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Heyters, Marc, Alain Carpentier, Jacques Duchateau e Karl Hainaut. "Twitch Analysis as an Approach to Motor Unit Activation During Electrical Stimulation". Canadian Journal of Applied Physiology 19, n.º 4 (1 de dezembro de 1994): 451–61. http://dx.doi.org/10.1139/h94-037.
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The mechanical twitch in response to increasing electrical stimulus intensity, delivered both over the motor point and motor nerve, was recorded in the first dorsal interosseous (FDI) and the adductor pollicis (AP), and only over the motor point in the soleus (Sol), lateral (LG), and medial (MG) gastrocnemius muscles of human subjects. The relationship between intensity of electrical stimulation (ES) and twitch torque showed a positive linear regression in all muscles. In the FDI and AP the relationship was not significantly different when ES was applied at the motor point or over the motor nerve. At small intensities of activation, ES induced larger twitch torques in the MG and LG, which contain a roughly equal proportion of slow and fast motor units (MUs) compared to the Sol, which is composed mainly of slow type fibres. Moreover, the relationship between ES intensity and twitch time-to-peak is best fitted in all muscles by a power curve that shows a greater twitch time-to-peak range in its initial part for muscles containing a larger proportion of fast MUs (LG, MG) than for muscles mainly composed of slow MUs (Sol). In conclusion, these results induced by ES at the motor point and/or over the motor nerve confirm the concept of a reversed sequence of MU activation, as compared to voluntary contractions, and document this viewpoint in muscles of different function and composition. The reversed sequence of MU activation is more clearly evident during motor point ES. Key words: muscle contraction, mechanical twitch, motor point, nerve
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Hansen, N. L., S. Hansen, L. O. D. Christensen, N. T. Petersen e J. B. Nielsen. "Synchronization of Lower Limb Motor Unit Activity During Walking in Human Subjects". Journal of Neurophysiology 86, n.º 3 (1 de setembro de 2001): 1266–76. http://dx.doi.org/10.1152/jn.2001.86.3.1266.
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Synchronization of motor unit activity was investigated during treadmill walking (speed: 3–4 km/h) in 25 healthy human subjects. Recordings were made by pairs of wire electrodes inserted into the tibialis anterior (TA) muscle and by pairs of surface electrodes placed over this muscle and a number of other lower limb muscles (soleus, gastrocnemius lateralis, gastrocnemius medialis, biceps femoris, vastus lateralis, and vastus medialis). Short-lasting synchronization (average duration: 9.6 ± 1.1 ms) was observed between spike trains generated from multiunit electromyographic (EMG) signals recorded by the wire electrodes in TA in eight of nine subjects. Synchronization with a slightly longer duration (12.8 ± 1.2 ms) was also found in 13 of 14 subjects for paired TA surface EMG recordings. The duration and size of this synchronization was within the same range as that observed during tonic dorsiflexion in sitting subjects. There was no relationship between the amount of synchronization and the speed of walking. Synchronization was also observed for pairs of surface EMG recordings from different ankle plantarflexors (soleus, medial gastrocnemius, and lateral gastrocnemius) and knee extensors (vastus lateralis and medialis of quadriceps), but not or rarely for paired recordings from ankle and knee muscles. The data demonstrate that human motor units within a muscle as well as synergistic muscles acting on the same joint receive a common synaptic drive during human gait. It is speculated that the common drive responsible for the motor unit synchronization during gait may be similar to that responsible for short-term synchronization during tonic voluntary contraction.
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Watanabe, Renato N., Fernando H. Magalhães, Leonardo A. Elias, Vitor M. Chaud, Emanuele M. Mello e André F. Kohn. "Influences of premotoneuronal command statistics on the scaling of motor output variability during isometric plantar flexion". Journal of Neurophysiology 110, n.º 11 (1 de dezembro de 2013): 2592–606. http://dx.doi.org/10.1152/jn.00073.2013.
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This study focuses on neuromuscular mechanisms behind ankle torque and EMG variability during a maintained isometric plantar flexion contraction. Experimentally obtained torque standard deviation (SD) and soleus, medial gastrocnemius, and lateral gastrocnemius EMG envelope mean and SD increased with mean torque for a wide range of torque levels. Computer simulations were performed on a biophysically-based neuromuscular model of the triceps surae consisting of premotoneuronal spike trains (the global input, GI) driving the motoneuron pools of the soleus, medial gastrocnemius, and lateral gastrocnemius muscles, which activate their respective muscle units. Two types of point processes were adopted to represent the statistics of the GI: Poisson and Gamma. Simulations showed a better agreement with experimental results when the GI was modeled by Gamma point processes having lower orders (higher variability) for higher target torques. At the same time, the simulations reproduced well the experimental data of EMG envelope mean and SD as a function of mean plantar flexion torque, for the three muscles. These results suggest that the experimentally found relations between torque-EMG variability as a function of mean plantar flexion torque level depend not only on the intrinsic properties of the motoneuron pools and the muscle units innervated, but also on the increasing variability of the premotoneuronal GI spike trains when their mean rates increase to command a higher plantar flexion torque level. The simulations also provided information on spike train statistics of several hundred motoneurons that compose the triceps surae, providing a wide picture of the associated mechanisms behind torque and EMG variability.
