Length and moment arm of human leg muscles as a function of knee and hip-joint angles

Lengths of muscle tendon complexes of the quadriceps femoris muscle and some of its heads, biceps femoris and gastrocnemius muscles, were measured for six limbs of human cadavers as a function of knee and hip-joint angles. Length-angle curves were fitted using second degree polynomials. Using these polynomials the relationships between knee and hip-joint angles and moment arms were calculated. The effect of changing the hip angle on the biceps femoris muscle length is much larger than that of changing the knee angle. For the rectus femoris muscle the reverse was found. The moment arm of the biceps femoris muscle was found to remain constant throughout the whole range of knee flexion as was the case for the medial part of the vastus medialis muscle. Changes in the length of the lateral part of the vastus medialis muscle as well as the medial part of the vastus lateralis muscle are very similar to those of vastus intermedius muscle to which they are adjacent, while those changes in the length of the medial part of the vastus medialis muscle and the lateral part of the vastus lateralis muscle, which are similar to each other, differ substantially from those of the vastus intermedius muscle. Application of the results to jumping showed that bi-articular rectus femoris and biceps femoris muscles, which are antagonists, both contract eccentrically early in the push off phase and concentrically in last part of this phase.     

Angle- and gender-specific quadriceps femoris muscle recruitment and knee extensor torque

The objectives were to examine knee angle-, and gender-specific knee extensor torque output and quadriceps femoris (QF) muscle recruitment during maximal effort, voluntary contractions. Fourteen young adult men and 15 young adult women performed three isometric maximal voluntary contractions (MVC), in a random order, with the knee at 0 degrees (terminal extension), 10 degrees, 30 degrees, 50 degrees, 70 degrees, and 90 degrees flexion. Knee extensor peak torque (PT), and average torque (AT) were expressed in absolute (N m), relative (N m kg(-1)) and allometric-modeled (N m kg(-n)) units. Vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) muscle EMG signals were full-wave rectified and integrated over the middle 3 s of each contraction, averaged over the three trials at each knee angle, and normalized to the activity recorded at 0 degrees. Muscle recruitment efficiency was calculated as the ratio of the normalized EMG of each muscle to the allometric-modeled average torque (normalized to the values at 0 degrees flexion), and expressed as a percent. Men generated significantly greater knee extensor PT and AT than women in absolute, relative and allometric-modeled units. Absolute and relative PT and AT were significantly highest at 70 degrees, while allometric-modeled values were observed to increase significantly across knee joint angles 10-90 degrees. VM EMG was significantly greater than the VL and RF muscles across all angles, and followed a similar pattern to absolute knee extensor torque. Recruitment efficiency improved across knee joint angles 10-90 degrees and was highest for the VL muscle. VM recruitment efficiency improved more than the VL and RF muscles across 70-90 degrees flexion. The findings demonstrate angle-, and gender-specific responses of knee extensor torque to maximal-effort contractions, while superficial QF muscle recruitment was most efficient at 90 degrees, and less dependent on gender.     

Kinematic, kinetic and EMG patterns during downward squatting.

The aim of this study was to investigate the kinematic, kinetic, and electromyographic pattern before, during and after downward squatting when the trunk movement is restricted in the sagittal plane. Eight healthy subjects performed downward squatting at two different positions, semisquatting (40 degrees knee flexion) and half squatting (70 degrees knee flexion). Electromyographic responses of the vastus medialis oblique, vastus medialis longus, rectus femoris, vastus lateralis, biceps femoris, semitendineous, gastrocnemius lateralis, and tibialis anterior were recorded. The kinematics of the major joints were reconstructed using an optoelectronic system. The center of pressure (COP) was obtained using data collected from one force plate, and the ankle and knee joint torques were calculated using inverse dynamics. In the upright position there were small changes in the COP and in the knee and ankle joint torques. The tibialis anterior provoked the disruption of this upright position initiating the squat. During the acceleration phase of the squat the COP moved posteriorly, the knee joint torque remained in flexion and there was no measurable muscle activation. As the body went into the deceleration phase, the knee joint torque increased towards extension with major muscle activities being observed in the four heads of the quadriceps. Understanding these kinematic, kinetic and EMG strategies before, during and after the squat is expected to be beneficial to practitioners for utilizing squatting as a task for improving motor function.     

