameters.
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Dhanda, Mohit, e Arpan Gupta. "Investigation of Jump Resonance of a Horizontal Axis Washing Machine for Nonlinear Vibration using the Harmonic Balance Method". IOP Conference Series: Materials Science and Engineering 1225, n. 1 (1 febbraio 2022): 012040. http://dx.doi.org/10.1088/1757-899x/1225/1/012040.
Testo completoAbstract (sommario):
Abstract In this work, a drum-type horizontal axis washing machine with two springs and two dampers has been considered to study the jump resonance phenomenon. Here two springs and two dampers are used and effects of nonlinear spring stiffness on vibration level in both horizontal and vertical directions have been investigated. In this study, a mathematical model of a horizontal axis washing machine is formed. A separate analysis is carried out in both vertical and horizontal directions by considering the whole system as a two degree of freedom system. As vibration magnitude is large due to unbalance mass of wet clothes, hence nonlinear stiffness of spring is considered. The analytical solution for the mathematical model is found using the harmonic balance method. One term expansion is used in this method to study the response of the system. To study the vibration response of the system, one term expansion is used. From this study, it has been found that the spring with cubic nonlinear stiffness increases the amplitude of vibration. Also, with the increase in cubic non-linear stiffness, the system tends to have multiple equilibrium positions. These equilibrium positions may be stable or unstable. It has been detected that the vibration amplitude has three different values for a few excitation frequencies due to cubic stiffness. So, the response of the system suddenly increases and then decreases due to cubic non-linearity which indicates the jump resonance.
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Reeves, N. Peter, Vanessa Q. Everding, Jacek Cholewicki e David C. Morrisette. "The effects of trunk stiffness on postural control during unstable seated balance". Experimental Brain Research 174, n. 4 (25 maggio 2006): 694–700. http://dx.doi.org/10.1007/s00221-006-0516-5.
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Chen, Yuan, Mingwang Pan, Yue Li, Jia-Zhuang Xu, Gan-Ji Zhong, Xu Ji, Zheng Yan e Zhong-Ming Li. "Core-shell nanoparticles toughened polylactide with excellent transparency and stiffness-toughness balance". Composites Science and Technology 164 (agosto 2018): 168–77. http://dx.doi.org/10.1016/j.compscitech.2018.04.037.
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Li, Eric, e Z. C. He. "Optimal balance between mass and smoothed stiffness in simulation of acoustic problems". Applied Mathematical Modelling 75 (novembre 2019): 1–22. http://dx.doi.org/10.1016/j.apm.2019.05.019.
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Liu, Hai Tao, Zhi Yu Wen, Zheng Guo Shang e Li Chen. "A New Method to Analyze the Stiffness of MEMS Accelerometer". Key Engineering Materials 609-610 (aprile 2014): 710–14. http://dx.doi.org/10.4028/www.scientific.net/kem.609-610.710.
Testo completoAbstract (sommario):
The finite element method to obtain the stiffness of MEMS (Micro-Electro-Mechanical Systems) accelerometer is difficult to give an exact expression, so a new method to analyze the stiffness of MEMS accelerometer was purposed. Both Mechanics and electricity analysis were used to calculate the stiffness of vacuum microelectronic accelerometer developed in our laboratory, and the control and detection circuits were design according to the result. Finally, the sensitivity, linearity and other performance were measured through the static gravitational field experiments; the least squares curve fitting correlation coefficient 0.9999 and linear 0.9%. The result shows that this method to analyze the stiffness of the stiffness of vacuum microelectronics accelerometer is correct and feasible, and this method can also be applied to other MEMS accelerometer with symmetrical structure and electrostatic force balance mode.
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Ramadurai, Sruthi, Michael Jacobson, Prakyath Kantharaju, Hyeongkeun Jeong, Heejin Jeong e Myunghee Kim. "Evaluation of Lower Limb Exoskeleton for Improving Balance during Squatting Exercise using Center of Pressure Metrics". Proceedings of the Human Factors and Ergonomics Society Annual Meeting 66, n. 1 (settembre 2022): 858–62. http://dx.doi.org/10.1177/1071181322661447.
Testo completoAbstract (sommario):
The foot center of pressure (COP) variability is an important indicator of balance, particularly relevant for rehabilitation and training using wearable lower limb exoskeletons. This study aimed to evaluate the effectiveness of our exoskeleton in assisting squatting motion using the COP variability as a metric. Six human subjects performed alternate squatting and standing movements while their foot pressure and COP trajectories were recorded using insole pressure sensors. The exercises were performed under three conditions: i) no device, ii) unpowered device, and iii) device with optimal stiffness. Results showed that the variability of the COP trajectory in the anterior-posterior direction of the foot during squatting tended to be lower for the optimal stiffness condition than the no device and unpowered device conditions, indicating the potential usefulness of the device in improving balance during squatting. This study has implications for human-inthe-loop optimization and balance control of the exoskeleton based on COP.
