Muscle spindles in human tibialis anterior encode muscle fascicle length changes

Muscle spindles provide exquisitely sensitive proprioceptive information regarding joint position and movement. Through passively driven length changes in the muscle-tendon unit (MTU), muscle spindles detect joint rotations because of their in-parallel mechanical linkage to muscle fascicles. In human microneurography studies, muscle fascicles are assumed to follow the MTU and, as such, fascicle length is not measured in such studies. However, under certain mechanical conditions, compliant structures can act to decouple the fascicles, and, therefore, the spindles, from the MTU. Such decoupling may reduce the fidelity by which muscle spindles encode joint position and movement. The aim of the present study was to measure, for the first time, both the changes in firing of single muscle spindle afferents and changes in muscle fascicle length in vivo from the tibialis anterior muscle (TA) during passive rotations about the ankle. Unitary recordings were made from 15 muscle spindle afferents supplying TA via a microelectrode inserted into the common peroneal nerve. Ultrasonography was used to measure the length of an individual fascicle of TA. We saw a strong correlation between fascicle length and firing rate during passive ankle rotations of varying rates (0.1–0.5 Hz) and amplitudes (1–9°). In particular, we saw responses observed at relatively small changes in muscle length that highlight the sensitivity of the TA muscle to small length changes. This study is the first to measure spindle firing and fascicle dynamics in vivo and provides an experimental basis for further understanding the link between fascicle length, MTU length, and spindle firing patterns. NEW & NOTEWORTHY Muscle spindles are exquisitely sensitive to changes in muscle length, but recordings from human muscle spindle afferents are usually correlated with joint angle rather than muscle fascicle length. In this study, we monitored both muscle fascicle length and spindle firing from the human tibialis anterior muscle in vivo. Our findings are the first to measure these signals in vivo and provide an experimental basis for exploring this link further.

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Tibialis anterior muscle fascicle dynamics adequately represent postural sway during standing balance

Journal of Applied Physiology ◽

10.1152/japplphysiol.00517.2013 ◽

2013 ◽

Vol 115 (12) ◽

pp. 1742-1750 ◽

Cited By ~ 23

Author(s):

James T. Day ◽

Glen A. Lichtwark ◽

Andrew G. Cresswell

Keyword(s):

Tibialis Anterior ◽

Tibialis Anterior Muscle ◽

Sensory Information ◽

Quiet Standing ◽

Emg Activity ◽

Proprioceptive Information ◽

Fascicle Length ◽

Muscle Fascicle ◽

Length Changes

To maintain a stable, upright posture, the central nervous system (CNS) must integrate sensory information from multiple sources and subsequently generate corrective torque about the ankle joint. Although proprioceptive information from the muscles that cross this joint has been shown to be vital in this process, the specific source of this information remains questionable. Recent research has been focused on the potential role of tibialis anterior (TA) muscle during standing, largely due to the lack of modulation of its activity throughout the sway cycle. Ten young, healthy subjects were asked to stand normally under varying conditions, for periods of 60 s. During these trials, intramuscular electromyographic (EMG) activity and the fascicle length of three distinct anatomical regions of TA were sampled synchronously with kinematic data regarding sway position. In the quiet standing conditions, TA muscle activity was unmodulated and fascicle length changes in each region were tightly coupled with changes in sway position. In the active sway condition, more EMG activity was observed in TA and the fascicle length changes were decoupled from sway position. No regional specific differences in correlation values were observed, contrasting previous observations. The ability of the fascicles to follow sway position builds upon the suggestion that TA is well placed to provide accurate, straightforward sensory information to the CNS. As previously suggested, through reciprocal inhibition, afferent information from TA could help to regulate plantar flexor torque at relevant phases of the sway cycle. The proprioceptive role of TA appears to become complicated during more challenging conditions.