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Dacko, S. M., A. J. Sokoloff e T. C. Cope. "Recruitment of triceps surae motor units in the decerebrate cat. I. Independence of type S units in soleus and medial gastrocnemius muscles". Journal of Neurophysiology 75, n.º 5 (1 de maio de 1996): 1997–2004. http://dx.doi.org/10.1152/jn.1996.75.5.1997.
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1. We tested the hypothesis that reflex inhibition of soleus motor units reflects selective inhibition of slow-twitch (type S) motor units throughout the triceps surae. Physiological properties including type, together with firing behavior, were measured from single motor units in the medial gastrocnemius (MG) muscle of decerebrate cats with the use of intra-axonal recording and stimulation. MG unit firing was contrasted during net inhibition or excitation of the slow-twitch soleus muscle produced by ramp-hold-release stretches of MG. 2. Stretch of the MG muscle increased the firing of type S motor units in the MG regardless of the reflex response of the soleus muscle. When stretch inhibited soleus, each of the 14 type S units sampled from MG either was newly recruited or exhibited increases in the rate of ongoing firing. Increased firing was observed in 320 of 321 stretch trials. For 8 of these 14 units, a total of 155 stretch trials evoked reflex excitation of soleus, and unit firing increased in all trials. 3. For the eight MG type S motor units studied during both reflex inhibition and excitation of soleus, firing rate tended to be higher during inhibition. The higher rates were also associated with the higher MG forces required to elicit soleus inhibition. For one MG type S unit it was possible to compare firing rates during soleus inhibition and excitation for trials of overlapping levels of MG force. For this unit, firing rate was similar, but still appreciably higher, during inhibition. 4. Soleus inhibition was also produced by stretch of the plantaris (PL) or lateral gastrocnemius (LG) muscles. Type S units in PL (n = 2) or in LG (n = 1) were recruited or increased firing rate even when stretch of these muscles produced soleus inhibition. 5. The firing behavior of 12 fast-twitch (type F) units was studied (11 from MG, 1 from PL). All type F units either were recruited or accelerated the rate of firing during soleus inhibition, as well as during soleus excitation. 6. These findings give evidence that reflex inhibition of type S motor units in the soleus muscle does not necessarily reflect an organizational scheme in which there is inactivation of type S units in other active muscles. In the DISCUSSION we point out the absence of direct evidence for selective inactivation of units on the basis of their type classification.
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Gordon, Keith E., Catherine R. Kinnaird e Daniel P. Ferris. "Locomotor adaptation to a soleus EMG-controlled antagonistic exoskeleton". Journal of Neurophysiology 109, n.º 7 (1 de abril de 2013): 1804–14. http://dx.doi.org/10.1152/jn.01128.2011.
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Locomotor adaptation in humans is not well understood. To provide insight into the neural reorganization that occurs following a significant disruption to one's learned neuromuscular map relating a given motor command to its resulting muscular action, we tied the mechanical action of a robotic exoskeleton to the electromyography (EMG) profile of the soleus muscle during walking. The powered exoskeleton produced an ankle dorsiflexion torque proportional to soleus muscle recruitment thus limiting the soleus' plantar flexion torque capability. We hypothesized that neurologically intact subjects would alter muscle activation patterns in response to the antagonistic exoskeleton by decreasing soleus recruitment. Subjects practiced walking with the exoskeleton for two 30-min sessions. The initial response to the perturbation was to “fight” the resistive exoskeleton by increasing soleus activation. By the end of training, subjects had significantly reduced soleus recruitment resulting in a gait pattern with almost no ankle push-off. In addition, there was a trend for subjects to reduce gastrocnemius recruitment in proportion to the soleus even though only the soleus EMG was used to control the exoskeleton. The results from this study demonstrate the ability of the nervous system to recalibrate locomotor output in response to substantial changes in the mechanical output of the soleus muscle and associated sensory feedback. This study provides further evidence that the human locomotor system of intact individuals is highly flexible and able to adapt to achieve effective locomotion in response to a broad range of neuromuscular perturbations.
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Finni, Taija, John A. Hodgson, Alex M. Lai, V. Reggie Edgerton e Shantanu Sinha. "Nonuniform strain of human soleus aponeurosis-tendon complex during submaximal voluntary contractions in vivo". Journal of Applied Physiology 95, n.º 2 (agosto de 2003): 829–37. http://dx.doi.org/10.1152/japplphysiol.00775.2002.
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The distribution of strain along the soleus aponeurosis tendon was examined during voluntary contractions in vivo. Eight subjects performed cyclic isometric contractions (20 and 40% of maximal voluntary contraction). Displacement and strain in the apparent Achilles tendon and in the aponeurosis were calculated from cine phase-contrast magnetic resonance images acquired with a field of view of 32 cm. The apparent Achilles tendon lengthened 2.8 and 4.7% in 20 and 40% maximal voluntary contraction, respectively. The midregion of the aponeurosis, below the gastrocnemius insertion, lengthened 1.2 and 2.2%, but the distal aponeurosis shortened 2.1 and 2.5%, respectively. There was considerable variation in the three-dimensional anatomy of the aponeurosis and muscle-tendon junction. We suggest that the nonuniformity in aponeurosis strain within an individual was due to the presence of active and passive motor units along the length of the muscle, causing variable force along the measurement site. Force transmission along intrasoleus connective tissue may also be a significant source of nonuniform strain in the aponeurosis.
