Viscoelastic behavior of plantar flexor muscle-tendon unit at rest

Muscle stretching as an exercise routine is widely used in orthopaedic and neurological rehabilitation. However, the muscle response to specific stretching parameters is still unclear. The aim of this study was to investigate the effect parameters, such as stretch velocity, stretch extent, and initial muscle-tendon resistance, on the plantar flexor response to passive movement. Eighteen healthy subjects (23-41 years) participated in this study. Five passive ankle dorsiflexions were randomly imposed at various velocities from 5 degrees/sec to 180 degrees/sec using a Kin-Com dynamometer, while unwanted activations of the soleus and tibialis anterior muscles were detected with surface electrodes. The resistive torque was averaged at -10 degrees and 0 degree of dorsiflexion. As shown by analyses of variance followed by Scheffé post hoc procedures, the resistive torque was significantly increased (p < 0.01) between 5 degrees/sec and higher velocities (60 degrees/sec or 120 degrees/sec and higher). A strong linear resistive torque-velocity relationship was also observed, as indicated by Pearson correlation coefficients of 0.92 (-10 degrees) and 0.91 (0 degree). The absolute resistive torque increment, calculated at 180 degrees/sec, was larger at 0 degree of dorsiflexion than at the -10 degrees of dorsiflexion position. Finally, subjects with larger initial plantar flexor resistance had a higher resistive torque increment (p < 0.05) at a high velocity of stretch (180 degrees/sec) than those with less initial muscle-tendon resistance. These results indicate that 1) the nonreflex resistive torque response to stretch is velocity-sensitive and 2) both a larger stretch extent and muscle initial resistance lead to greater resistive torque increments at high velocity. These observations suggest that slow and gradual stretching procedures, rather than rapid or ballistic movements, should be used, especially with stiff muscles to reduce the chance of injury from excessively high tension.