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Activity patterns in individual hindlimb primary and secondary muscle spindle afferents during normal movements in unrestrained cats

Journal of Neurophysiology ◽

10.1152/jn.1979.42.2.420 ◽

1979 ◽

Vol 42 (2) ◽

pp. 420-440 ◽

Cited By ~ 120

Author(s):

G. E. Loeb ◽

J. Duysens

Keyword(s):

Muscle Spindle ◽

Activity Patterns ◽

Muscle Length ◽

Muscle Spindle Afferents ◽

General Observation ◽

Postural Changes ◽

Motor Activities ◽

Specific Muscle ◽

Length Changes ◽

Rapid Modulation

1. Chronically implanted microelectrode wires in the L7 and S1 dorsal root ganglia were used to record unit activity from cat hindlimb primary and secondary muscle spindle afferents. Units could be reliably recorded for several days, permitting comparison of their activity with homonymous muscle EMG and length during a variety of normal, unrestrained movements. 2. The general observation was that among both primary and secondary endings there was a broad range of different patterns of activity depending on the type of muscle involved and the type of movement performed. 3. During walking, the activity of a given spindle primary was usually consistent among similar step cycles. However, the activity was usually poorly correlated with absolute muscle length, apparently unrealted to velocity of muscle stretch, and could change markedly for similar movements performed under different conditions. 4. Spindle activity modulation not apparently related to muscle length changes was assumed to be influenced by fusimotor activity. In certain muscles, this presumption leads to the conclusion that gamma-motoneurons may be activated out of phase with homonymous alpha-motoneurons as well as by more conventional alpha-gamma-motoneuron coactivation. 5. Simultaneous recordings of two spindle primary afferents from extensor digitorum longus indicated that spindles within the same muscle may differ considerably with respect to this presumed gamma-motoneuron drive. 6. Spindle secondary endings appeared to be predominantly passive indicators of muscle length during walking, but could demonstrate apparently strong fusimotor modulation during other motor activities such as postural changes and paw shaking. 7. Both primary and secondary endings were observed to undergo very rapid modulation of firing rates in response to presumed reflexly induced intrafusal contractions. 8. It is suggested that the pattern of fusimotor control of spindles may be tailored to the specific muscle and task being performed, rather than necessarily dominated by rigid alpha-gamma coactivation.

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Effects of tonic muscle pain on fusimotor control of human muscle spindles during isometric ankle dorsiflexion

Journal of Neurophysiology ◽

10.1152/jn.00862.2018 ◽

2019 ◽

Vol 121 (4) ◽

pp. 1143-1149

Author(s):

Lyndon J. Smith ◽

Vaughan G. Macefield ◽

Ingvars Birznieks ◽

Alexander R. Burton

Keyword(s):

Muscle Spindle ◽

Muscle Pain ◽

Tibialis Anterior Muscle ◽

Muscle Spindles ◽

Muscle Spindle Afferents ◽

Firing Rates ◽

Leg Muscles ◽

Tonic Muscle ◽

Voluntary Contractions ◽

Ankle Dorsiflexors

Studies on anesthetized animals have revealed that nociceptors can excite fusimotor neurons and thereby change the sensitivity of muscle spindles to stretch; such nociceptive reflexes have been suggested to underlie the mechanisms that lead to chronic musculoskeletal pain syndromes. However, the validity of the “vicious cycle” hypothesis in humans has yielded results contrasting with those found in animals. Given that spindle firing rates are much lower in humans than in animals, it is possible that some of the discrepancies between human experimental data and those obtained in animals could be explained by differences in background fusimotor drive when the leg muscles are relaxed. We examined the effects of tonic muscle pain during voluntary contractions of the ankle dorsiflexors. Unitary recordings were obtained from 10 fusimotor-driven muscle spindle afferents (6 primary, 4 secondary) supplying the ankle dorsiflexors via a microelectrode inserted percutaneously into the common peroneal nerve. A series of 1-min weak contractions was performed at rest and during 1 h of muscle pain induced by intramuscular infusion of 5% hypertonic saline into the tibialis anterior muscle. We did not observe any statistically significant increases in muscle spindle firing rates of six afferents followed during tonic muscle pain, although discharge variability increased slightly. Furthermore, a participant’s capacity to maintain a constant level of force, while relying on proprioceptive feedback in the absence of visual feedback, was not compromised during pain. We conclude that nociceptive inputs from contracting muscle do not excite fusimotor neurons during voluntary isometric contractions in humans. NEW & NOTEWORTHY Data obtained in the cat have shown that muscle pain causes a marked increase in the firing of muscle spindles, attributed to a nociceptor-driven fusimotor reflex. However, our studies of muscle spindles in relaxed leg muscles failed to find any effect on spindle discharge. Here we showed that experimental muscle pain failed to increase the firing of muscle spindle afferents during weak voluntary contractions, when fusimotor drive sufficient to increase their firing is present.

