The torque-velocity relationship in large human muscles: maximum voluntary versus electrically stimulated behaviour

The in vivo maximum voluntary torque-velocity profile for large muscle groups differs from the in vitro tetanic profile with lower than expected eccentric torques. Using sub-maximal transcutaneous electrical stimulation has given torque-velocity profiles with an eccentric torque plateau ∼1.4 times the isometric value. This is closer to, but still less than, the in vitro tetanic profiles with plateaus between 1.5 and 1.9 times isometric. This study investigated the maximum voluntary and sub-maximum transcutaneous electrical stimulated torque-angle-angular velocity profiles for the knee extensors and flexors in a group of healthy males. Fifteen male subjects performed maximum voluntary and sub-maximum electrically stimulated (∼40% for extensors and ∼20% for flexors) eccentric and concentric knee extension and flexions on an isovelocity dynamometer at velocities ranging from ±50°s(-1) to ±400°s(-1). The ratio of peak eccentric to peak isometric torque (T(ecc)/T(0)) was compared between the maximum voluntary and electrically stimulated conditions for both extensors and flexors, and between muscle groups. Under maximum voluntary conditions the peak torque ratio, T(ecc)/T(0), remained close to 1 (0.9-1.2) while for the electrically stimulated conditions it was significantly higher (1.4-1.7; p<0.001) and within the range of tetanic values reported from in vitro studies. In all but one case there was no significant difference in ratios between the extensors and flexors. The results showed that even the largest muscle groups have an intrinsic T(ecc)/T(0) comparable with in vitro muscle tests, and it can be ascertained from appropriate in vivo testing.