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W Benjamin, Raghavendra V Pisale, SA Premchand, Edward Indla, Seema Valsalan Ennazhiyil, VR Akshara e Lovely S Livingston. "A Study of Tibial Nerve in the Popliteal Fossa Along With Its Variations in Its Branching Pattern". Academia Anatomica International 6, n.º 2 (22 de dezembro de 2020): 29–34. http://dx.doi.org/10.21276/aanat.2020.6.2.6.
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Background: A detailed Knowledge of these variations in motor branching patterns will help the surgeons when certain procedures are done for calf reduction and also when selective neurectomy is required. It is also required by the anesthetists to give neurolytic blocks. Subjects and Methods: 40 formalin-fixed lower limbs of adult human cadavers were selected. The origin of the tibial nerve, variations in a branching pattern, number of muscular branches given was studied by dissection. The Level of origin of these nerves was taken to the apex of the head of the fibula (AHF). Results: In 70 % of specimens the origin of the Tibial Nerve was < 12 cm and in 30 % it was between 12-24 cm above the level of AHF. In 10% of cases, the sural nerve originated from the nerve to the medial head of gastrocnemius (MHG). In 82.5% of specimens, the MHG received one branch from the tibial nerve and in 17.5% it received two branches. The lateral head of Gastrocnemius (LHG) received one branch from the tibial nerve. In 10%, there was a common branch for the LHG and the soleus muscle. 90% of specimens had one branch and 10% had two branches that supplied the soleus muscle. A single branch supplied the plantaris muscle. The popliteus muscle also received a single branch. Conclusion : The results in the study provide information that is required by the anatomists, surgeons, radiologists and anesthetists.
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Tamaki, H., K. Kitada, T. Akamine, F. Murata, T. Sakou e H. Kurata. "Alternate activity in the synergistic muscles during prolonged low-level contractions". Journal of Applied Physiology 84, n.º 6 (1 de junho de 1998): 1943–51. http://dx.doi.org/10.1152/jappl.1998.84.6.1943.
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The purpose of this study was to investigate the functional interrelationship between synergistic muscle activities during low-level fatiguing contractions. Six human subjects performed static and dynamic contractions at an ankle joint angle of 110° plantar flexion and within the range of 90–110° (anatomic position = 90°) under constant load (10% maximal voluntary contraction) for 210 min. Surface electromyogram records from lateral gastrocnemius (LG), medial gastrocnemius (MG), and soleus (Sol) muscles showed high and silent activities alternately in the three muscles and a complementary and alternate activity between muscles in the time course. In the second half of all exercise times, the number of changes in activity increased significantly ( P < 0.05) in each muscle. The ratios of active to silent periods of electromyogram activity were significantly higher ( P< 0.05) in MG (4.5 ± 2.2) and Sol (4.3 ± 2.8) than in the LG (0.4 ± 0.1), but no significant differences were observed between MG and Sol. These results suggest that the relative activation of synergistic motor pools are not constant during a low-level fatiguing task.
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Kukulka, C. G. "The reflex effects of nonnoxious sural nerve stimulation on human triceps surae motor neurons". Journal of Neurophysiology 71, n.º 5 (1 de maio de 1994): 1897–906. http://dx.doi.org/10.1152/jn.1994.71.5.1897.
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1. The effects of low-intensity electrical stimulation of the ipsilateral sural nerve on the reflex response of human triceps surae motor neurons were examined in 169 motor units recorded in 11 adult volunteers: 69 units from soleus (SOL), 48 units from lateral gastrocnemius (LG), and 52 units from medial gastrocnemius (MG). The reflex effects were assessed by the peristimulus time histogram (PSTH) technique, categorized according to onset latencies, and the magnitudes of effects were calculated as percent changes in baseline firing rates. 2. Sural stimulation evoked complex changes in motor-unit firing at onset latencies between 28 and 140 ms. The two most common responses seen in all muscles were a short-latency depression (D1) in firing (mean onset latency = 40 ms) in 42% of all units studied and a secondary enhancement (E2) in firing (mean onset latency = 72 ms) in 43% of all units. In LG, the D1 effect represented a mean decrease in firing of 52% which was statistically different from the changes in MG (42% decrease) and SOL (38% decrease). The magnitudes of E2 effects were similar across muscles with an average of 47% increase in firing. 3. No differences were found in the frequencies of occurrence for the enhancements in firing among the muscles studied. The main difference in reflex responses was the occurrence of an intermediate latency depression (D2) in 27% of the LG units with a mean onset latency of 72 ms. 4. Based on estimates of conduction times for activation of low-threshold cutaneous afferents, the short-latency D1 response likely represents an oligosynaptic spinal reflex with transmission times similar to the Ia reciprocal inhibitory pathway. These findings raise the question as to the possibility of low-threshold cutaneous afferents sharing common interneurons with low-threshold muscle afferent reflexes that have identical onset latencies. The complex reflex effects associated with low-level stimulation of a cutaneous nerve indicate a rich assortment of peripheral responses that may influence a given movement. The predominance of a specific effect is most likely determined by the interaction of this input with other peripheral signals and descending commands specific to a given motor task.
