Architecture of the hind limb muscles of cats: Functional significance

Force, velocity, and displacement properties of a muscle are determined in large part by its architectural design. The relative effect of muscle architecture on these physiological variables was studied by determining muscle weight, fiber length, average sarcomere length, and approximate angle of pinnation for 24 cat hind limb muscles. Muscle lengths ranged from 28.3 to 144 mm, whereas fiber lengths ranged from 8.4 to 105.5 mm. Generally, fiber to muscle length ratios were similar throughout a muscle. Estimated angles of pinnation of muscle fibers varied from 0 to 21° with most having an angle of less than 10°. The cross-sectional area of the knee extensors was similar to the knee flexors (16.43 vs. 16.83 cm²) whereas the cross-sectional area of the ankle extensors was more than six times greater than the ankle flexors (18.59 vs. 2.83 cm²). There was a 6.7-fold difference in the maximal force between muscles, when normalized to a constant weight, that could be attributed to architectural features. Rations of wet weight to predicted maximal tetanic tension for each muscle and group were calculated to compare the relative priority of muscle force versus muscle length-velocity for a given mass of muscle. These ratios varied from 0.4 to 4.84. The ratios suggest that velocity and/or displacement is a priority for the hamstrings, whereas force is a priority for the quadriceps and lower leg muscles. As much as a 12.6-fold difference in maximal velocity between muscles can be attributed to differences in fiber lengths. This can be compared to approximately a 2.5-fold difference in maximal velocity reported to occur as a result of biochemical (intrinsic) differences.     

江豚鼻道肌的解剖和构筑研究

江豚的鼻部肌共分为后外肌、前外肌、后内肌、前内肌和深肌5层,无间肌和大小内肌较退化,无对角膜肌。通过测定各肌的肌重、平均肌纤维长、平均肌小节长以及肌纤维角度,计算了各肌的生理横截面积,估计最大强直张力和肌鲜重对估计最大强直张力之比值等指标。鼻部肌各肌的相对肌纤维长度相似。各鼻部肌的肌纤维角度均为零。前部肌比后部肌具有较大的收缩速度和收缩位移优势,后部肌则具有较强的张力产生能力。着于额隆和唇部吻肌的张力产生能力很强。     

Microvascular architecture of the pampiniform plexus-testicular artery system in the rat: a scanning electron microscope study of corrosion casts

Because the architectural and biochemical properties of skeletal muscle dictate its force, velocity, and displacement properties, the major extensors (triceps brachii) and flexors (biceps brachii, brachialis, and brachioradialis) of the elbow in a primate (cynomolgus, monkey) were studied. Functional cross-sectional areas (CSA) were calculated from muscle mass, mean fiber length (normalized to a 2.20 microns sarcomere length), and angle of fiber pinnation measurements from each muscle. Fiber-type distributions were determined and used as a gross index of the biochemical capacities of the muscle. The extensor group had a shorter mean fiber length (31 vs. 47 mm), a larger CSA (13 vs. 8 cm2), and a higher overall percentage of slow-twitch fibers (47 vs. 26%). Consequently, the elbow extensors had a relatively greater potential for force production and force maintenance than the flexors. In contrast, the flexors were designed to optimize their length-velocity potentials; i.e., they had relatively long fibers and a higher fast-twitch fiber composition than the extensors. These morphologic differences between antagonistic muscle groups should be considered when evaluating the motor control mechanisms regulating reciprocal movements about the elbow.     

Functional bases of fiber length and angulation in muscle

The differences in angulation and length observed for the fibers of anatomical muscles may reflect two distinct mechanical requirements: arrangement for pinnation, reflecting an increase in physiological cross-section and arrangement for equivalent placement of sarcomeres, possibly associated with coordination. The observed differences in fiber angulation and length have different effects upon the responses of sarcomeres, specifically on their extent and rate of shortening and on the force they may generate. The basic mechanisms governing these effects and the various arrangements of muscles are reviewed. Fiber length and angulation in the complex M. adductor mandibulae externus 2 of a lizard were measured stereotactically; these values correlate well with the hypothesis that the muscle shows equivalence and demonstrate that angulation for pinnation is less constant. An outline for the study of muscle architecture and function, detailing the kinds of information require to estimate forces and evaluate muscle and fiber placements, is presented.     