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Functional properties of human muscle spindles

Journal of Neurophysiology ◽

10.1152/jn.00071.2018 ◽

2018 ◽

Vol 120 (2) ◽

pp. 452-467 ◽

Cited By ~ 19

Author(s):

Vaughan G. Macefield ◽

Thomas P. Knellwolf

Keyword(s):

Functional Properties ◽

Somatosensory System ◽

Muscle Length ◽

Muscle Spindles ◽

Human Muscle ◽

Independent Control ◽

Fascicle Length ◽

Muscle Fascicle ◽

The Central Nervous System ◽

Human Muscles

Muscle spindles are ubiquitous encapsulated mechanoreceptors found in most mammalian muscles. There are two types of endings, primary and secondary, and both are sensitive to changes in muscle length and velocity, with the primary endings having a greater dynamic sensitivity. Unlike other mechanoreceptors in the somatosensory system, muscle spindles are unique in possessing motor innervation, via γ-motoneurons (fusimotor neurons), that control their sensitivity to stretch. Much of what we know about human muscles spindles comes from studying the behavior of their afferents via intraneural microelectrodes (microneurography) inserted into accessible peripheral nerves. We review the functional properties of human muscle spindles, comparing and contrasting with what we know about the functions of muscle spindles studied in experimental animals. As in the cat, many human muscle spindles possess a background discharge that is related to the degree of muscle stretch, but mean firing rates are much lower (~10 Hz). They can faithfully encode changes in muscle fascicle length in passive conditions, but higher level extraction of information is required by the central nervous system to measure changes in muscle length during muscle contraction. Moreover, although there is some evidence supporting independent control of human muscle spindles via fusimotor neurons, any effects are modest compared with the clearly independent control of fusimotor neurons observed in the cat.

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Machine Learning to Extract Muscle Fascicle Length Changes from Dynamic Ultrasound Images in Real-Time

10.1101/2021.01.25.428061 ◽

2021 ◽

Author(s):

Luis G. Rosa ◽

Jonathan S. Zia ◽

Omer T. Inan ◽

Gregory S. Sawicki

Keyword(s):

Machine Learning ◽

Real Time ◽

Muscle Length ◽

Ground Truth ◽

Length Change ◽

Fascicle Length ◽

Automated Tracking ◽

Direct Training ◽

Muscle Fascicle ◽

Length Changes

AbstractBackground and objectiveDynamic muscle fascicle length measurements through B-mode ultrasound have become popular for the non-invasive physiological insights they provide regarding musculoskeletal structure-function. However, current practices typically require time consuming post-processing to track muscle length changes from B-mode images. A real-time measurement tool would not only save processing time but would also help pave the way toward closed-loop applications based on feedback signals driven by in vivo muscle length change patterns. In this paper, we benchmark an approach that combines traditional machine learning (ML) models with B-mode ultrasound recordings to obtain muscle fascicle length changes in real-time. To gauge the utility of this framework for ‘in-the-loop’ applications, we evaluate accuracy of the extracted muscle length change signals against time-series’ derived from a standard, post-hoc automated tracking algorithm.MethodsWe collected B-mode ultrasound data from the soleus muscle of six participants performing five defined ankle motion tasks: (a) seated, constrained ankle plantarflexion, (b) seated, free ankle dorsi/plantarflexion, (c) weight-bearing, calf raises (d) walking, and then a (e) mix. We trained machine learning (ML) models by pairing muscle fascicle lengths obtained from standardized automated tracking software (UltraTrack) with the respective B-mode ultrasound image input to the tracker, frame-by-frame. Then we conducted hyperparameter optimizations for five different ML models using a grid search to find the best performing parameters for a combination of high correlation and low RMSE between ML and UltraTrack processed muscle fascicle length trajectories. Finally, using the global best model/hyperparameter settings, we comprehensively evaluated training-testing outcomes within subject (i.e., train and test on same subject), cross subject (i.e., train on one subject, test on another) and within/direct cross task (i.e., train and test on same subject, but different task).ResultsSupport vector machine (SVM) was the best performing model with an average r = 0.70 ±0.34 and average RMSE = 2.86 ±2.55 mm across all direct training conditions and average r = 0.65 ±0.35 and average RMSE = 3.28 ±2.64 mm when optimized for all cross-participant conditions. Comparisons between ML vs. UltraTrack (i.e., ground truth) tracked muscle fascicle length versus time data indicated that ML tracked images reliably capture the salient qualitative features in ground truth length change data, even when correlation values are on the lower end. Furthermore, in the direct training, calf raises condition, which is most comparable to previous studies validating automated tracking performance during isolated contractions on a dynamometer, our ML approach yielded 0.90 average correlation, in line with other accepted tracking methods in the field.ConclusionsBy combining B-mode ultrasound and classical ML models, we demonstrate it is possible to achieve real-time tracking of human soleus muscle fascicles across a number of functionally relevant contractile conditions. This novel sensing modality paves the way for muscle physiology in-the-loop applications that could be used to modify gait via biofeedback or unlock novel wearable device control techniques that could enable restored or augmented locomotion performance.

