Isometric muscle length-tension curves do not predict angle-torque curves of human wrist in continuous active movements

In this study we tested the hypothesis that during steady contractions of h uman wrist extensors or flexors, the torque-angle relationship during movem ents imposed about the wrist is predicted by the classical isometric muscle length-tension curve, with ascending, descending and ascending limbs. Angl e-torque relationships were measured during steady muscle activation (10% o f maximal voluntary contraction: MVC), elicited either by electrical stimul ation or voluntary regulation of the electromyogram (EMG). Flexion-extensio n movements of constant speed (+/- 10 degrees /s) were imposed on the subje cts' hands with a servo actuator, either through the full physiological ran ge of motion +/- 50 degrees, or through +/- 10 degrees. During extensor con tractions, angle-torque curves in +/- 50 degrees movements had ascending, d escending and ascending limbs, as in isometric contractions. However, in +/ - 10 degrees movements, torque always increased with increasing muscle leng th and decreased with decreasing length, even over angles corresponding to the descending limb of isometric curves. For flexor activation, angle-torqu e curves had similar properties, though descending limbs were less obvious or absent. During imposed movements, hysteresis was observed in the angle-t orque curves. This was attributed to non-linearities of the active muscles. Hysteresis reached a maximum at intermediate wrist angles and declined at maximal muscle length, contradicting the recent hypothesis that sarcomere n on-uniformity is responsible for the hysteresis. We conclude that the class ical isometric length-tension curve, with its prominent descending limb, do es not predict angle-torque curves of human wrist muscles in continuous mov ements. A more appropriate model is one in which stiffness about the wrist is always positive and hysteresis is a significant factor. (C) 2000 Elsevie r Science Ltd. All rights reserved.