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-sensitive changes in the extra force produced during and after subsequent stretching has not been studied. In the present study, repeated dorsiflexion of the foot of rats with maximally contracting plantar flexor muscles was performed at two angular velocities 0.87 (slow muscle stretch) and 10.47rads(-1) (fast muscle stretch)] to examine the active force of the muscles during and following dorsiflexion. Dorsiflexion was performed 30 times with a rest period of 3min between the stretches to minimize muscle fatigue. The ability of rat plantar flexor muscles to produce additional force during the stretch was not velocity sensitive. In contrast, repeated dorsiflexion with fast muscle stretches, but not with slow muscle stretches, resulted in an increase in the force decay with time following the stretches (i.e. increased 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 movements; repeated fast, but not slow dorsiflexion, alters the stress relaxation process of active skeletal muscles exposed to stretches which create a force deficit. The change in time constant of force decay during stress relaxation in response to a series of repeated stretches might provide information on the sarcomere length distribution in skeletal muscles.