Contractile properties of human skeletal muscle in childhood and adolescence

Using a combination of single maximal stimuli and maximum voluntary contractions, a comparison has been made of muscle properties in pre- and post-pubertal male subjects. In the dorsiflexor and plantarflexor muscles of the ankle, the twitch and maximum voluntary torques were approximately twice as large in the older subjects; the mean height and mean weight increased by factors of 1.20 and 1.86 respectively. The only other muscle parameter that changed, as a function of age, was the contraction time of the ankle dorsiflexors; the mean value was significantly longer in the older subjects. In the younger subjects, there were already clear differences between the dorsiflexor and plantarflexor muscles, the former developing smaller torques and having shorter contraction and half-relaxation times, greater post-activation potentiation and more susceptibility to fatigue. Even in the youngest subject, motor unit activation was complete in the ankle dorsiflexors; although this was not always true of the plantarflexors, the difference between the two subject groups was not significant.     

Muscular design in the equine interosseus muscle

We studied the forelimb interosseus muscle in horses, Equus caballus, to determine the muscular properties inherent in its function. Some authors have speculated that the equine interosseus contains muscle fibers at birth only to undergo loss of these fibers through postnatal ontogeny. We describe the muscle fibers in eight interosseus specimens from adult horses. These fibers were studied histochemically using myosin ATPase studies and immunocytochemically using several antibodies directed against type I and type II myosin heavy chain antibodies. We determined that 95% of the fibers were type I, presumed slow-twitch fibers. All fibers exhibited normal morphological appearance in terms of fiber diameter and cross-sectional area, suggesting that the muscles are undergoing normal cycles of recruitment. SDS-PAGE studies of myosin heavy chain isoforms were consistent with these observations of primarily slow-twitch muscle. Fibers were determined to be approximately 800 microm long when studied using nitric acid digestion protocols. Short fiber length combined with high pinnation angles suggest that the interosseus muscle is able to generate large amounts of force but can produce little work (measured as pulling the distal tendon proximally). While the equine interosseus muscle has undergone a general reduction of muscle content during its evolution, it remains composed of a significant muscular component that likely contributes to forelimb stability and elastic storage of energy during locomotion.     

Fiber architecture of the intrinsic muscles of the shoulder and arm in semiterrestrial and arboreal guenons

The internal organization of myofibers and connective tissues has important physiologic implications for muscle function and for naturalistic behavior. In this study of forelimb muscle morphology and primate locomotion, fiber architecture is examined in the intrinsic muscles of the shoulder (musculi deltoideus, infraspinatus, supraspinatus, subscapularis, teres major, and t. minor) and arm (m. coracobrachialis, biceps brachii, brachialis, and triceps brachii) in the semiterrestrial vervets (Chlorocebus aethiops) and arboreal red-tailed guenons (Cercopithecus ascanius). Wet weights and lengths of whole muscles, lengths of fasciculi and their associated proximal and distal tendons, and angles of pinnation were measured to estimate morphologic correlates of physiologic properties of individual muscles: force, velocity/excursion, energy expense, and relative isometric or isotonic contraction. Neither mean total-shoulder:total-arm ratios for muscle mass nor total reduced physiological cross-sectional area exhibited significant (P < 0.05) interspecific differences, thus emphasizing the importance of fine-tuning musculoskeletal analyses by the data collected here. The results generally support those previously published for quadriceps femoris and triceps surae of the hind limb in these species (Anapol and Barry [1996] Am. J. Phys. Anthropol. 99:429-447). The fiber architecture of the semiterrestrial vervets is largely suited for higher velocity while running on the ground. By contrast, the architectural configuration of red-tailed monkeys implies relatively isometric muscle contraction and passive storage of elastic strain energy for exploitation of the compliant canopy, where substrate components are situated beneath the sagittal plane of the animal. With respect to relative distribution of maximum potential force output among muscles of either shoulder or arm groups in these otherwise hind limb-dominated quadrupedal primates, statistically significant interspecific differences are best interpreted in light of braking, climbing, and, for vervets, the transition between ground and canopy.The interspecific differences shown here for the intrinsic muscles of the shoulder and arm underscore the significance of intramuscular morphology in reconciling structure and function with regard to locomotor behavior. Its analysis and interpretation lend support to consideration of "semiterrestrial" as a bona fide locomotor category uniquely different from what is practiced by dedicated arboreal and terrestrial quadrupeds that occasionally visit the habitat of one another. Data from a more committed terrestrial species would clarify this enigma.     