Deficits in the way to achieve balance related to mechanisms of dynamic stability control in the elderly

The purpose of this study was to examine the postural corrections related to components of dynamic stability aimed to increase our understanding of successful postural control among the elderly population. This was done by comparing balance behaviour of older adults who were able to recover stability (stable) and others who failed to regain stability (unstable) with a single step after a forward fall. Thirty-eight old male adults (64+/-3yr, 176+/-6cm, 78.5+/-7.8kg) had to recover balance after a sudden induced forward fall. All participants performed maximal isometric ankle plantarflexion and knee extension contractions on a dynamometer. The elongation of the gastrocnemius medialis and the vastus lateralis tendon and aponeuroses during isometric contraction was examined by ultrasonography. There were no differences in leg-extensor muscle strength or tendon stiffness between the two groups showing that the muscle tendon capacities may not be the reason for the observed differences in dynamic stability control. The unstable participants created a higher horizontal ground reaction push-off force of the support limb in the second part ( approximately 260ms after release) of the phase until touchdown leading to an unstable body position at touchdown. The results indicate deficits in the way to achieve balance related to mechanisms responsible for dynamic stability control within the elderly population.     

Kinematic,kinetic and electromyographic differences between young adults with and without chronic ankle instability during walking

The objective of this study was to quantify the kinematic, kinetic and electromyography differences between individuals with and without chronic ankle instability (CAI) during comfortable (CW) and fast (FW) walking. Twenty-one individuals with CAI and 21 healthy controls were recruited to walk at CW and FW speeds. The dependent variables were gluteus medius, vastus lateralis, gastrocnemius lateralis, gastrocnemius medialis, peroneus longus and tibialis anterior muscles mean activity, ankle and knee angles and moments. Kinematic, kinetic and electromyography variables were compared between groups with a one-dimensional statistical non-parametric mapping analysis. The CAI group exhibited no significant difference for ankle angles and moments compared to the control group. However, the CAI group showed less external knee rotation from 56 to 100% (CW) and 51 to 98% (FW) and more knee abduction moment from 1 to 6% and 7 to 9% (CW) and 1 to 2% (FW) of the stance phase. Less gluteus medius muscle activity was also observed from 6 to 9% and 99 to 100% (CW) of the stance phase for the CAI group. These results suggest proximal biomechanical compensations and will help better understand the underlying deficits associated with CAI. They also indicate that regardless of walking speeds, individuals with CAI exhibit similar differences compared to healthy participants.     

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.     

Skeletal muscle responses to lower limb suspension in humans.

Eight subjects participated in a 6-wk unilateral lower limb suspension (ULLS) study to determine the influence of reduced weight bearing on human skeletal muscle morphology. The right shoe was outfitted with a platform sole that prevented the left foot from bearing weight while walking with crutches, yet it allowed freedom of movement about the ankle, knee, and hip. Magnetic resonance images pre- and post-ULLS showed that thigh muscle cross-sectional area (CSA) decreased (P less than 0.05) 12% in the suspended left lower limb, whereas right thigh muscle CSA did not change. Likewise, magnetic resonance images collected post-ULLS showed that muscle CSA was 14% smaller (P less than 0.05) in the left than in the right leg. The decrease in muscle CSA of the thigh was due to a twofold greater response of the knee extensors (-16%, P less than 0.05) than knee flexors (-7%, P less than 0.05). The rectus femoris muscle of the knee extensors showed no change in CSA, whereas the three vastus muscles showed similar decreases of approximately 16% (P less than 0.05). The apparent atrophy in the leg was due mainly to reductions in CSA of the soleus (-17%) and gastrocnemius muscles (-26%). Biopsies of the left vastus lateralis pre- and post-ULLS showed a 14% decrease (P less than 0.05) in average fiber CSA. The decrease was evident in both type I (-12%) and II (-15%) fibers. The number of capillaries surrounding the different fiber types was unchanged after ULLS.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of 20 days bed rest on muscle morphology

