Muscle Fiber Length Research Articles

Neuromusculoskeletal Model Calibration Significantly Affects Predicted Knee Contact Forces for Walking.

Though walking impairments are prevalent in society, clinical treatments are often ineffective at restoring lost function. For this reason, researchers have begun to explore the use of patient-specific computational walking models to develop more effective treatments. However, the accuracy with which models can predict internal body forces in muscles and across joints depends on how well relevant model parameter values can be calibrated for the patient. This study investigated how knowledge of internal knee contact forces affects calibration of neuromusculoskeletal model parameter values and subsequent prediction of internal knee contact and leg muscle forces during walking. Model calibration was performed using a novel two-level optimization procedure applied to six normal walking trials from the Fourth Grand Challenge Competition to Predict In Vivo Knee Loads. The outer-level optimization adjusted time-invariant model parameter values to minimize passive muscle forces, reserve actuator moments, and model parameter value changes with (Approach A) and without (Approach B) tracking of experimental knee contact forces. Using the current guess for model parameter values but no knee contact force information, the inner-level optimization predicted time-varying muscle activations that were close to experimental muscle synergy patterns and consistent with the experimental inverse dynamic loads (both approaches). For all the six gait trials, Approach A predicted knee contact forces with high accuracy for both compartments (average correlation coefficient r = 0.99 and root mean square error (RMSE) = 52.6 N medial; average r = 0.95 and RMSE = 56.6 N lateral). In contrast, Approach B overpredicted contact force magnitude for both compartments (average RMSE = 323 N medial and 348 N lateral) and poorly matched contact force shape for the lateral compartment (average r = 0.90 medial and -0.10 lateral). Approach B had statistically higher lateral muscle forces and lateral optimal muscle fiber lengths but lower medial, central, and lateral normalized muscle fiber lengths compared to Approach A. These findings suggest that poorly calibrated model parameter values may be a major factor limiting the ability of neuromusculoskeletal models to predict knee contact and leg muscle forces accurately for walking.

Muscle architectural changes after massive human rotator cuff tear.

Rotator cuff (RC) tendon tears lead to negative structural and functional changes in the associated musculature. The structural features of muscle that predict function are termed "muscle architecture." Although the architectural features of "normal" rotator cuff muscles are known, they are poorly understood in the context of cuff pathology. The purpose of this study was to investigate the effects of tear and repair on RC muscle architecture. To this end thirty cadaveric shoulders were grouped into one of four categories based on tear magnitude: Intact, Full-thickness tear (FTT), Massive tear (MT), or Intervention if sutures or hardware were present, and key parameters of muscle architecture were measured. We found that muscle mass and fiber length decreased proportionally with tear size, with significant differences between all groups. Conversely, sarcomere number was reduced in both FTT and MT with no significant difference between these two groups, in large part because sarcomere length was significantly reduced in MT but not FTT. The loss of muscle mass in FTT is due, in part, to subtraction of serial sarcomeres, which may help preserve sarcomere length. This indicates that function in FTT may be impaired, but there is some remaining mechanical loading to maintain "normal" sarcomere length-tension relationships. However, the changes resulting from MT suggest more severe limitations in force-generating capacity because sarcomere length-tension relationships are no longer normal. The architectural deficits observed in MT muscles may indicate deeper deficiencies in muscle adaptability to length change, which could negatively impact RC function despite successful anatomical repair. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:2089-2095, 2016.

Sensitivity of subject-specific models to Hill muscle–tendon model parameters in simulations of gait

Subject-specific musculoskeletal (MS) models of the lower extremity are essential for applications such as predicting the effects of orthopedic surgery. We performed an extensive sensitivity analysis to assess the effects of potential errors in Hill muscle–tendon (MT) model parameters for each of the 56 MT parts contained in a state-of-the-art MS model. We used two metrics, namely a Local Sensitivity Index (LSI) and an Overall Sensitivity Index (OSI), to distinguish the effect of the perturbation on the predicted force produced by the perturbed MT parts and by all the remaining MT parts, respectively, during a simulated gait cycle. Results indicated that sensitivity of the model depended on the specific role of each MT part during gait, and not merely on its size and length. Tendon slack length was the most sensitive parameter, followed by maximal isometric muscle force and optimal muscle fiber length, while nominal pennation angle showed very low sensitivity. The highest sensitivity values were found for the MT parts that act as prime movers of gait (Soleus: average OSI=5.27%, Rectus Femoris: average OSI=4.47%, Gastrocnemius: average OSI=3.77%, Vastus Lateralis: average OSI=1.36%, Biceps Femoris Caput Longum: average OSI=1.06%) and hip stabilizers (Gluteus Medius: average OSI=3.10%, Obturator Internus: average OSI=1.96%, Gluteus Minimus: average OSI=1.40%, Piriformis: average OSI=0.98%), followed by the Peroneal muscles (average OSI=2.20%) and Tibialis Anterior (average OSI=1.78%) some of which were not included in previous sensitivity studies. Finally, the proposed priority list provides quantitative information to indicate which MT parts and which MT parameters should be estimated most accurately to create detailed and reliable subject-specific MS models.