Fiber architecture of the extensors of the hindlimb in semiterrestrial and arboreal guenons

Fiber architecture of the extensor musculature of the knee and ankle is examined in two African guenon species—the semiterrestrial Cercopithecus aethiops, and the arboreal C. ascanius. Using histologic and microscopic techniques to measure lengths of sarcomeres, the original lengths of muscle fasciculi and angles of pinnation in quadriceps femoris and triceps surae are reconstructed from direct measurements on cadavers. Calculations of reduced physiological cross-sectional area, mass/predicted effective tetanic tension, maximum excursion, and tendon length/fasciculus + tendon lengths are correlated to preferred locomotor modalities in the wild. For both species, greater morphological differences occur among the bellies of quadriceps femoris—rectus femoris, vastus intermedius, v. lateralis, and v. medialis—than among the bellies of triceps surae—gastrocnemius lateralis, g. medialis, plantaris, and soleus. With regard to quadriceps femoris, few differences occur between species. Interspecific differences in the triceps surae indicate (1) redirection of muscle force to accommodate arboreality in which the substrate is less than body width; (2) muscles more suited for velocity in the semiterrestrial vervets; and (3) muscles used more isotonically in vervets and more isometrically in red-tailed monkeys. The inherent flexibility of muscle may be preadaptive to a primary species shift in locomotor modality until the bony morphology is able to adapt through natural selection. © 1996 Wiley-Liss, Inc.     

Asymmetric deformation of contracting human gastrocnemius muscle

Muscle fiber deformation is related to its cellular structure, as well as its architectural arrangement within the musculoskeletal system. While playing an important role in aponeurosis displacement, and efficiency of force transmission to the tendon, such deformation also provides important clues about the underlying mechanical structure of the muscle. We hypothesized that muscle fiber cross section would deform asymmetrically to satisfy the observed constant volume of muscle during a contraction. Velocity-encoded, phase-contrast, and morphological magnetic resonance imaging techniques were used to measure changes in fascicle length, pinnation angle, and aponeurosis separation of the human gastrocnemius muscle during passive and active eccentric ankle joint movements. These parameters were then used to subsequently calculate the in-plane muscle area subtended by the two aponeuroses and fascicles and to calculate the in-plane (dividing area by fascicle length), and through-plane (dividing muscle volume by area) thicknesses. Constant-volume considerations of the whole-muscle geometry require that, as fascicle length increases, the muscle fiber cross-sectional area must decrease in proportion to the length change. Our empirical findings confirm the definition of a constant-volume rule that dictates that changes in the dimension perpendicular to the plane, i.e., through-plane thickness, (-6.0% for passive, -3.3% for eccentric) equate to the reciprocal of the changes in area (6.8% for passive, 3.7% for eccentric) for both exercise paradigms. The asymmetry in fascicle cross-section deformation for both passive and active muscle fibers is established in this study with a ～22% in-plane and ～6% through-plane fascicle thickness change. These fiber deformations have functional relevance, not only because they affect the force production of the muscle itself, but also because they affect the characteristics of adjacent muscles by deflecting their line of pull.     

Transcutaneous FES of the paralyzed quadriceps: is knee torque affected by unintended activation of the hamstrings?

This study addresses the question whether unintended response of the knee flexors (hamstrings) accompanies transcutaneous functional electrical stimulation (FES) of the quadriceps and whether the knee torque is hereby affected. Transcutaneous FES of the right quadriceps of two paraplegic subjects was applied and measurements were made of the net torque and of the myoelectric activities of the quadriceps and hamstrings muscles of the right leg. A low correlation was obtained between the peak-to-peak amplitudes of the M-waves of the two muscles. This correlation decreased further with the development of fatigue, which indicated that the electromyography (EMG) signals from the hamstrings were not the result of cross-talk between adjacent recording sites. The force profile of each muscle was determined from a developed model incorporating EMG-based activation, muscle anthropometry as obtained from in vivo magnetic resonance imaging of the thigh, and metabolic fatigue function, based on data acquired by 31P nuclear magnetic resonance spectroscopy. A sensitivity analysis revealed that the muscle specific tension and the muscle moment arms have a major influence on the resulting muscle forces and should therefore be accurately provided. The results show that during the unfatigued phase of contraction the estimated maximal force in the hamstrings was lower than 20% of that in the quadriceps and could be considered to be practically negligible. As fatigue progressed the hamstrings-to-quadriceps force ratio increased, reaching up to 45%, and the effect of co-activation on the torque partition between the two muscles was no longer negligible.     