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Hobbs, S. F., e D. I. McCloskey. "Effects of blood pressure on force production in cat and human muscle". Journal of Applied Physiology 63, n.º 2 (1 de agosto de 1987): 834–39. http://dx.doi.org/10.1152/jappl.1987.63.2.834.
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In anesthetized cats reducing local arterial pressure from 125 to 75 Torr decreased blood flow (53 +/- 5%) and force production (57 +/- 7%) in soleus and medial gastrocnemius. Force was produced in these muscles by aerobic, slowly fatiguing fibers. Similar reductions in arterial pressure did not affect force production in caudofemoralis, which contains mainly fast-fatiguing fibers. In human subjects the electromyogram produced by the ankle extensors during rhythmic constant-force contractions increased as the contracting muscles were raised above the heart during legs-up tilt. This suggests that force production of active muscle fibers at a given level of activation fell with muscle perfusion pressure, thus requiring augmentation of muscle activity to sustain the standard contractions. Because aerobic fibers contributed to these contractions, it appears that force production of human muscle fibers is sensitive to small changes in perfusion pressure and, presumably, blood flow. The critical dependence of developed muscular force on blood pressure is of importance to motor control and may also play a significant role in cardiovascular control during exercise.
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Trajano, Gabriel S., Laurent B. Seitz, Kazunori Nosaka e Anthony J. Blazevich. "Can passive stretch inhibit motoneuron facilitation in the human plantar flexors?" Journal of Applied Physiology 117, n.º 12 (15 de dezembro de 2014): 1486–92. http://dx.doi.org/10.1152/japplphysiol.00809.2014.
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The purpose of the present study was to examine the possible inhibitory effect of passive plantar flexor muscle stretching on the motoneuron facilitatory system. Achilles tendon vibration (70 Hz) and triceps surae electrical stimulation (20 Hz) were imposed simultaneously in 11 subjects to elicit contraction through reflexive pathways in two experiments. In experiment 1, a vibration-stimulation protocol was implemented with the ankle joint plantar flexed (+10°), neutral (0°), and dorsiflexed (−10°). In experiment 2, the vibration-stimulation protocol was performed twice before (control), then immediately, 5, 10, and 15 min after a 5-min intermittent muscle stretch protocol. Plantar flexor torque and medial and lateral gastrocnemius and soleus (EMGSol) EMG amplitudes measured during and after (i.e., self-sustained motor unit firing) the vibration protocol were used as an indicator of this facilitatory pathway. In experiment 1, vibration torque, self-sustained torque and EMGSol were higher with the ankle at −10° compared with 0° and +10°, suggesting that this method is valid to assess motoneuronal facilitation. In experiment 2, torque during vibration was reduced by ∼60% immediately after stretch and remained depressed by ∼35% at 5 min after stretch ( P < 0.05). Self-sustained torque was also reduced by ∼65% immediately after stretch ( P < 0.05) but recovered by 5 min. Similarly, medial gastrocnemius EMG during vibration was reduced by ∼40% immediately after stretch ( P < 0.05), and EMGSol during the self-sustained torque period was reduced by 44% immediately after stretch ( P < 0.05). In conclusion, passive stretch negatively affected the motoneuronal amplification for at least 5 min, suggesting that motoneuron disfacilitation is a possible mechanism influencing the stretch-induced torque loss.
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Kennedy, Paul M., Andrew G. Cresswell, Romeo Chua e J. Timothy Inglis. "Vestibulospinal influences on lower limb motoneurons". Canadian Journal of Physiology and Pharmacology 82, n.º 8-9 (1 de julho de 2004): 675–81. http://dx.doi.org/10.1139/y04-080.
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Galvanic vestibular stimulation (GVS) is a research tool used to activate the vestibular system in human subjects. When a low-intensity stimulus (1–4 mA) is delivered percutaneously to the vestibular nerve, a transient electromyographic response is observed a short time later in lower limb muscles. Typically, galvanically evoked responses are present when the test muscle is actively engaged in controlling standing balance. However, there is evidence to suggest that GVS may be able to modulate the activity of lower limb muscles when subjects are not in a free-standing situation. The purpose of this review is to examine 2 studies from our laboratory that examined the effects of GVS on the lower limb motoneuron pool. For instance, a monopolar monaural galvanic stimulus modified the amplitude of the ipsilateral soleus H-reflex. Furthermore, bipolar binaural GVS significantly altered the onset of activation and the initial firing frequency of gastrocnemius motor units. The following paper examines the effects of GVS on muscles that are not being used to maintain balance. We propose that GVS is modulating motor output by influencing the activity of presynaptic inhibitory mechanisms that act on the motoneuron pool.Key words: galvanic vestibular stimulation, h-reflex, motor unit, vestibulospinal, human.
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Aniss, A. M., S. C. Gandevia e D. Burke. "Reflex responses in active muscles elicited by stimulation of low-threshold afferents from the human foot". Journal of Neurophysiology 67, n.º 5 (1 de maio de 1992): 1375–84. http://dx.doi.org/10.1152/jn.1992.67.5.1375.