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Boonstra, T. A., A. C. Schouten, J. P. P. van Vugt, B. R. Bloem e H. van der Kooij. "Parkinson's disease patients compensate for balance control asymmetry". Journal of Neurophysiology 112, n. 12 (15 dicembre 2014): 3227–39. http://dx.doi.org/10.1152/jn.00813.2013.
Testo completoAbstract (sommario):
In Parkinson's disease (PD) subtle balance abnormalities can already be detected in early-stage patients. One feature of impaired balance control in PD is asymmetry: one leg produces more corrective joint torque than the other. We hypothesize that in mild to moderately affected PD patients, the least impaired leg compensates for the more impaired leg. Twenty PD patients and eleven healthy matched control subjects participated. Clinical asymmetry was determined by the difference between the left and right body side scores on the Unified Parkinson's Disease Rating Scale. Balance was perturbed with two independent continuous multisine perturbations in the forward-backward direction. Subsequently, we applied closed-loop system identification, which determined the spectral estimate of the stabilizing mechanisms, for each leg. Balance control behavior was similar in PD patients and control subjects at the ankle, but at the hip stiffness was increased. Control subjects exhibited symmetric balance control, but in PD patients the balance contribution of the leg of the clinically least affected body side was higher whereas the leg of the clinically most affected body side contributed less. The ratio between the legs helped to preserve a normal motor output at the ankle. Our results suggest that PD patients compensate for balance control asymmetries by increasing the relative contribution of the leg of their least affected body side. This compensation appears to be successful at the ankle but is accompanied by an increased stiffness at the hip. We discuss the possible implications of these findings for postural stability and fall risk in PD patients.
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Zhang, Qingshan, Robin Trama, Alexandre Fouré e Christophe A. Hautier. "The Immediate Effects of Self‐Myofacial Release on Flexibility, Jump Performance and Dynamic Balance Ability". Journal of Human Kinetics 75, n. 1 (31 ottobre 2020): 139–48. http://dx.doi.org/10.2478/hukin-2020-0043.
Testo completoAbstract (sommario):
Abstract Self-myofascial release (SMR) is a popular method to potentially increase the compliance and extensibility of the fascia and reduce muscle stiffness. The purpose of this study was to examine the acute effects of posterior muscle chain SMR on flexibility, vertical jump performance and balance ability. Eighteen young participants volunteered to take part in this crossover design study. They performed two self-massage sessions in randomized order separated by at least one week. One session consisted of posterior muscle chain SMR whereas the other one was performed on the upper limbs as a control intervention (CON). Flexibility was measured with the Toe Touch Test (TTT), Weight-Bearing Lunge Test (WBLT), and Straight Leg Raise Test (SLR). Jump performance was evaluated during a squat jump, a counter movement jump and a stiffness jump. Dynamic balance ability was assessed through the Star Excursion Balance Test. All these variables were measured before and after each intervention. A significant increase in flexibility (+3.5 ± 1.8 cm, +1.6 ± 1.0°, and +7.7 ± 4.0° for the TTT, WLBT, and SLR, respectively, p < 0.003) and balance performance (4.8 ± 3.9 cm, p < 0.003) was observed following SMR intervention compared to CON. Conversely, jumping performance was unchanged in both groups. SMR improves joint flexibility and dynamic balance ability.
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Muldoon, Jeanette, Sylvie Hampton, Sarah Gray e Trish Cosham. "Compression hosiery for venous disorders and oedema: a question of balance". British Journal of Community Nursing 25, Sup9 (1 settembre 2020): S26—S32. http://dx.doi.org/10.12968/bjcn.2020.25.sup9.s26.
Testo completoAbstract (sommario):
Compression therapy for venous and lymphatic conditions may be delivered via a range of treatment modalities using many different technologies, depending on the patient's condition and needs. Clinical decision-making relies on accurate assessment of the patient, their presenting and underlying clinical condition, skill and training of the applier and the available resources. However, changes in the patient's condition or lifestyle may necessitate re-evaluation of the treatment pathway. Generally, compression bandages and Velcro wraps are used in the intensive acute phase of treatment, with self-management using compression hosiery or wraps being used for long-term maintenance to prevent recurrence. Although guidelines recommend the highest class of compression hosiery for maximum effectiveness, clinical evidence shows practical challenges associated with application and tolerance of higher pressures and stiffness. An audit of a new type of compression garment was conducted, and it showed that incorporating stiffness into circular knitted hosiery helped overcome some of these challenges with improvements in limb size, skin softening and wound size. Additionally, self-management was facilitated by the ease of donning and doffing.
46
Dong, Hao, Libang Wang, Haoqin Zhang e Xiao-long Zhao. "Nonlinear Frequency Response Analysis of Double-helical Gear Pair Based on the Incremental Harmonic Balance Method". Shock and Vibration 2021 (15 aprile 2021): 1–20. http://dx.doi.org/10.1155/2021/6687467.