Understanding muscle coordination of the human leg with dynamical simulations

Muscles coordinate multijoint motion by generating forces that cause reaction forces throughout the body. Thus, a muscle can redistribute existing segmental energy by accelerating some segments and decelerating others. In the process, a muscle may also produce or absorb energy, in which case its summed energetic effect on the segments is positive or negative, respectively. This Borelli Lecture shows how dynamical simulations derived from musculoskeletal models reveal muscle-induced segmental energy redistribution and muscle co-functions and synergies. Synergy occurs when co-excited muscles distribute segmental energy differently to execute the motor task. In maximum height jumping, high vertical velocity at lift-off occurs desirably at full body extension because biarticular leg muscles redistribute the energy produced by the uniarticular leg muscles. In pedaling, synergistic ankle plantarflexor force generation during leg extension allows the high energy produced by the uniarticular hip and knee extensors to be delivered to the crank. An analogous less-powerful flexor synergy exists during leg flexion. Hamstrings reduce crank deceleration during the leg extension-to-flexion transition by not only producing energy but delivering it to the crank through its contribution to the tangential (accelerating) crank force, though this hamstrings function occurs at the opposite (flexion-extension) transition when pedaling backwards. In walking, the eccentric quadriceps activity in early stance not only decelerates the leg but also accelerates the trunk. In mid-stance, the uni- and biarticular plantarflexors, by having opposite segmental energetic effects, act in synergy to support the whole body, so segmental potential and kinetic energy exchange can occur. To conclude, the extraction of unmeasurable variables from dynamical simulations emulating task kinematics, kinetics, and EMGs shows how the production of force and energy by individual muscles contribute to the energy flow among the individual segments during task execution.     

Comparison of muscle weight and force ratios in New and Old World monkeys

Thin mandibles and small incisors found in New World monkeys as compared with Old World monkeys suggest that there may be differences in craniofacial loading patterns between these two groups, particularly in levels of mandibular corpus twisting (Hylander, 1975, 1979a; Eaglen, 1984; Bouvier, 1986a,b). This study examined the hypothesis that changes in the relative force contributions of the masticatory muscles were responsible for lowering torsion on the mandibular corpus in New World monkeys. Muscle weight and physiological cross-sections were compared using data from the literature (Schumacher, 1960: Turnbull, 1970; Cachel, 1979) as well as new data on adult male Cebus apella and Macaca mulatta. Both age and sex had an effect on muscle ratios. Mixed samples such as those used by Schumacher and Turnbull probably are not appropriate for drawing conclusions concerning species or group differences in muscle ratios. In addition, biomechanical conclusions based on muscle weight ratios alone to estimate muscle force may be misleading because fiber length inversely affects the amount of force a muscle can exert. A comparison of ratios based on physiological cross-section as an estimator of muscle force in New and Old World monkeys does not support the hypothesis that alterations in force contribution by individual masticatory muscles are responsible for minimizing mandibular corpus twisting in New World monkeys. Therefore, if twisting has been minimized in New World monkeys as suggested by their thin corpora, other changes in the craniofacial musculoskeletal complex, such as different muscle recruitment or pinnation patterns, may be responsible.     

Relationship between muscle fiber types and sizes and muscle architectural properties in the mouse hindlimb

Skeletal muscle fiber and architectural properties both contribute to the functional behavior of a muscle. This study uses discriminant analysis and mathematical modeling to identify the structurally and functionally significant properties. The architectural properties of fiber length, muscle length, and pennation angle are found to be the most structurally significant parameters, whereas fiber length, muscle length, and fiber type distribution are found to be most functionally determining. Architectural speed and fiber type do not appear to be complimentary (i.e., the architectural determinant of sspeed, fiber length, is not associated with fibers of high intrinsic velocity). However, there does seem to be a synergistic relation between the two property classes and force production. Muscles with large physiological cross sectional areas (PCSAs) tend to contain a greater proportion of larger, faster fibers. Structurally or morphologically significant parameters are not always found to have a large functional effect. Pennation angle, though one of the most structurally significant variables, was found to have very little functional effect. © 1994 Wiley-Liss, Inc.     