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1. Reflex responses were elicited in muscles that act at the ankle by electrical stimulation of low-threshold afferents from the foot in human subjects who were reclining supine. During steady voluntary contractions, stimulus trains (5 pulses at 300 Hz) were delivered at two intensities to the sural nerve (1.2-4.0 times sensory threshold) or to the posterior tibial nerve (1.1-3.0 times motor threshold for the intrinsic muscles of the foot). Electromyographic (EMG) recordings were made from tibialis anterior (TA), peroneus longus (PL), soleus (SOL), medial gastrocnemius (MG), and lateral gastrocnemius (LG) muscles by the use of intramuscular wire electrodes. 2. As assessed by averages of rectified EMG, stimulation of the sural or posterior tibial nerves at nonpainful levels evoked a complex oscillation with onset latencies as early as 40 ms and lasting up to 200 ms in each muscle. The most common initial responses in TA were a decrease in EMG activity at an onset latency of 54 ms for sural stimuli, and an increase at an onset latency of 49 ms for posterior tibial stimuli. The response of PL to stimulation of the two nerves began with a strong facilitation of 44 ms (sural) and 49 ms (posterior tibial). With SOL, stimulation of both nerves produced early inhibition beginning at 45 and 50 ms, respectively. With both LG and MG, sural stimuli produced an early facilitation at 52-53 ms. However, posterior tibial stimuli produced different initial responses in these two muscles: facilitation in LG at 50 ms and inhibition in MG at 51 ms. 3. Perstimulus time histograms of the discharge of 61 single motor units revealed generally similar reflex responses as in multiunit EMG. However, different reflex components were not equally apparent in the responses of different single motor units: an individual motor unit could respond slightly differently with a change in stimulus intensity or background contraction level. The multiunit EMG record represents a global average that does not necessarily depict the precise pattern of all motor units contributing to the average. 4. When subjects stood erect without support and with eyes closed, reflex patterns were seen only in active muscles, and the patterns were similar to those in the reclining posture. 5. It is concluded that afferents from mechanoreceptors in the sole of the foot have multisynaptic reflex connections with the motoneuron pools innervating the muscles that act at the ankle. When the muscles are active in standing or walking, cutaneous feedback may play a role in modulating motoneuron output and thereby contribute to stabilization of stance and gait.
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Lai, Adrian K. M., Taylor J. M. Dick, Andrew A. Biewener e James M. Wakeling. "Task-dependent recruitment across ankle extensor muscles and between mechanical demands is driven by the metabolic cost of muscle contraction". Journal of The Royal Society Interface 18, n.º 174 (janeiro de 2021): 20200765. http://dx.doi.org/10.1098/rsif.2020.0765.
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The nervous system is faced with numerous strategies for recruiting a large number of motor units within and among muscle synergists to produce and control body movement. This is challenging, considering multiple combinations of motor unit recruitment may result in the same movement. Yet vertebrates are capable of performing a wide range of movement tasks with different mechanical demands. In this study, we used an experimental human cycling paradigm and musculoskeletal simulations to test the theory that a strategy of prioritizing the minimization of the metabolic cost of muscle contraction, which improves mechanical efficiency, governs the recruitment of motor units within a muscle and the coordination among synergist muscles within the limb. Our results support our hypothesis, for which measured muscle activity and model-predicted muscle forces in soleus—the slower but stronger ankle plantarflexor—is favoured over the weaker but faster medial gastrocnemius (MG) to produce plantarflexor force to meet increased load demands. However, for faster-contracting speeds induced by faster-pedalling cadence, the faster MG is favoured. Similar recruitment patterns were observed for the slow and fast fibres within each muscle. By contrast, a commonly used modelling strategy that minimizes muscle excitations failed to predict force sharing and known physiological recruitment strategies, such as orderly motor unit recruitment. Our findings illustrate that this common strategy for recruiting motor units within muscles and coordination between muscles can explain the control of the plantarflexor muscles across a range of mechanical demands.
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Akay, Turgay. "Long-term measurement of muscle denervation and locomotor behavior in individual wild-type and ALS model mice". Journal of Neurophysiology 111, n.º 3 (1 de fevereiro de 2014): 694–703. http://dx.doi.org/10.1152/jn.00507.2013.
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The increasing number of mouse models of human degenerative and injury-related diseases that affect motor behavior raises the importance of in vivo methodologies allowing measurement of physiological and behavioral changes over an extended period of time in individual animals. A method that provides long-term measurements of muscle denervation and its behavioral consequences in individual mice for several months is presented in this article. The method is applied to mSod1G93A mice, which model human amyotrophic lateral sclerosis (ALS). The denervation process of gastrocnemius and soleus muscles in mSod1G93A mice is demonstrated for up to 3 mo. The data suggest that as muscle denervation progresses, massive behavioral compensation occurs within the spinal cord that allows animals to walk almost normally until late ages. Only around the age of 84 days is the first sign of abnormal movement during walking behavior detected as an abnormal tibialis anterior activity profile that is manifested in subtle but abnormal swing movement during walking. Additionally, this method can be used with other mouse models of human diseases, such as spinal cord injury, intracerebral hemorrhage, Parkinson's diseases, and spinal muscular atrophy.