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Three-dimensional architecture of the whole human soleus muscle in vivo

PeerJ ◽

10.7717/peerj.4610 ◽

2018 ◽

Vol 6 ◽

pp. e4610 ◽

Cited By ~ 25

Author(s):

Bart Bolsterlee ◽

Taija Finni ◽

Arkiev D’Souza ◽

Junya Eguchi ◽

Elizabeth C. Clarke ◽

...

Keyword(s):

Soleus Muscle ◽

Diffusion Tensor ◽

Three Dimensional ◽

Muscle Length ◽

Fascicle Length ◽

Cross Sectional ◽

Quantitative Measurements ◽

Muscle Fascicle ◽

Total Muscle

Background Most data on the architecture of the human soleus muscle have been obtained from cadaveric dissection or two-dimensional ultrasound imaging. We present the first comprehensive, quantitative study on the three-dimensional anatomy of the human soleus muscle in vivo using diffusion tensor imaging (DTI) techniques. Methods We report three-dimensional fascicle lengths, pennation angles, fascicle curvatures, physiological cross-sectional areas and volumes in four compartments of the soleus at ankle joint angles of 69 ± 12° (plantarflexion, short muscle length; average ± SD across subjects) and 108 ± 7° (dorsiflexion, long muscle length) of six healthy young adults. Microdissection and three-dimensional digitisation on two cadaveric muscles corroborated the compartmentalised structure of the soleus, and confirmed the validity of DTI-based muscle fascicle reconstructions. Results The posterior compartments of the soleus comprised 80 ± 5% of the total muscle volume (356 ± 58 cm3). At the short muscle length, the average fascicle length, pennation angle and curvature was 37 ± 8 mm, 31 ± 3° and 17 ± 4 /m, respectively. We did not find differences in fascicle lengths between compartments. However, pennation angles were on average 12° larger (p < 0.01) in the posterior compartments than in the anterior compartments. For every centimetre that the muscle-tendon unit lengthened, fascicle lengths increased by 3.7 ± 0.8 mm, pennation angles decreased by −3.2 ± 0.9° and curvatures decreased by −2.7 ± 0.8 /m. Fascicles in the posterior compartments rotated almost twice as much as in the anterior compartments during passive lengthening. Discussion The homogeneity in fascicle lengths and inhomogeneity in pennation angles of the soleus may indicate a functionally different role for the anterior and posterior compartments. The data and techniques presented here demonstrate how DTI can be used to obtain detailed, quantitative measurements of the anatomy of complex skeletal muscles in living humans.

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Machine learning to extract muscle fascicle length changes from dynamic ultrasound images in real-time

PLoS ONE ◽

10.1371/journal.pone.0246611 ◽

2021 ◽

Vol 16 (5) ◽

pp. e0246611

Author(s):

Luis G. Rosa ◽

Jonathan S. Zia ◽

Omer T. Inan ◽

Gregory S. Sawicki

Keyword(s):