Changes in the Capillarity of the Rat Extensor Digitorum Longus Muscle 4 Weeks after Nerve Injury Studied by 2D Measurement Methods

We have previously shown by 3D study that 2 weeks after nerve injury there was no change in the length of capillaries per muscle fibre length in rat extensor digitorum longus muscle (EDL). The primary goal of the present 2D study was to determine the capillarity of rat EDL 4 weeks after various modes of nerve injury. Additionally, we wished to calculate the same capillary/fibre parameters that were used in our 3D stereological study. EDL muscles derived from denervated (4 weeks after nerve injury), re-innervated (4 weeks after two successive nerve crushes) and age-matched controls from the beginning (CON-1) and the end (CON-2) of the experiment were analysed in two ways. Global indices of capillarity, such as capillary density (CD) and capillary/fibre (C/F) ratio, were determined by automatic analysis, local indices as the number (CAF) and the length of capillaries around individual muscle fibres (Lcap) in relation to muscle fibre size were estimated manually by tracing the muscle fibre outlines and the transversally and longitudinally cut segments of capillaries seen in 5-µm-thin muscle cross sections. Four weeks after both types of nerve injury, CD increased in comparison to the CON-2 group (p < 0.001) due to atrophied muscle fibres in denervated muscles and probably proliferation of capillaries in re-innervated ones. Higher C/F, CAF (both p < 0.001) and Lcap (p < 0.01) in re-innervated than denervated EDL confirmed this assumption. Calculated capillary/fibre parameters were comparable to our previous 3D study, which strengthens the practical value to the adapted 2D method used in this study.

Passive Muscle-Tendon Unit Gearing Is Joint Dependent in Human Medial Gastrocnemius.

Skeletal muscles change length and develop force both passively and actively. Gearing allows muscle fiber length changes to be uncoupled from those of the whole muscle-tendon unit. During active contractions this process allows muscles to operate at mechanically favorable conditions for power or economical force production. Here we ask whether gearing is constant in passive muscle; determining the relationship between fascicle and muscle-tendon unit length change in the bi-articular medial gastrocnemius and investigating the influence of whether motion occurs at the knee or ankle joint. Specifically, the same muscle-tendon unit length changes were elicited by rotating either the ankle or knee joint whilst simultaneously measuring fascicle lengths in proximal and distal muscle regions using B-mode ultrasound. In both the proximal and distal muscle region, passive gearing values differed depending on whether ankle or knee motion occurred. Fascicle length changes were greater with ankle motion, likely reflecting anatomical differences in proximal and distal passive tendinous tissues, as well as shape changes of the adjacent mono-articular soleus. This suggests that there is joint-dependent dissociation between the mechanical behavior of muscle fibers and the muscle-tendon unit during passive joint motions that may be important to consider when developing accurate models of bi-articular muscles.

Phosphorylation and Calcium Antagonistically Tune Myosin-Binding Protein C's Molecular Structure and Function