Using ultrasound, muscle thickness and fascicle angles from aponeurosis were evaluated before, during and after 20 days bed rest (BR). Subjects were healthy adults (4 women and 4 men). Measurements were carried out before and after BR and after 10 weeks of recovery, respectively. Muscle measurements were taken at nine sites in trunk and upper and lower extremities, respectively. For the m. triceps brachii, m. vastus lateralis, and m. gastrocnemius medialis, fascicle angles from the aponeurosis as well as muscle thickness were measured. There was a high statistical significant correlation between muscle thickness and cross-sectional area for quadriceps muscles, suggesting applicability of muscle thickness for evaluation of muscle size. Muscle thickness decreased in muscles of the lower extremity by 2.1-4.4 % after bed rest. In triceps brachii and vastus lateralis muscles, there were no prominent changes in muscle thickness and fascicle angles. It was concluded that muscle morphology deteriorates with changes in muscle architecture by bed rest but the response is small and muscle-specific. It was also suggested that bed rest affects not only muscle mass but muscle tone as well.     

Can the patellar tendon moment arm be predicted from anthropometric measurements?

The purpose of this study was to examine the relations between patellar tendon moment arm length and several relevant anthropometric characteristics of 22 healthy men. The patellar tendon moment arm length was measured using magnetic resonance imaging with two different methods: (1) measurement of patellar tendon moment arm length (d(PT)) with respect to the tibiofemoral contact point (d(PTCP)) and (2) measurement of d(PT) with respect to the intersection point of the anterior and posterior cruciate ligament (d(PTIP)). Pearson correlation coefficients and a stepwise linear regression analysis were used to examine the relationships between the d(PT) and anthropometric measurements taken. Furthermore, a Student's t-test was used to determine differences between the d(PTCP) and d(PTIP) values. Only knee circumference was a significant d(PTCP) predictor (P < 0.05) but with a very low R2 (0.139). None of the anthropometric parameters examined was found to be a significant d(PTIP) predictor. The correlation coefficients ranged from -0.04 to 0.42. The d(PTIP) values were significantly higher (by 0.84-1.89 cm) than the d(PTCP) values (P < 0.05). These results are in disagreement with previous in vitro findings that d(PT) variance may be explained by knee joint size differences. Hence, existing imaging-based methodologies remain necessary for accurate quantification of the patellar tendon moment arm.     

Mechanical and morphological properties of human quadriceps femoris and triceps surae muscle-tendon unit in relation to aging and running

The purpose of this study was to examine the effects of aging and endurance running on the mechanical and morphological properties of different muscle-tendon units (MTUs) in vivo. The investigation was conducted on 30 elderly and 19 young adult males. For the analysis of possible MTU adaptation in response to endurance running the subjects were divided into two subgroups: non-active vs. endurance-runners. All subjects performed isometric maximal voluntary plantarflexion and knee extension contractions on a dynamometer. The distal aponeurosis of the gastrocnemius medialis (GM) and vastus lateralis (VL) during plantarflexion and knee extensions and the muscle architecture of the GM and VL were visualized by ultrasonography. The maximal knee and ankle joint moment were higher for the young compared to the elderly population (p<0.05). No identifiable differences in muscle architecture between young and elderly subjects were detected in VL and GM. Aging results in a reduced (p<0.05) normalized stiffness of the quadriceps femoris tendon and aponeurosis, which were not identifiable for the triceps surae. In contrast, the properties of both MTUs showed no major differences between endurance-runners and the non-active group (p>0.05). Only pennation angle at the GM were higher for the runners compared to the non-active group (p<0.05). The present results indicate that tendon changes related to aging do not occur proportionally in different MTUs. Furthermore, it seems that the extra stress and load imposed on high-load-bearing MTUs during endurance running may not be sufficient to produce significant adaptative processes in the mechanical parameters analyzed.     

