Force output during and following active stretches of rat plantar flexor muscles: effect of velocity of ankle rotation

During the development of force deficits by repeated stretches, velocity-se nsitive changes in the extra force produced during and after subsequent str etching has not been studied. Tn the present study, repeated dorsiflexion o f the foot of rats with maximally contracting planter flexor muscles was pe rformed at two angular velocities [0.87 (slow muscle stretch) and 10.47 rad s(-1) (fast muscle stretch)] to examine the active force of the muscles du ring and following dorsiflexion. Dorsiflexion was performed 30 times with a rest period of 3 min between the stretches to minimize muscle fatigue. The ability of rat plantar flexor muscles to produce additional force during t he stretch was not velocity sensitive. In contrast, repeated dorsiflexion w ith fast muscle stretches, but not with slow muscle stretches, resulted in an increase in the force decay with time following the stretches (i.e. incr eased stress relaxation), as indicated by a change in the time constant of force decay during stress relaxation. Apparently, the stress-relaxation of rat plantar flexor muscles is sensitive to angular velocity of ankle moveme nts; repeated fast, but not slow dorsiflexion, alters the stress relaxation process of active skeletal muscles exposed to stretches which create a for ce deficit. The change in time constant of force decay during stress relaxa tion in response to a series of repeated stretches might provide informatio n on the sarcomere length distribution in skeletal muscles. (C) 2000 Elsevi er Science Ltd. All rights reserved.