Torque-angle relationships of human wrist muscles

In intact human subjects we studied the relationship between active torque and angle about the wrist using either surface electrical stimulation or voluntary effort to contract the muscles. The wrist was moved at a constant 10° s^-1 by a torque motor from 50° flexion to 50° extension (full range-of-motion: ROM), or over 20° portions of this range. Isometric torque was measured at five wrist angles. The extensor angle-torque profile in full ROM trials was similar to the classical isometric length-tension curve, with ascending and descending limbs, the latter representing negative stiffness. Flexor torque increased monotonically with flexion angle over full ROM. However, in 20° movements, wrist extensors showed positive stiffnesses throughout. In addition, a prominent hysteresis separated torque for muscle lengthening and shortening which could not be totally explained by force-velocity effects