During each heartbeat, calcium activates the sliding of actin-thin filaments towards the centers of myosin-thick filaments to shorten the overall length of cardiac muscle cells. Cardiac myosin binding protein C (cMyBP-C) tunes these interactions. cMyBP-C's C terminus is strongly bound to the thick filament backbone and its N-terminal domains transiently interact with actin and/or the myosin S2 domain, sensitizing thin filaments to calcium and governing maximal sliding velocities. Both mechanisms are tunable by phosphorylation of 4 serines within an extensible, intrinsically disordered region of cMyBP-C's N-terminus, the M-domain. Does phosphorylation impact the structure and molecular mechanics of cMyBP-C's N terminus? Rotary shadowing electron microscopy showed that M-domain phosphorylation caused N-terminal fragments (domains C0-C3) to shift from an extended, rod-like structure to a compact conformation. Atomic force microscopy studies of N-terminal fragments (C1-C2), showed that phosphorylation reduced the M-domain's contour length and extensibility. Taken together, M-domain phosphorylation and its impact on cMyBP-C's N-terminal structure, suggest a mechanism for tuning cMyBP-C's function in motility assays. Interestingly, we found that free calcium (0.1 mM) mitigated the structural effects of phosphorylation by shifting the phosphorylated-C0C3 fragments back to the extended, rod-like state, with a freely extensible M-domain, as if unphosphorylated. Functionally, even though phosphorylation reduced cMyBP-C's ability to inhibit actin filament sliding velocity in the motility assay (24% vs. 40% inhibition), addition of calcium ablated the impact of phosphorylation, fully restoring cMyBP-C's inhibitory capacity. We conclude that phosphorylation may have its greatest effect on cardiac contractility by tuning cMyBP-C's ability to sensitize actin-thin filaments to calcium at the low levels present during the onset of contraction. Importantly, calcium levels at the peak contraction would allow cMyBP-C to remain a potent contractile modulator, regardless of the cMyBP-C's phosphorylation state.

Fascicle length does increase in response to longitudinal resistance training and in a contraction-mode specific manner.

Fascicle length does increase in response to longitudinal resistance training and in a contraction-mode specific manner.

Physiological and architectural changes in the ageing muscle and their relation to strength and function in sarcopenia

In the ageing muscle, many changes occur. Some are on an architectural level, like alterations in muscle composition, or modifications in the characteristics of the muscle fiber itself, where muscle fiber length, orientation and type change. Other changes are neuronal, which occur on all levels, from the central activation over the spinal properties down to the level of the motor unit and the neuromuscular junction. There are also hormonal factors that undergo agerelated concentration variations. All these alterations in the muscle have an effect on both strength and function. In this matter, they contribute to the process of sarcopenia. Although many different components are identified, it is still unclear to what degree these components contribute to the loss of muscle mass, strength and function. This review summarizes the occurring physiological and anatomical changes within the ageing muscle and links them to outcomes such as strength and function.

Functional difference between the proximal and distal compartments of the semitendinosus muscle.

[Purpose] The tendinous inscription divides the semitendinosus muscle into the proximal and distal compartments. It was hypothesized that there are functional differences between those compartments. [Subjects and Methods] Seven adult males performed knee flexion and hip extension in the prone position. An ultrasound device measured the decrease in the length of muscle fibers in the two compartments during these movements. The knee and hip joint angles were concurrently measured using a video camera. Pearson’s correlation coefficients were calculated between the decrease in muscle fiber length in each compartment and joint angle. [Results] During knee flexion, decreased muscle fiber length was significantly correlated with increased knee flexion angle. During hip extension, there were no significant correlations for either compartment. Only the decrease in muscle fiber length in the distal compartment during hip extension tended to be negative; the other decreases in muscle fiber length tended to be positive. [Conclusion] Correlations did not reveal any functional differences. However, only the distal compartment elongated during hip extension. This result might show a functional difference and could be applied in clinical contexts during hip extension.

Anatomical parameters for musculoskeletal modeling of the hand and wrist

A musculoskeletal model of the hand and wrist can provide valuable biomechanical and neurophysiological insights, relevant for clinicians and ergonomists. Currently, no consistent data-set exists comprising the full anatomy of these upper extremity parts. The aim of this study was to collect a complete anatomical data-set of the hand and wrist, including the intrinsic and extrinsic muscles. One right lower arm, taken from a fresh frozen female specimen, was studied. Geometrical data for muscles and joints were digitized using a 3D optical tracking system. For each muscle, optimal fiber length and physiological cross-sectional area were assessed based on muscle belly mass, fiber length, and sarcomere length. A brief description of model, in which these data were imported as input, is also provided. Anatomical data including muscle morphology and joint axes (48 muscles and 24 joints) and mechanical representations of the hand are presented. After incorporating anatomical data in the presented model, a good consistency was found between outcomes of the model and the previous experimental studies.

Role of calpain system in post-mortem tenderization of meat and its influence on quality-A review

Tenderness is considered to be a single most important factor considerably affecting consumer's dissatisfaction by reducing eating quality. Meat tenderization process is a complex mechanism could be affected by several pathways including pre-and post-slaughter handling and processing alone or both. Toughness of meat could be due to development of crosslinkages of collagen connective tissue, intramuscular fat deposition, muscle fibre or sarcomere length etc. But ultimate tenderness is determined by the extent of proteolysis of key target myofibrillar proteins by the inherent enzymatic activity. So, main objective of this review is to provide ample information relating to role calpain proteolytic system in post-mortem proteolysis of muscle tissues and how it is affecting the meat quality.