Limb and sex-related differences in knee muscle co-contraction exist 3 months after anterior cruciate ligament reconstruction

Interlimb and sex-based differences in gait mechanics and neuromuscular control are common after anterior cruciate ligament reconstruction (ACLR). Following ACLR, individuals typically exhibit elevated co-contraction of knee muscles, which may accelerate knee osteoarthritis (OA) onset. While directed (medial/lateral) co-contractions influence tibiofemoral loading in healthy people, it is unknown if directed co-contractions are present early after ACLR and if they differ across limbs and sexes. The purpose of this study was to compare directed co-contraction indices (CCIs) of knee muscles in both limbs between men and women after ACLR. Forty-five participants (27 men) completed overground walking at a self-selected speed 3 months after ACLR during which quadriceps, hamstrings, and gastrocnemii muscle activities were collected bilaterally using surface electromyography. CCIs of six muscle pairs were calculated during the weight acceptance interval. The CCIs of the vastus lateralis/biceps femoris muscle pair (lateral musculature) was greater in the involved limb (vs uninvolved; p = 0.02). Compared to men, women exhibited greater CCIs in the vastus medialis/lateral gastrocnemius and vastus lateralis/lateral gastrocnemius muscle pairs (p < 0.01 and p = 0.01, respectively). Limb- and sex-based differences in knee muscle co-contractions are detectable 3 months after ACLR and may be responsible for altered gait mechanics.     

Influences of repetitive drop jump and isometric leg press exercises on tendon properties in knee extensors

The present study aimed to investigate the effects of repetitive drop jumps (DJ) and isometric leg presses (LP) on the tendon properties in knee extensors. Before and after each endurance test, the elongation (L) of the tendon and aponeurosis of the vastus lateralis muscle was measured directly by ultrasonography while the subjects performed ramp isometric knee extensions up to maximum voluntary isometric contraction. Eight men performed 100 repetitions of the DJ and 50 repetitions of the LP for 10 seconds with 10 seconds relaxation. In the DJ, there were no significant differences in L values at any force production levels before and after each endurance test. In LP, however, the L values above 500 N were significantly greater after the endurance test than before. These results suggest that the tendon properties in knee extensors change to become more compliant after the repeated longer-duration contractions, but not after repeated ballistic exercises.     

In vivo changes in the human patellar tendon moment arm length with different modes and intensities of muscle contraction

The purpose of this study was to examine the effect of different muscle contraction modes and intensities on patellar tendon moment arm length (d(PT)). Five men performed isokinetic concentric, eccentric and passive knee extensions at an angular velocity of 60 deg/s and six men performed gradually increasing to maximum effort isometric muscle contractions at 90( composite function) and 20( composite function) of knee flexion. During the tests, lateral X-ray fluoroscopy imaging was used to scan the knee joint. The d(PT) differences between the passive state and the isokinetic concentric and extension were quantified at 15( composite function) intervals of knee joint flexion angle. Furthermore, the changes of the d(PT) as a function of the isometric muscle contraction intensities were determined during the isometric knee extension at 90( composite function) and 20( composite function) of knee joint flexion. Muscle contraction-induced changes in knee joint flexion angle during the isometric muscle contraction were also taken into account for the d(PT) measurements. During the two isometric knee extensions, d(PT) increased from rest to maximum voluntary muscle contraction (MVC) by 14-15%. However, when changes in knee joint flexion angle induced by the muscle contraction were taken into account, d(PT) during MVC increased by 6-26% compared with rest. Moreover, d(PT) increased during concentric and eccentric knee extension by 3-15%, depending on knee flexion angle, compared with passive knee extension. These findings have important implications for estimating musculoskeletal loads using modelling under static and dynamic conditions.     

Type IIC fibres in certain muscles of the adult rat (sedentary and exercised)

Samples were taken, at fixed levels, of the vastus lateralis, the caput lateralis of the gastrocnemius muscle and the longissimus lumbaris of 72 Wistar rats which were either sedentary or subjected to various exercise schedules. The samples were analyzed using the histochemical technique of myosin ATPase (m-ATPase) after preincubation at pH 4.2, and the fibre-types I, II (IIA and IIB) and IIC were identified, calculating the percentage of type IIC fibres as well as their minimum diameter. The percentages of these IIC fibres found in the red and mixed parts of the gastrocnemius (caput lateralis) and the longissimus lumbaris were between 0.7% and 2.6%. However, their presence was not detected in the vastus lateralis or in the white part of the gastrocnemius (caput lateralis). The lack of differences in this fibre type between the males and females of the population was shown statistically. Likewise, no significant modification of the IIC fibres between sedentary and exercised animals was seen. With regard to fibrillar size, females showed a smaller minimum diameter than males, the results showing a small increase in the size of these fibres in both sexes after exercise, although in most cases this was not statistically significant.     