Machine Learning ◽

Real Time ◽

Muscle Length ◽

Ground Truth ◽

Length Change ◽

Fascicle Length ◽

Automated Tracking ◽

Direct Training ◽

Muscle Fascicle ◽

Length Changes

Background and objective Dynamic muscle fascicle length measurements through B-mode ultrasound have become popular for the non-invasive physiological insights they provide regarding musculoskeletal structure-function. However, current practices typically require time consuming post-processing to track muscle length changes from B-mode images. A real-time measurement tool would not only save processing time but would also help pave the way toward closed-loop applications based on feedback signals driven by in vivo muscle length change patterns. In this paper, we benchmark an approach that combines traditional machine learning (ML) models with B-mode ultrasound recordings to obtain muscle fascicle length changes in real-time. To gauge the utility of this framework for ‘in-the-loop’ applications, we evaluate accuracy of the extracted muscle length change signals against time-series’ derived from a standard, post-hoc automated tracking algorithm. Methods We collected B-mode ultrasound data from the soleus muscle of six participants performing five defined ankle motion tasks: (a) seated, constrained ankle plantarflexion, (b) seated, free ankle dorsi/plantarflexion, (c) weight-bearing, calf raises (d) walking, and then a (e) mix. We trained machine learning (ML) models by pairing muscle fascicle lengths obtained from standardized automated tracking software (UltraTrack) with the respective B-mode ultrasound image input to the tracker, frame-by-frame. Then we conducted hyperparameter optimizations for five different ML models using a grid search to find the best performing parameters for a combination of high correlation and low RMSE between ML and UltraTrack processed muscle fascicle length trajectories. Finally, using the global best model/hyperparameter settings, we comprehensively evaluated training-testing outcomes within subject (i.e., train and test on same subject), cross subject (i.e., train on one subject, test on another) and within/direct cross task (i.e., train and test on same subject, but different task). Results Support vector machine (SVM) was the best performing model with an average r = 0.70 ±0.34 and average RMSE = 2.86 ±2.55 mm across all direct training conditions and average r = 0.65 ±0.35 and average RMSE = 3.28 ±2.64 mm when optimized for all cross-participant conditions. Comparisons between ML vs. UltraTrack (i.e., ground truth) tracked muscle fascicle length versus time data indicated that ML tracked images reliably capture the salient qualitative features in ground truth length change data, even when correlation values are on the lower end. Furthermore, in the direct training, calf raises condition, which is most comparable to previous studies validating automated tracking performance during isolated contractions on a dynamometer, our ML approach yielded 0.90 average correlation, in line with other accepted tracking methods in the field. Conclusions By combining B-mode ultrasound and classical ML models, we demonstrate it is possible to achieve real-time tracking of human soleus muscle fascicles across a number of functionally relevant contractile conditions. This novel sensing modality paves the way for muscle physiology in-the-loop applications that could be used to modify gait via biofeedback or unlock novel wearable device control techniques that could enable restored or augmented locomotion performance.

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Load, length, and velocity of load-moving tibialis anterior muscle of the cat

Journal of Applied Physiology ◽

10.1152/jappl.1996.80.6.2243 ◽

1996 ◽

Vol 80 (6) ◽

pp. 2243-2249 ◽

Cited By ~ 7

Author(s):

R. V. Baratta ◽

M. Solomonow ◽

G. Nguyen ◽

R. D'Ambrosia

Keyword(s):

Tibialis Anterior ◽

Tibialis Anterior Muscle ◽

Three Dimensional ◽

Muscle Length ◽

Muscle Performance ◽

Maximal Velocity ◽

Load Spectrum ◽

Velocity Of Shortening ◽

Length Changes ◽

Nerve Recordings

Three-dimensional relationships of load, length, and velocity of shortening of the tibialis anterior muscle in the cat were derived experimentally and fitted with an analytic model. Gravitational loads were applied to the isolated muscle, which arrived at an equilibrium with the passive forces before supramaximal tetanic stimulation was delivered to its nerve. Recordings of initial passive muscle length at equilibrium and length changes throughout the shortening phase up to the final length at active equilibrium were taken and numerically differentiated to obtain each load's instantaneous velocity. A three-dimensional surface was constructed by using instantaneous length and the corresponding velocity for each of several loads. Maximal velocity of shortening was shown to gradually decrease, occurring earlier in the shortening phase (at larger muscle lengths) as loads increased. Whereas load-velocity curves were hyperbolic for middle and short muscle lengths, they were nonmonotonic during shortening above the optimal length. The model was found to correlate well with the experimental data (R = 0.98) and allowed for prediction of both muscle performance boundaries and instantaneous shortening velocity for a given length across the physiological load spectrum, thus offering a realistic estimation of the contractile properties exhibited by the tibialis anterior muscle in functions similar to naturally occurring movements against gravitational loads, which are accelerated and decelerated during the movement.