Effects of epimuscular myofascial force transmission on sarcomere length of passive muscles in the rat hindlimb

Results from imaging studies and finite element models suggest epimuscular myofascial effects on sarcomere lengths in series within muscle fibers. However, experimental evidence is lacking. We evaluated epimuscular myofascial effects on (1) muscle belly, fiber, and mean sarcomere length and (2) sarcomere length distribution within passive fibers of the rat tibialis anterior (TA) and soleus (SO) muscles. Hindlimbs (n = 24) were positioned in predefined knee (55°, 90°, 125°, 160°) and ankle (either 90° or 125°) angles, and fixed in a formaldehyde solution. Varying knee joint angle causes changes in muscle–tendon unit length of SO and TA’s synergists, but not of SO and TA. Whole fibers were taken from SO and TA and photographed along their length. Mean sarcomere length was assessed for the entire fiber and for the proximal, intermediate, and distal thirds (fiber segments) separately. Mean sarcomere length of the fiber was not affected by knee angle, neither for SO (mean: 2.44 ± 0.03 μm and 2.19 ± 0.05 μm for ankle angles of 90° and 125°, respectively) nor for TA (mean: 2.33 ± 0.05 μm and 2.51 ± 0.07 μm for ankle angle set to 90° and 125°, respectively). Only for TA, a significant interaction between knee angle and fiber segment was found, indicating changes in the distribution of lengths of in-series sarcomeres. Thus, while epimuscular myofascial force transmission did not cause mean sarcomere length changes within passive SO and TA, it did alter the length distribution of sarcomeres within passive TA.

Proximate determinants of bite force in Anolis lizards.

Performance measures associated with the vertebrate jaw system may provide important insights into vertebrate ecology and evolution because of their importance in many ecologically relevant tasks. Previous studies have shown that in many taxa, evolution toward higher bite force has gone hand in hand with the evolution of larger body size. However, independent of differences in overall body size, bite force may vary depending on head size and shape as well. Moreover, the underlying musculature may also drive variation in bite force. Here, we investigate the proximate determinants of bite force in lizards of the genus Anolis. We dissected the jaw muscles and quantified muscle mass, fibre length, and cross-sectional area. Data were analysed for both sexes independently given the sexual dimorphism detected in the dataset. Our results show that the traits that explain bite force are similar in both males and females with overall body size and muscle mass being the principal determinants. Among the different muscles examined, the adductor externus and the pseudotemporalis groups were the best determinants of bite force. However, models run for males predicted the variation in bite force better than models for females, suggesting that selection on morphology improving bite force may be stronger in males.

A computational approach to calculate personalized pennation angle based on MRI: effect on motion analysis.

Muscles are the primary component responsible for the locomotion and change of posture of the human body. The physiologic basis of muscle force production and movement is determined by the muscle architecture (maximum muscle force, [Formula: see text], optimal muscle fiber length, [Formula: see text], tendon slack length, [Formula: see text], and pennation angle at optimal muscle fiber length, [Formula: see text]). The pennation angle is related to the maximum force production and to the range of motion. The aim of this study was to investigate a computational approach to calculate subject-specific pennation angle from magnetic resonance images (MRI)-based 3D anatomical model and to determine the impact of this approach on the motion analysis with personalized musculoskeletal models. A 3D method that calculates the pennation angle using MRI was developed. The fiber orientations were automatically computed, while the muscle line of action was determined using approaches based on anatomical landmarks and on centroids of image segmentation. Three healthy male volunteers were recruited for MRI scanning and motion capture acquisition. This work evaluates the effect of subject-specific pennation angle as musculoskeletal parameter in the lower limb, focusing on the quadriceps group. A comparison was made for assessing the contribution of personalized models on motion analysis. Gait and deep squat were analyzed using neuromuscular simulations (OpenSim). The results showed variation of the pennation angle between the generic and subject-specific models, demonstrating important interindividual differences, especially for the vastus intermedius and vastus medialis muscles. The pennation angle variation between personalized and generic musculoskeletal models generated significant variation in muscle moments and forces during dynamic motion analysis. A MRI-based approach to define subject-specific pennation angle was proposed and evaluated in motion analysis models. The significant differences obtained for the moments and muscle forces in quadriceps muscles indicate that a personalized approach in modeling the pennation angle can provide more individual details when investigating motion behaviors in specific subjects.