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Blackburn, Marjan, Paulette van Vliet e Simon P. Mockett. "Reliability of Measurements Obtained With the Modified Ashworth Scale in the Lower Extremities of People With Stroke". Physical Therapy 82, n.º 1 (1 de janeiro de 2002): 25–34. http://dx.doi.org/10.1093/ptj/82.1.25.
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Abstract Background and Purpose. Abnormal muscle tone is a common motor disorder following stroke, which may require rehabilitation. The Modified Ashworth Scale is a 6-point rating scale that is used to measure muscle tone. The interrater and intrarater reliability of measurements obtained with the scale remain equivocal. The purpose of this study was to investigate the reliability of measurements obtained with the scale in the lower limb of patients with stroke. Subjects. Twenty patients were tested 2 weeks after their stroke, and 12 patients were tested 12 weeks after their stroke. Methods. Gastrocnemius, soleus, and quadriceps femoris muscles on the hemiplegic side were tested. Results. Interrater reliability for 2 raters was poor, with a Kendall tau-b correlation for the combined muscle group of .062 (P=.461). For intrarater reliability, the Kendall tau-b correlation was .567 (P<.001). The agreement within one rater occurred mostly on the grade of 0. Discussion and Conclusion. The Modified Ashworth Scale yielded reliable measurements in the lower limb for a single examiner, and agreement was best on the grade of 0. The reliability between examiners was not good, which may bring into question the validity of measurements obtained with the scale.
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Dy, Christine J., Yury P. Gerasimenko, V. Reggie Edgerton, Poul Dyhre-Poulsen, Grégoire Courtine e Susan J. Harkema. "Phase-Dependent Modulation of Percutaneously Elicited Multisegmental Muscle Responses After Spinal Cord Injury". Journal of Neurophysiology 103, n.º 5 (maio de 2010): 2808–20. http://dx.doi.org/10.1152/jn.00316.2009.
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Phase-dependent modulation of monosynaptic reflexes has been reported for several muscles of the lower limb of uninjured rats and humans. To assess whether this step-phase-dependent modulation can be mediated at the level of the human spinal cord, we compared the modulation of responses evoked simultaneously in multiple motor pools in clinically complete spinal cord injury (SCI) compared with noninjured (NI) individuals. We induced multisegmental responses of the soleus, medial gastrocnemius, tibialis anterior, medial hamstring, and vastus lateralis muscles in response to percutaneous spinal cord stimulation over the Th11–Th12 vertebrae during standing and stepping on a treadmill. Individuals with SCI stepped on a treadmill with partial body-weight support and manual assistance of leg movements. The NI group demonstrated phase-dependent modulation of evoked potentials in all recorded muscles with the modulation of the response amplitude corresponding with changes in EMG amplitude in the same muscle. The SCI group demonstrated more variation in the pattern of modulation across the step cycle and same individuals in the SCI group could display responses with a magnitude as great as that of modulation observed in the NI group. The relationship between modulation and EMG activity during the step cycle varied from noncorrelated to highly correlated patterns. These findings demonstrate that the human lumbosacral spinal cord can phase-dependently modulate motor neuron excitability in the absence of functional supraspinal influence, although with much less consistency than that in NI individuals.
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Prilutsky, Boris I., e Robert J. Gregor. "Swing- and support-related muscle actions differentially trigger human walk–run and run–walk transitions". Journal of Experimental Biology 204, n.º 13 (1 de julho de 2001): 2277–87. http://dx.doi.org/10.1242/jeb.204.13.2277.
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SUMMARY There has been no consistent explanation as to why humans prefer changing their gait from walking to running and from running to walking at increasing and decreasing speeds, respectively. This study examined muscle activation as a possible determinant of these gait transitions. Seven subjects walked and ran on a motor-driven treadmill for 40s at speeds of 55, 70, 85, 100, 115, 130 and 145% of the preferred transition speed. The movements of subjects were videotaped, and surface electromyographic activity was recorded from seven major leg muscles. Resultant moments at the leg joints during the swing phase were calculated. During the swing phase of locomotion at preferred running speeds (115, 130, 145%), swing-related activation of the ankle, knee and hip flexors and peaks of flexion moments were typically lower (P<0.05) during running than during walking. At preferred walking speeds (55, 70, 85%), support-related activation of the ankle and knee extensors was typically lower during stance of walking than during stance of running (P<0.05). These results support the hypothesis that the preferred walk–run transition might be triggered by the increased sense of effort due to the exaggerated swing-related activation of the tibialis anterior, rectus femoris and hamstrings; this increased activation is necessary to meet the higher joint moment demands to move the swing leg during fast walking. The preferred run–walk transition might be similarly triggered by the sense of effort due to the higher support-related activation of the soleus, gastrocnemius and vastii that must generate higher forces during slow running than during walking at the same speed.
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Harkema, Susan J., Seanna L. Hurley, Uday K. Patel, Philip S. Requejo, BRUCE H. Dobkin e V. Reggie Edgerton. "Human Lumbosacral Spinal Cord Interprets Loading During Stepping". Journal of Neurophysiology 77, n.º 2 (1 de fevereiro de 1997): 797–811. http://dx.doi.org/10.1152/jn.1997.77.2.797.