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Length-force characteristics of in vivo human muscle reflected by supersonic shear imaging

Journal of Applied Physiology ◽

10.1152/japplphysiol.01058.2013 ◽

2014 ◽

Vol 117 (2) ◽

pp. 153-162 ◽

Cited By ~ 29

Author(s):

Kazushige Sasaki ◽

Sho Toyama ◽

Naokata Ishii

Keyword(s):

Shear Modulus ◽

Tibialis Anterior ◽

Muscle Force ◽

Tibialis Anterior Muscle ◽

Human Muscle ◽

Joint Angles ◽

Fascicle Length ◽

Voluntary Contractions ◽

Force Relationship

Recently, an ultrasound-based elastography technique has been used to measure stiffness (shear modulus) of an active human muscle along the axis of contraction. Using this technique, we explored 1) whether muscle shear modulus, like muscle force, is length dependent; and 2) whether the length dependence of muscle shear modulus is consistent between electrically elicited and voluntary contractions. From nine healthy participants, ankle joint torque and shear modulus of the tibialis anterior muscle were measured at five different ankle joint angles during tetanic contractions and during maximal voluntary contractions. Fascicle length, pennation angle, and tendon moment arm length of the tetanized tibialis anterior calculated from ultrasound images were used to reveal the length-dependent changes in muscle force and shear modulus. Over the range of joint angles examined, both force and shear modulus of the tetanized muscle increased with increasing fascicle length. Regression analysis of normalized data revealed a significant linear relationship between force and shear modulus ( R2 = 0.52, n = 45, P < 0.001). Although the length dependence of shear modulus was consistent, irrespective of contraction mode, the slope of length-shear modulus relationship was steeper during maximal voluntary contractions than during tetanic contractions. These results provide novel evidence that length-force relationship, one of the most fundamental characteristics of muscle, can be inferred from in vivo imaging of shear modulus in the tibialis anterior muscle. Furthermore, the estimation of length-force relationship may be applicable to voluntary contractions in which neural and mechanical interactions of multiple muscles are involved.

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Distinct muscle fascicle length changes in feline medial gastrocnemius and soleus muscles during slope walking

Journal of Applied Physiology ◽

10.1152/japplphysiol.01306.2007 ◽

2009 ◽

Vol 106 (4) ◽

pp. 1169-1180 ◽

Cited By ~ 29

Author(s):

Huub Maas ◽

Robert J. Gregor ◽

Emma F. Hodson-Tole ◽

Brad J. Farrell ◽

Boris I. Prilutsky

Keyword(s):

Inverse Dynamics ◽

Geometric Model ◽

Muscle Spindles ◽

Sensory Function ◽

Medial Gastrocnemius ◽

Overground Walking ◽

Force Output ◽

Fascicle Length ◽

Muscle Fascicle ◽

Length Changes

On the basis of differences in physiology, e.g., histochemical properties and spindle density, and the structural design of the cat soleus (SO) and medial gastrocnemius (MG) muscles, we hypothesized that 1) fascicle length changes during overground walking would be both muscle and slope dependent, which would have implications for the muscles' force output as well as sensory function, and that 2) muscle-tendon unit (MTU) and fascicle length changes would be different, in which case MTU length could not be used as an indicator of muscle spindle strain. To test these hypotheses, we quantified muscle fascicle length changes and compared them with length changes of the whole MTU in the SO and MG during overground walking at various slopes (0, ± 25, ± 50, +75, and +100%). The SO and MG were surgically instrumented with sonomicrometry crystals and fine-wire electromyogram electrodes to measure changes in muscle fascicle length and muscle activity, respectively. MTU lengths were calculated using recorded ankle and knee joint angles and a geometric model of the hindlimb. The resultant joint moments were calculated using inverse dynamics analysis to infer muscle loading. It was found that although MTU length and velocity profiles of the SO and MG appeared similar, length changes and velocities of muscle fascicles were substantially different between the two muscles. Fascicle length changes of both SO and MG were significantly affected by slope intensity acting eccentrically in downslope walking (−25 to −50%) and concentrically in upslope walking (+25 to +100%). The differences in MTU and fascicle behaviors in both the SO and MG muscles during slope walking were explained by the three distinct features of these muscles: 1) the number of joints spanned, 2) the pennation angle, and 3) the in-series elastic component. It was further suggested that the potential role of length feedback from muscle spindles is both task and muscle dependent.

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