Stride frequency in relation to allometric growth in ghost crabs

AbstractBody size has significant impacts on fundamental processes of locomotion, including the operational frequency of skeletal muscle contraction, which declines systematically with increasing size. Although this shift in operation frequency and contractile kinetics is well documented in the literature, the mechanisms responsible for these changes are still incompletely understood. One important factor is that the mechanical properties of the musculoskeletal system possess resonant properties that favor higher frequencies in small animals. Another significant element is the physiological properties of the skeletal muscles, which may be tuned for faster contractions in smaller animals. These two components are interrelated, but precisely how muscle physiology and musculoskeletal mechanics interact to shape patterns of locomotion is complex. Ghost crabs Ocypode quadrata present an interesting model to study these processes because they are proficient runners that exhibit systematic changes in stride frequency as they grow. In the current study, we focused on anatomical changes that might occur with allometric growth in ghost crabs to test the hypothesis that changes in mechanical parameters contribute to the slowing of stride frequency. We paired basic anatomical measurements with kinematic analyses of crabs running at top speeds on a treadmill and experimentally weighted crabs to determine if the relative mass of larger crabs affects running frequency. We found that biologically relevant mechanics of the leg joints do not change with growth, as mechanical advantage and muscle fiber length relative to joint moment arm were unaffected by body size. Loading crabs had similar effects on stride frequency in both large and small animals alike. In contrast, muscle shortening velocity, estimated directly from angular velocity of the leg joints, decreased significantly with increasing body size. These data suggest that fundamental changes to the contractile properties of skeletal muscles during growth are primarily responsible for the changes in stride frequency observed in ghost crabs.

Osteotomy of the greater trochanter: effect on gluteus medius function.

Advancement of the greater trochanter alters the function of the gluteus medius muscle. However, with the exception of clinical studies and biomechanical lever arm studies, no publications that analyze the consequences of advancement of the greater trochanter on the muscle function exist. The aim of the study was to analyze the mechanical changes of gluteus medius after osteotomy of the greater trochanter in a lab setting. An anatomical study of origin and insertion of the gluteus medius was carried out on four hips. Based on the dissections, a string model was developed dividing the muscle into five sectors. Changes in muscle fiber length were measured for every 10° of flexion, internal and external rotation and abduction with the trochanter in anatomic, proximalized and distalized positions. Distalization of the trochanter leads to an imbalance of muscle action, moving the isometric sector of the muscle anteriorly with more muscle sectors being active during flexion and less during extension. Stretching of the muscle increases passive forces but decreases the force generation capacity of the muscle and at the same time increased muscle fiber excursion may require more energy consumption, which may explain earlier fatigue of the abductor musculature after distalization of the trochanter. For abduction, distalization of the muscle attachment leads to a change in contraction pattern from isometric to isotonic. Optimal balancing and excursion of the muscle is when the tip of the greater trochanter is at level with the hip rotation center. In hips with high riding trochanter, the optimal position is at the level of the center of hip rotation. Excessive distalization should be avoided. As the conclusions and considerations are based on a lab setting, transfer to clinical practice may not necessarily apply.

Bipedally trained rats have an increased hindlimb muscle force‐generating capacity compared to quadrupedally trained rats

The shift from bipedalism to quadrupedalism in human evolution led to major morphological changes because of new demands placed on the locomotor system including include larger forces on each limb. The ability for a muscle to generate forces and length change to respond to these demands depends on the muscle's architecture, or the arrangement of fibers within a muscle. Muscle force is determined by the cross-sectional area of its fibers (physiological cross-sectional area or PCSA), while muscle excursion is determined by fiber length. In this study, we asked whether changes in muscle architecture respond to the demands of bipedal locomotion and if this response can occur during the lifetime of an individual. To test this question, we trained rats to stand or walk bipedally on a treadmill for one hour a day for five days a week over 90 days using a harness that loaded the hindlimbs to 90% of body mass. A control group was trained quadrupedally over the same time period. The rats were sacrificed and their hindlimbs were fixed in formalin. Muscles were dissected from one hindlimb each of the bipedally (BI) and quadrupedally trained rats (QU). Measurements of muscle mass, length, fiber length, sarcomere length, and pennation angle were obtained and used to calculate normalized fiber length and PCSA. PCSA was normalized to body mass. We found an increase in the mass-specific PCSA of muscles in the BI rats compared to QU rat muscles. Furthermore, the larger PCSA in BI rats was achieved through a decrease in fiber length as opposed to an increase in muscle mass. These results indicate that muscle architecture responds to the varied demands of bipedal locomotion and these changes are a plastic response that can occur over a short time period.