Macaque Pterygoid Muscles: Internal Architecture, Fiber length, and Cross-Sectional Area

Models of mastication require knowledge of fiber lengths and physiological cross-sectional area (PCS), a proxy for muscle force. I dissected 36 medial pterygoid and 36 lateral pterygoid muscles from 30 adult females of 3 macaque species (Macaca fascicularis, M. mulatta, M. nemestrina) using gross and chemical techniques and calculated PCS. These macaques have mechanically similar dietary niches and exhibit no significant difference in muscle architecture or fiber length. Fiber length does not scale with body size (mass) for either total pterygoid muscle or for medial pterygoid muscle mass. However, fiber length scales weakly with lateral pterygoid muscle mass. In each case, differences in PCS among species result from differences in muscle mass not fiber length. Medial pterygoid PCS scales isometrically with body size; larger animals have greater force production capabilities. Medial and lateral pterygoid PCS scale positively allometrically with facial size; individuals with more prognathic faces and taller mandibular corpora have greater PCS, and hence force, values. This positive allometry counters the less efficient positioning of masticatory muscles in longer-faced macaques. PCS is only weakly correlated with bone proxies previously used to estimate muscle force. Thus, predictions of muscle force from bone parameters will entail large margins of error and should be used with caution.     

Changes in fascicle lengths and pennation angles do not contribute to residual force enhancement/depression in voluntary contractions

Force enhancement following muscle stretching and force depression following muscle shortening are well-accepted properties of skeletal muscle contraction. However, the factors contributing to force enhancement/depression remain a matter of debate. In addition to factors on the fiber or sarcomere level, fiber length and angle of pennation affect the force during voluntary isometric contractions in whole muscles. Therefore, we hypothesized that differences in fiber lengths and angles of pennation between force-enhanced/depressed and reference states may contribute to force enhancement/depression during voluntary contractions. The purpose of this study was to test this hypothesis. Twelve subjects participated in this study, and force enhancement/depression was measured in human tibialis anterior. Fiber lengths and angles of pennation were quantified using ultrasound imaging. Neither fiber lengths nor angles of pennation were found to differ between the isometric reference contractions and any of the force-enhanced or force-depressed conditions. Therefore, we rejected our hypothesis and concluded that differences in fiber lengths or angles of pennation do not contribute to the observed force enhancement/depression in human tibialis anterior, and speculate that this result is likely true for other muscles too.     

Velocity of ultrasound in active and passive cat medial gastrocnemius muscle.

The lengths and pinnation angles of muscle fibers in the medial gastrocnemius (MG) muscle have recently been measured in freely moving cats [Hoffer et al., Progr. Brain Res. 80, 75-85 (1989); Muscle Afferents and Spinal Control of Movement (1992)] using an ultrasound transit-time (USTT) technique. This method assumed that the velocity of ultrasound through intact muscles was constant, independent of fiber orientation, muscle activity, load, belly shape, or fiber movement. However, the velocity of ultrasound along and across the fibers has been reported to depend on the state of muscle activation in frog muscle experiments in vitro [Hatta et al., J. Physiol. 403, 193-209 (1988)]. In the present study, the assumption of constant velocity of ultrasound in the cat MG muscle was evaluated. In acute experiments, done in situ with intact blood supply, the USTT was measured along and across cat MG muscle fibers in the passive, reflexly activated and tetanically activated states, with and without changes in muscle fiber length, for situations that reproduced the length and force ranges normally used by cats during locomotion. The velocity of ultrasound was found to be independent of the state of activation or motion of the muscle, and independent of the direction of the measurement with respect to the fiber orientation, within a measurement uncertainty less than or equal to 0.2%. These results validate the use of the USTT technique for the measurement of intramuscular dimensions in freely moving animals.     

Aging alters contractile properties and fiber morphology in pigeon skeletal muscle

In this study, we tested the hypothesis that skeletal muscle from pigeons would display age-related alterations in isometric force and contractile parameters as well as a shift of the single muscle fiber cross-sectional area (CSA) distribution toward smaller fiber sizes. Maximal force output, twitch contraction durations and the force–frequency relationship were determined in tensor propatagialis pars biceps muscle from young 3-year-old pigeons, middle-aged 18-year-old pigeons, and aged 30-year-old pigeons. The fiber CSA distribution was determined by planimetry from muscle sections stained with hematoxylin and eosin. Maximal force output of twitch and tetanic contractions was greatest in muscles from young pigeons, while the time to peak force of twitch contractions was longest in muscles from aged pigeons. There were no changes in the force–frequency relationship between the age groups. Interestingly, the fiber CSA distribution in aged muscles revealed a greater number of larger sized muscle fibers, which was verified visually in histological images. Middle-aged and aged muscles also displayed a greater amount of slow myosin containing muscle fibers. These data demonstrate that muscles from middle-aged and aged pigeons are susceptible to alterations in contractile properties that are consistent with aging, including lower force production and longer contraction durations. These functional changes were supported by the appearance of slow myosin containing muscle fibers in muscles from middle-aged and aged pigeons. Therefore, the pigeon may represent an appropriate animal model for the study of aging-related alterations in skeletal muscle function and structure.     

