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

Abstract

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 10.47 rad s⁻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 3 min 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.

Introduction

The production of extra force during and following stretches of activated skeletal muscles has been described for undamaged muscles (Abbott and Aubert, 1952; Cook and McDonagh, 1995; Edman et al., 1978) but not for muscles in which prior stretching produced force deficits indicative of muscle injury (Hesselink et al., 1996). For normal (undamaged) skeletal muscles, there is a relationship between velocity of stretching and the extra force produced during the stretch at low velocities of stretch such that the faster the stretch the greater the extra force produced. At higher velocities of stretching, no such relationship exists as force remains constant with increasing stretching velocity. In addition to velocity, the magnitude of the extra force depends on initial length, length changes and final muscle length (Edman et al., 1978). The magnitude of the extra force produced by stretching also correlates with the force deficit produced. The force deficit caused by exposure of muscle to unaccustomed stretches is taken as indirect evidence of muscle injury (e.g. McCully and Faulkner, 1986) because there is no correlation between the amount of structural damage (e.g. sarcomere disruption) and the force deficit.

For the extra force recorded after the stretch (i.e. stress-relaxation), velocity of stretching was without effect for undamaged muscles (Edman et al., 1978). Stress-relaxation has not been studied in muscles exposed to repeated stretching or following other types of muscle injury. For injured muscles where structural and functional parameters are changing, the response to subsequent stretching could be quite different than for undamaged muscles. Since repeated stretches with and without muscle damage are common during sports and occupational tasks, knowledge of any altered biomechanics would be useful in predicting functional outcomes.

Using velocity-controlled joint rotations to produce the stretches, the extra force during and following stretches was monitored as a function of repetition number to determine the effects of angular velocity of repeated stretches on the capacity for extra force production and stress-relaxation. It was found that stress-relaxation following stretches became sensitive to the angular velocity with repeated fast but not slow stretches.