Developing, Testing and Optimising a Mouse Hindlimb Musculoskeletal Model

Stability during locomotion arises from many complex interactions which are not yet fully understood. Studies into how sensory feedback from muscle spindles contributes to this have used neuromechanical simulations, however their accuracy is limited due to the lack of available physiological data. This presentation outlines the methods used to develop a musculoskeletal model of the mouse hindlimb, which will be used to predict responses to perturbations which occur with or without sensory feedback.The 3D musculoskeletal model, which represents bones, joints and 39 muscles of the mouse hindlimb, was created using SIMM (Musculographics, USA) through a combination of muscle architecture data and I2KI enhanced microCT scanning. Architecture data, such as muscle mass, pennation angle and fibre length were gathered from microdissections, while muscle geometry was determined using I2KI microCT scanning, a non‐destructive method of determining muscle paths and attachment points. Muscle moment arms predicted by the model were found to agree with measured data from previous studies.In future work, in vivo optogenetics will be used to characterise changes in gait and muscle forces which occur in free running mice in response to induced neural and mechanical perturbations with or without sensory feedback from hindlimb muscles. These physiological data will be used to update and refine the musculoskeletal model.

Pathophysiological principles for evaluating hemodynamic

The diagnosis and treatment of patients with hemodynamic disorders is basing of the understanding the physiology and pathophysiology of cardiovascular system. The relationship between length of muscle fibers and power reduction were first time revealed by Fick. Otto Frank was formulated fundamental principles of contractility of cardiomyocytes. Straub and Wiggers in 1914 was described the mail principles of the right ventricle work. Ernest Starling was performed a series of experiments, which explore a dependence of left ventricular ejection from venous inflow and elastic resistance of the aorta. In 1914 Ernest Starling was published research result, which describe how the mechanical energy of heart beats is depend from length fiber. Another essential part of knowledge of this problem was discovered by Aurtur Guyton experiments. Guyton has been established that there is a linear relationship between the pressure in the right atrium and the venous return. The lack of quantitative assessment of volume status has led to a qualitative approach, de-scribed by the term “response to the volume load.” However, as pointed out by some authors [16, 29], it is important to remember that the response to the preload is not a pathological condition. A quantitative approach to the assessment of volume status is based on the concept of Guyton on average system pressure filling, theoretically independent of cardiac function. This approach is used clinically. In this review article we describe possibility of clinical application of all knowledge of this questions.

Spinal mechanisms may provide a combination of intermittent and continuous control of human posture: predictions from a biologically based neuromusculoskeletal model.

Several models have been employed to study human postural control during upright quiet stance. Most have adopted an inverted pendulum approximation to the standing human and theoretical models to account for the neural feedback necessary to keep balance. The present study adds to the previous efforts in focusing more closely on modelling the physiological mechanisms of important elements associated with the control of human posture. This paper studies neuromuscular mechanisms behind upright stance control by means of a biologically based large-scale neuromusculoskeletal (NMS) model. It encompasses: i) conductance-based spinal neuron models (motor neurons and interneurons); ii) muscle proprioceptor models (spindle and Golgi tendon organ) providing sensory afferent feedback; iii) Hill-type muscle models of the leg plantar and dorsiflexors; and iv) an inverted pendulum model for the body biomechanics during upright stance. The motor neuron pools are driven by stochastic spike trains. Simulation results showed that the neuromechanical outputs generated by the NMS model resemble experimental data from subjects standing on a stable surface. Interesting findings were that: i) an intermittent pattern of muscle activation emerged from this posture control model for two of the leg muscles (Medial and Lateral Gastrocnemius); and ii) the Soleus muscle was mostly activated in a continuous manner. These results suggest that the spinal cord anatomy and neurophysiology (e.g., motor unit types, synaptic connectivities, ordered recruitment), along with the modulation of afferent activity, may account for the mixture of intermittent and continuous control that has been a subject of debate in recent studies on postural control. Another finding was the occurrence of the so-called “paradoxical” behaviour of muscle fibre lengths as a function of postural sway. The simulations confirmed previous conjectures that reciprocal inhibition is possibly contributing to this effect, but on the other hand showed that this effect may arise without any anticipatory neural control mechanism.