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Harkema, Susan J., Seanna L. Hurley, Uday K. Patel, Philip S. Requejo, Bruce H. Dobkin, and V. Reggie Edgerton. Human lumbosacral spinal cord interprets loading during stepping. J. Neurophysiol. 77: 797–811, 1997. Studies suggest that the human lumbosacral spinal cord can generate steplike oscillating electromyographic (EMG) patterns, but it remains unclear to what degree these efferent patterns depend on the phasic peripheral sensory information associated with bilateral limb movements and loading. We examined the role of sensory information related to lower-extremity weight bearing in modulating the efferent motor patterns of spinal-cord-injured (SCI) subjects during manually assisted stepping on a treadmill. Four nonambulatory subjects, each with a chronic thoracic spinal cord injury, and two nondisabled subjects were studied. The level of loading, EMG patterns, and kinematics of the lower limbs were studied during manually assisted or unassisted stepping on a treadmill with body weight support. The relationships among lumbosacral motor pool activity [soleus (SOL), medial gastrocnemius (MG), and tibialis anterior (TA)], limb load, muscle-tendon length, and velocity of muscle-tendon length change were examined. The EMG mean amplitude of the SOL, MG, and TA was directly related to the peak load per step on the lower limb during locomotion. The effects on the EMG amplitude were qualitatively similar in subjects with normal, partial, or no detectable supraspinal input. Responses were most consistent in the SOL and MG at load levels of <50% of a subject's body weight. The modulation of the EMG amplitude from the SOL and MG, both across steps and within a step, was more closely associated with limb peak load than muscle-tendon stretch or the velocity of muscle-tendon stretch. Thus stretch reflexes were not the sole source of the phasic EMG activity in flexors and extensors during manually assisted stepping in SCI subjects. The EMG amplitude within a step was highly dependent on the phase of the step cycle regardless of level of load. These data suggest that level of loading on the lower limbs provides cues that enable the human lumbosacral spinal cord to modulate efferent output in a manner that may facilitate the generation of stepping. These data provide a rationale for gait rehabilitation strategies that utilize the level of load-bearing stepping to enhance the locomotor capability of SCI subjects.
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Tansey, K. E., e B. R. Botterman. "Activation of type-identified motor units during centrally evoked contractions in the cat medial gastrocnemius muscle. II. Motoneuron firing-rate modulation". Journal of Neurophysiology 75, n.º 1 (1 de janeiro de 1996): 38–50. http://dx.doi.org/10.1152/jn.1996.75.1.38.
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1. The aim of this study was to examine the nature of motoneuron firing-rate modulation in type-identified motor units during smoothly graded contractions of the cat medial gastrocnemius (MG) muscle evoked by stimulation of the mesencephalic locomotor region (MLR). Motoneuron discharge patterns, firing rates, and the extent of firing-rate modulation in individual units were studied, as was the extent of concomitant changes in firing rates within pairs of simultaneously active units. 2. In 21 pairs of simultaneously active motor units, studied during 41 evoked contractions, the motoneurons' discharge rates and patterns were measured by processing the cells' recorded action potentials through windowing devices and storing their timing in computer memory. Once recruited, most motoneurons increased their firing rates over a limited range of increasing muscle tension and then maintained a fairly constant firing rate as muscle force continued to rise. Most motoneurons also decreased their firing rates over a slightly larger, but still limited, range of declining muscle force before they were derecruited. Although this was the most common discharge pattern recorded, several other interesting patterns were also seen. 3. The mean firing rate for slow twitch (type S) motor units (27.8 imp/s, 5,092 activations) was found to be significantly different from the mean firing rate for fast twitch (type F) motor units (48.4 imp/s, 11,272 activations; Student's t-test, P < 0.001). There was no significant difference between the mean firing rates of fast twitch, fatigue-resistant (type FR) and fast twitch, fatigable (type FF) motor units. When the relationship between motoneuron firing rate and whole-muscle force was analyzed, it was noted that, in general, smaller, lower threshold motor units began firing at lower rates and reached lower peak firing rates than did larger, higher threshold motor units. These results confirm both earlier experimental observations and predictions made by other investigators on the basis of computer simulations of the cat MG motor pool, but are in contrast to motor-unit discharge behavior recorded in some human motor-unit studies. 4. The extent of concomitant changes in firing rate within pairs of simultaneously active motor units was examined to estimate the extent of simultaneous motoneuron firing-rate modulation across the motoneuron pool. A smoothed (5 point sliding average) version of the two motoneurons' instantaneous firing rates was plotted against each other, and the slope and statistical significance of the relationship was determined. In 16 motor-unit pairs, the slope of the motoneurons' firing-rate relationship was significantly distinct from 0. Parallel firing-rate modulation (< 10-fold difference in firing rate change reflected by a slope of > 0.1) was noted only in pairs containing motor units of like physiological type and then only if they were of similar recruitment threshold. 5. Other investigators have demonstrated that changes in a motoneuron's "steady-state" firing rate predictably reflect changes in the amount of effective synaptic current that cell is receiving. The finding in the present study of limited parallel firing-rate modulation between simultaneously active motoneurons would suggest that changes in the synaptic drive to the various motoneurons of the pool is unevenly distributed. This finding, in addition to the findings of orderly motor-unit recruitment and the relationship between motor-unit recruitment threshold and motoneuron firing rate, cannot be adequately accommodated for by the existing models of the synaptic organization in motoneuron pools. Therefore a new model of the synaptic organization within the motoneuron pool has been proposed.