Muscle activation patterns of selected lower extremity muscles during stepping and cutting tasks

Lower extremity muscle activations during crossover and side step cut tasks are hypothesized to play an important role in controlling knee motion, and therefore, impact the design of knee injury prevention and rehabilitation programs. However, the contribution of lower extremity muscles to frontal and transverse plane moments during cutting tasks is unclear. The purpose of this study was to compare the muscle activation patterns of selected lower extremity muscles (vastus lateralis, medial/lateral hamstrings and medial/lateral gastrocnemius) of subjects performing a stepping down and side step cut, a stepping down and crossover cut and an equivalent straight ahead task. Ground reaction force was used to determine the cut angle, stance time and compare the lower limb loading during each task. Electromyography data during all tasks were normalized to the average activation during the straight ahead tasks to determine relative changes in muscle activation between the straight ahead and different cut styles (crossover and side step). There were no differences in the pattern of muscle activation of the vastus lateralis, or lateral hamstring muscles when comparing the cutting tasks to the equivalent straight ahead task. However, the crossover cut task resulted in significantly higher muscle activation of the medial hamstrings and lateral gastrocnemius muscles relative to both the side step cut and straight ahead tasks. These results suggest the medial/lateral hamstrings and medial/lateral gastrocnemius play a role in transverse and frontal plane control during cut tasks.     

Soleus muscle and Achilles tendon compressive stiffness is related to knee and ankle positioning

Changes in fascicle length and tension of the soleus (SOL) muscle have been observed in humans using B-mode ultrasound to examine the knee from different angles. An alternative technique of assessing muscle and tendon stiffness is myometry, which is non-invasive, accessible, and easy to use. This study aimed to estimate the compressive stiffness of the distal SOL and Achilles tendon (AT) using myometry in various knee and ankle joint positions. Twenty-six healthy young males were recruited. The Myoton-PRO device was used to measure the compressive stiffness of the distal SOL and AT in the dominant leg. The knee was measured in two positions (90° of flexion and 0° of flexion) and the ankle joint in three positions (10° of dorsiflexion, neutral position, and 30° of plantar flexion) in random order. A three-way repeated-measures ANOVA test was performed. Significant interactions were found for structure × ankle position, structure × knee position, and structure × ankle position × knee position (p < 0.05). The AT and SOL showed significant increases in compressive stiffness with knee extension over knee flexion for all tested ankle positions (p < 0.05). Changes in stiffness relating to knee positioning were larger in the SOL than in the AT (p < 0.05). These results indicate that knee extension increases the compressive stiffness of the distal SOL and AT under various ankle joint positions, with a greater degree of change observed for the SOL. This study highlights the relevance of knee position in passive stiffness of the SOL and AT.     

Nature of the coupling between neural drive and force-generating capacity in the human quadriceps muscle

The force produced by a muscle depends on both the neural drive it receives and several biomechanical factors. When multiple muscles act on a single joint, the nature of the relationship between the neural drive and force-generating capacity of the synergistic muscles is largely unknown. This study aimed to determine the relationship between the ratio of neural drive and the ratio of muscle force-generating capacity between two synergist muscles (vastus lateralis (VL) and vastus medialis (VM)) in humans. Twenty-one participants performed isometric knee extensions at 20 and 50% of maximal voluntary contractions (MVC). Myoelectric activity (surface electromyography (EMG)) provided an index of neural drive. Physiological cross-sectional area (PCSA) was estimated from measurements of muscle volume (magnetic resonance imaging) and muscle fascicle length (three-dimensional ultrasound imaging) to represent the muscles'' force-generating capacities. Neither PCSA nor neural drive was balanced between VL and VM. There was a large (r = 0.68) and moderate (r = 0.43) correlation between the ratio of VL/VM EMG amplitude and the ratio of VL/VM PCSA at 20 and 50% of MVC, respectively. This study provides evidence that neural drive is biased by muscle force-generating capacity, the greater the force-generating capacity of VL compared with VM, the stronger bias of drive to the VL.     

