Shortening velocity of human triceps surae muscle measured with the slack test in vivo.

Unloaded shortening velocity (V(0)) of human triceps surae muscle was measured in vivo by applying the 'slack test', originally developed for determining V(0) of single muscle fibres, to voluntary contractions at varied activation levels (ALs). V(0) was measured from 10 subjects at five different ALs defined as a fraction (5, 10, 20, 40 and 60%) of the maximum voluntary contraction (MVC) torque. Although individual variability was apparent, V(0) tended to increase with AL (R(2) = 0.089; P = 0.035) up to 60%MVC (8.6 +/- 2.6 rad s(-1)). This value of V(0) at 60%MVC was comparable to the maximum shortening velocity of plantar flexors reported in the previous studies. Electromyographic analysis showed that the activities of soleus, medial gastrocnemius and lateral gastrocnemius muscles increased with AL during isometric contraction and after the application of quick release in a similar manner. Also, it showed that the activity of an antagonist, tibialis anterior muscle, was negligible, even though a slight increase took place after the quick release of agonist. Correlation analysis showed that there were no significant correlations between V(0) and MVC torque normalized with respect to body mass, although the correlation coefficient was relatively high at low ALs. The results suggest that in human muscle, V(0) represents the unloaded velocity of the fastest muscle fibres recruited, and increases with AL possibly because of progressive recruitment of faster fibres. Individual variability may be explained, at least partially, by the difference in fibre-type composition.