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Hug, François, Alessandro Del Vecchio, Simon Avrillon, Dario Farina e Kylie J. Tucker. "Muscles from the same muscle group do not necessarily share common drive: evidence from the human triceps surae". Journal of Applied Physiology, 26 de novembro de 2020. http://dx.doi.org/10.1152/japplphysiol.00635.2020.
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It has been proposed that movements are produced through groups of muscles, or motor modules, activated by common neural commands. However, the neural origin of motor modules is still debated. Here, we used complementary approaches to determine: i) whether three muscles of the same muscle group (soleus, gastrocnemius medialis [GM] and lateralis [GL]) are activated by a common neural drive ; and ii) whether the neural drive to GM and GL could be differentially modified by altering the mechanical requirements of the task. Eighteen human participants performed an isometric standing heel raise and submaximal isometric plantarflexions (10%, 30%, 50% of maximal effort). High-density surface electromyography recordings were decomposed into motor unit action potentials and coherence analysis was applied on the motor units spike trains. We identified strong common drive to each muscle, but minimal common drive between the muscles. Further, large between-muscle differences were observed during the isometric plantarflexions, such as a delayed recruitment time of GL compared to GM and soleus motor units and opposite time-dependent changes in the estimates of neural drive to muscles during the torque plateau. Finally, the feet position adopted during the heel raise task (neutral vs internally rotated) affected only the GL neural drive with no change for GM. These results provide conclusive that not all anatomically defined synergist muscles are controlled by strong common neural drive. Independent drive to some muscles from the same muscle group may allow for more flexible control to comply with secondary goals such as joint stabilization.
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Hodson-Tole, E. F., e A. K. M. Lai. "Ultrasound-derived changes in thickness of human ankle plantar flexor muscles during walking and running are not homogeneous along the muscle mid-belly region". Scientific Reports 9, n.º 1 (21 de outubro de 2019). http://dx.doi.org/10.1038/s41598-019-51510-4.
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Abstract Skeletal muscle thickness is a valuable indicator of several aspects of a muscle’s functional capabilities. We used computational analysis of ultrasound images, recorded from 10 humans walking and running at a range of speeds (0.7–5.0 m s−1), to quantify interactions in thickness change between three ankle plantar flexor muscles (soleus, medial and lateral gastrocnemius) and quantify thickness changes at multiple muscle sites within each image. Statistical analysis of thickness change as a function of stride cycle (1d statistical parametric mapping) revealed significant differences between soleus and both gastrocnemii across the whole stride cycle as they bulged within the shared anatomical space. Within each muscle, changes in thickness differed between measurement sites but not locomotor condition. For some of the stride, thickness measures taken from the distal-mid image region represented the mean muscle thickness, which may therefore be a reliable region for these measures. Assumptions that muscle thickness is constant during a task, often made in musculoskeletal models, do not hold for the muscles and locomotor conditions studied here and researchers should not assume that a single thickness measure, from one point of the stride cycle or a static image, represents muscle thickness during dynamic movements.
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Dick, Taylor J. M., Christofer J. Clemente, Laksh K. Punith e Gregory S. Sawicki. "Series elasticity facilitates safe plantar flexor muscle–tendon shock absorption during perturbed human hopping". Proceedings of the Royal Society B: Biological Sciences 288, n.º 1947 (17 de março de 2021). http://dx.doi.org/10.1098/rspb.2021.0201.
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In our everyday lives, we negotiate complex and unpredictable environments. Yet, much of our knowledge regarding locomotion has come from studies conducted under steady-state conditions. We have previously shown that humans rely on the ankle joint to absorb energy and recover from perturbations; however, the muscle–tendon unit (MTU) behaviour and motor control strategies that accompany these joint-level responses are not yet understood. In this study, we determined how neuromuscular control and plantar flexor MTU dynamics are modulated to maintain stability during unexpected vertical perturbations. Participants performed steady-state hopping and, at an unknown time, we elicited an unexpected perturbation via rapid removal of a platform. In addition to kinematics and kinetics, we measured gastrocnemius and soleus muscle activations using electromyography andin vivofascicle dynamics using B-mode ultrasound. Here, we show that an unexpected drop in ground height introduces an automatic phase shift in the timing of plantar flexor muscle activity relative to MTU length changes. This altered timing initiates a cascade of responses including increased MTU and fascicle length changes and increased muscle forces which, when taken together, enables the plantar flexors to effectively dissipate energy. Our results also show another mechanism, whereby increased co-activation of the plantar- and dorsiflexors enables shortening of the plantar flexor fascicles prior to ground contact. This co-activation improves the capacity of the plantar flexors to rapidly absorb energy upon ground contact, and may also aid in the avoidance of potentially damaging muscle strains. Our study provides novel insight into how humans alter their neural control to modulatein vivomuscle–tendon interaction dynamics in response to unexpected perturbations. These data provide essential insight to help guide design of lower-limb assistive devices that can perform within varied and unpredictable environments.