Testo completoAbstract (sommario):
The torsional dynamic model of double-helical gear pair considering time-varying meshing stiffness, constant backlash, dynamic backlash, static transmission error, and external dynamic excitation was established. The frequency response characteristics of the system under constant and dynamic backlashes were solved by the incremental harmonic balance method, and the results were further verified by the numerical integration method. At the same time, the influence of time-varying meshing stiffness, damping, static transmission error, and external load excitation on the amplitude frequency characteristics of the system was analyzed. The results show that there is not only main harmonic response but also superharmonic response in the system. The time-varying meshing stiffness and static transmission error can stimulate the amplitude frequency response of the system, while the damping can restrain the amplitude frequency response of the system. Changing the external load excitation has little effect on the amplitude frequency response state change of the system. Compared with the constant backlash, increasing the dynamic backlash amplitude can further control the nonlinear vibration of the gear system.
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Guo, Fuchen, Ke Li, Jiaxin Wu, Linli He e Linxi Zhang. "Effects of Topological Constraints on Penetration Structures of Semi-Flexible Ring Polymers". Polymers 12, n. 11 (11 novembre 2020): 2659. http://dx.doi.org/10.3390/polym12112659.
Testo completoAbstract (sommario):
The effects of topological constraints on penetration structures of semi-flexible ring polymers in a melt are investigated using molecular dynamics simulations, considering simultaneously the effects of the chain stiffness. Three topology types of rings are considered: 01-knot (the unknotted), 31-knot and 61-knot ring polymers, respectively. With the improved algorithm to detect and quantify the inter-ring penetration (or inter-ring threading), the degree of ring threading does not increase monotonously with the chain stiffness, existing a peak value at the intermediate stiffness. It indicates that rings interpenetrate most at intermediate stiffness where there is a balance between coil expansion (favoring penetrations) and stiffness (inhibiting penetrations). Meanwhile, the inter-ring penetration would be suppressed with the knot complexity of the rings. The analysis of effective potential between the rings provides a better understanding for this non-monotonous behavior in inter-ring penetration.
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Li, Ming, Wei Cheng e Ruili Xie. "Design and experiments of a quasi–zero-stiffness isolator with a noncircular cam-based negative-stiffness mechanism". Journal of Vibration and Control 26, n. 21-22 (19 febbraio 2020): 1935–47. http://dx.doi.org/10.1177/1077546320908689.
Testo completoAbstract (sommario):
This article presents a quasi–zero-stiffness isolator with a cam-based negative-stiffness mechanism, where the cam has a user-defined noncircular profile to generate negative stiffness to counterbalance the positive stiffness of the vertical spring and yield the quasi–zero-stiffness characteristic around the equilibrium position. Unlike previous studies, the proposed quasi–zero-stiffness isolator has the preferable feature that the desired cubic restoring force can be directly obtained through the well-designed profile of the cam in the negative-stiffness mechanism with the friction considered during the model design, rather than through the Taylor expansion and friction-ignoring assumption, which can avoid the approximation error between the theoretical design and the specific realization. The pure-cubic nonlinear differential equation of motion of the quasi–zero-stiffness isolator is derived and solved with the harmonic balance method, followed by the discussion of the relevant dynamic characteristics. Experimental studies are carried out based on the physical prototype of the quasi–zero-stiffness isolator. The results show that the quasi–zero-stiffness isolator can greatly extend the isolation frequency bandwidth and has a much lower resonance peak. In the low-frequency band, the quasi–zero-stiffness isolator greatly outperforms the corresponding linear system but is equivalent or even inferior in the high-frequency range with the increase of excitation force.
49
Liu, Zhenwei, Hongwei Bai, Yuanlin Luo, Qin Zhang e Qiang Fu. "Achieving a low electrical percolation threshold and superior mechanical performance in poly(l-lactide)/thermoplastic polyurethane/carbon nanotubes composites via tailoring phase morphology with the aid of stereocomplex crystallites". RSC Advances 7, n. 18 (2017): 11076–84. http://dx.doi.org/10.1039/c6ra27401c.
Testo completoAbstract (sommario):
We demonstrate a facile strategy to fabricate highly conductive PLLA/TPU/CNTs composites with very low percolation threshold and good stiffness–toughness balance via constructing stereocomplex crystallites in PLLA melt to tailor phase morphology.
50
Liu, Zuo Shi, e Yi Guo Wei. "Analysis and Simulation for Balance of Rigid Rotors Based on ADAMS". Advanced Materials Research 655-657 (gennaio 2013): 412–15. http://dx.doi.org/10.4028/www.scientific.net/amr.655-657.412.
Testo completoAbstract (sommario):
Conventional models for mechanical rotors based on Automatic Dynamic Analysis of Mechanical Systems (ADAMS) are over-constraint. This paper proposes a new method for simulating the stiffness and damping of bearings using bushing and other kinematic pairs. Analysis and Simulation for balance of rigid rotors by ADAMS were performed. To reduce the imbalance inertial force, the balance weight of a take-up machine has been optimized. The results were verified by the prototype test. Our results suggest a more effective method for balance of mechanical rotors in the design stage.