A three-dimensional muscle model: a quantified relation between form and function of skeletal muscles

A three-dimensional muscle model with complex geometry is described and tested against experimental data. Using this model, several muscles were constructed. These muscles have equal optimum length but differ in architecture. The force exerted by the constructed muscles, in relation to their actual length and velocity of shortening, is discussed. Generally speaking, the constructed muscles with considerable pennation have great fiber angles, a great physiological cross section, a narrow active and steep passive length-force relation, and a low maximal velocity of shortening. The maximal power (force times velocity) delivered by the constructed muscles is shown to be almost independent of the architecture of the muscles. The steepness of the passive length-force relation is determined mainly by the shortest fibers within the group of constructed muscles, whereas maximal velocity of shortening and the width of the active length-force relation are determined mainly by the longest fibers. The validity of the three-dimensional muscle model with respect to some morphological and functional characteristics is tested. Length-force relations of constructed muscles are compared with the actual length-force relations of mm. gastrocnemii mediales and mm. semimembranosi of male Wistar rats. Moreover, actual fiber angle, fiber length, and muscle thickness of three mm. gastrocnemii mediales are compared with values found for constructed muscles. It is concluded that the three-dimensional muscle model closely approximates the actual muscle form and function.     

Functional muscle synergies to support the knee against moment specific loads while weight bearing

ObjectiveExternally applied abduction and rotational loads are major contributors to the knee joint injury mechanism; yet, how muscles work together to stabilize the knee against these loads remains unclear. Our study sought to evaluate lower limb functional muscle synergies in healthy young adults such that muscle activation can be directly related to internal knee joint moments.MethodsConcatenated non-negative matrix factorization extracted muscle and moment synergies of 22 participants from electromyographic signals and joint moments elicited during a weight-bearing force matching protocol.ResultsTwo synergy sets were extracted: Set 1 included four synergies, each corresponding to a general anterior, posterior, medial, or lateral force direction. Frontal and transverse moments were coupled during medial and lateral force directions. Set 2 included six synergies, each corresponding to a moment type (extension/flexion, ab/adduction, internal/external rotation). Hamstrings and quadriceps dominated synergies associated with respective flexion and extension moments while quadriceps-hamstring co-activation was associated with knee abduction. Rotation moments were associated with notable contributions from hamstrings, quadriceps, gastrocnemius, and hip ab/adductors, corresponding to a general co-activation muscle synergy.ConclusionOur results highlight the importance of muscular co-activation of all muscles crossing the knee to support it during injury-inducing loading conditions such as externally applied knee abduction and rotation. Functional muscle synergies can provide new insight into the relationship between neuromuscular control and knee joint stability by directly associating biomechanical variables to muscle activation.     

In vivo moment arm lengths for hip extensor muscles at different angles of hip flexion

Moment arm lengths of three hip extensor muscles, the gluteus maximus, the hamstrings and the adductor magnus, were determined at hip flexion angles from 0 degrees to 90 degrees by combining data from ten autopsy specimens and from twenty patients, the latter examined by computed tomography. A straight-line muscle model for muscle force was used for the hamstrings and adductor magnus, and for the gluteus maximus a two-segment straight-line muscle force model was used. With the joint in its anatomical position the moment arm of the gluteus maximus to the bilateral motion axis averaged 79 mm, for the hamstrings 61 mm and for the adductor magnus 15 mm. The moment arm of gluteus maximus decreased with increasing hip flexion angle. The hamstrings showed an increase in moment arm length up to an average of 35 degrees hip flexion and then a decrease with increasing hip flexion angle. The corresponding figures for the adductor magnus moment arm showed an increase up to 75 degrees and then a decrease. Statistical analysis revealed significant differences in moment arm length between men and women.