Effects of isometric training on the elasticity of human tendon structures in vivo.

The present study aimed to investigate the effect of isometric training on the elasticity of human tendon structures. Eight subjects completed 12 wk (4 days/wk) of isometric training that consisted of unilateral knee extension at 70% of maximal voluntary contraction (MVC) for 20 s per set (4 sets/day). Before and after training, the elongation of the tendon structures in the vastus lateralis muscle was directly measured using ultrasonography while the subjects performed ramp isometric knee extension up to MVC. The relationship between the estimated muscle force and tendon elongation (L) was fitted to a linear regression, the slope of which was defined as stiffness of the tendon structures. The training increased significantly the volume (7.6+/-4.3%) and MVC torque (33.9+/-14.4%) of quadriceps femoris muscle. The L values at force production levels beyond 550 N were significantly shorter after training. The stiffness increased significantly from 67.5+/-21.3 to 106.2+/-33.4 N/mm. Furthermore, the training significantly increased the rate of torque development (35.8 +/- 20.4%) and decreased electromechanical delay (-18.4+/-3.8%). Thus the present results indicate that isometric training increases the stiffness and Young's modulus of human tendon structures as well as muscle strength and size. This change in the tendon structures would be assumed to be an advantage for increasing the rate of torque development and shortening the electromechanical delay.     

Sprint performance is related to muscle fascicle length in male 100-m sprinters.

The purpose of this study was to investigate the relationship between muscle fascicle length and sprint running performance in 37 male 100-m sprinters. The sample was divided into two performance groups by the personal-best 100-m time: 10.00-10.90 s (S10; n = 22) and 11.00-11.70 s (S11; n = 15). Muscle thickness and fascicle pennation angle of the vastus lateralis and gastrocnemius medialis and lateralis muscles were measured by B-mode ultrasonography, and fascicle length was estimated. Standing height, body weight, and leg length were similar between groups. Muscle thickness was similar between groups for vastus lateralis and gastrocnemius medialis, but S10 had a significantly greater gastrocnemius lateralis muscle thickness. S10 also had a greater muscle thickness in the upper portion of the thigh, which, given similar limb lengths, demonstrates an altered "muscle shape." Pennation angle was always less in S10 than in S11. In all muscles, S10 had significantly greater fascicle length than did S11, which significantly correlated with 100-m best performance (r values from -0.40 to -0.57). It is concluded that longer fascicle length is associated with greater sprinting performance.     

Gear ratios at the limb joints of jumping dogs

An increase in gear ratio of the limb extensor muscles during joint extension has been suggested to be a mechanism that facilitates optimal power production by skeletal muscles. The objectives of this study were to: (1) measure gear ratios at the wrist, elbow, shoulder, ankle, knee, and hip joints of jumping dogs, (2) compute the work performed by each of these joints, and (3) measure muscle shortening velocity for a joint exhibiting an increasing gear ratio during joint extension. The gear ratio out-lever was computed by dividing the ground reaction force (GRF) moment by the GRF, whereas the in-lever was directly measured as the perpendicular distance from the joint center to the line of action of the extensor muscle. In addition, changes in fascicle length were measured from the vastus lateralis muscle using sonomicrometry. Of the joints examined, only the gear ratios at the shoulder and knee joints increased during jumping in a manner that could facilitate peak power production of actively shortening muscles. The vastus lateralis was found to shorten at an average velocity of 3.20 muscle lengths per second. This is similar to estimates of shortening velocity that produce peak muscular power in mammals the size of dogs. Additionally, the knee extensors were found to produce a large proportion (26.6%) of the positive external work of the limbs. These observations suggest that dynamic gearing in jumping dogs may allow the extensor muscles of the knee joint to shorten in a way that maximizes their power production.