Muscle architecture and force-velocity relationships in humans.

The in vivo torque-velocity relationships of the knee extensors (KE), knee flexors (KF), ankle plantarflexors (PF), and ankle dorsiflexors (DF) were determined in 12 untrained subjects using an isokinetic testing device (Cybex II). These data were then matched to the predicted maximum forces and shortening velocities derived from muscle architectural determinations made on three hemipelvectomies (36). The torque-velocity curves of all muscle groups resembled that predicted by Hill's (19, 20) equation except at the higher forces and lower velocities. The peak torques occurred at mean velocities ranging from 41-62 rad X s-1 for the KE, KF, and PF. Although the peak torque of the DF occurred at the isometric loading condition, it was also lower than that predicted by Hill's equation. The muscle fiber length and physiological cross-sectional area measurements indicate that the architecture of the human leg musculature has a major influence on the torque-velocity characteristics. These data corroborate previous findings (24) that some neural inhibitory mechanism exists in the control of the leg musculature, which limits the maximum forces that could be produced under optimal stimulating conditions.

Training-induced alterations of the in vivo force-velocity relationship of human muscle.

Seventeen male and female subjects (ages 20-38 yr) were tested pre- and posttraining for maximal knee extension torque at seven specific velocities (0, 0.84, 1.68, 2.51, 3.35, 4.19, and 5.03 rad . s-1) with an isokinetic dynamometer. Maximal knee extension torques were recorded at a specific joint angle (0.52 rad below the horizontal plane) for all test speeds. Subjects were randomly assigned to one of three experimental groups: group A, control, n = 7; group B, training at 1.68 rad . s-1, n = 5; or group C, training at 4.19 rad . s-1, n = 5. Subjects trained the knee extensors by performing two sets of 10 single maximal voluntary efforts three times a week for 4 wk. Before training, each training group exhibited a leveling-off of muscular tension in the slow velocity-high force region of the in vivo force-velocity relationship. Training at 1.68 rad . s-1 resulted in significant (P less than 0.05) improvements at all velocities except for 5.03 rad . s-1 and markedly affected the leveling-off in the slow velocity-high force region. Training at 4.19 rad . s-1 did not affect the leveling-off phenomenon but brought about significant improvements (P less than 0.05) at velocities of 2.51, 3.35, and 4.19 rad . s-1. The changes seen in the leveling-off phenomenon suggest that training at 1.68 rad . s-1 might have brought about an enhancement of motoneuron activation.

Torque-velocity relationships and muscle fiber composition in elite female athletes.

The relationship between the predominance of fast and slow muscle fibers of the vastus lateralis and "in vivo" torque velocity properties in 22 female athletes was studied. Fiber types were classified according to the histochemical myofibrillar adenosine triphosphatase technique at a basic pH. Maximal extensor troques were recorded at 30 degrees from full extension at four selected velocities. While results confirm earlier reports on muscle fiber type and performance, an additional finding was that as knee extension velocities increased from 0 to 95 degrees/s angle specific extensor torque production did not decline as seen in in vitro muscle preparations. The difference in extensor torque between 0 and 96 degrees/s appeared far more critical than the differences observed between 96 and 288 degrees/s. Significant differences in torque were seen at 96, 192, and 288 degrees/s in thos with greater than 50% and less than 50% slow-twitch fibers. When expressed per kilogram of body weight the subjects with greater than 50% fast-twitch fiber produced the greatest torque at 192 degrees/s. These results suggest that the velocity at which torque begins to decline in vivo is related to the proportion of slow-twitch fibers in the vastus lateralismuscle.

Muscle force-velocity and power-velocity relationships under isokinetic loading.

Various studies have indicated that human muscles in-vivo manifest a substantially similar, if not the identical force-velocity relationship established for isolated, maximally stimulated animal muscles. In the present study, fifteen healthy males and females, 18 to 38 years old and representing varied activity patterns from sedentary to athletic, performed maximal dynamic knee extensions on an isokinetic loading dynamometer. Maximal torque forces attained at a specific point in the range (30 degrees before full extension) and at seven loading velocities from 0 (isometric) to 288 degrees/sec were recorded. The maximum 30 degrees torques exhibited by the various subjects ranged from 29 to 245 Newton-meters. However, over the four lower test velocities (0, 48, 96 & 144 degrees/sec), all subjects exhibited less than a 15% deviation from their respective maximum 30 degrees torque values, which occurred most often at the 96 degree/sec test velocity. Maximal instantaneous power output at the 30 degree position ranged from 98 to 680 Watts. In all 15 subjects this was attained at and remained generally constant over the three highest test velocities (192 to 288 degrees/sec). A neural mechanism that restricts a muscle's maximal tension in-vivo is postulated as being responsible for the marked difference between the force-velocity relationship found for human muscles in-vivo and that exhibited by isolated animal muscles.