[Close interdependence between passive and active soleus muscle torques in man: approach to the coregulation of the number of sarcomeres and length of the connective tissue].

Soleus muscle has, to as great an extent as possible, been functionally isolated in man. A device previously described permitted measurement of the soleus torque as a function of the tibia-calcaneum angle, and not as a function of tibia-foot angle. This latter angle cannot be correctly related to soleus length, since the arch of the foot can not be considered as rigid throughout the experiment. Ankle movements were performed on a horizontal plane by successive increments of 5 degrees from full extension (plantar flexion) up to full flexion (dorsiflexion). Passive torques were measured for every angle. At the same angles, the total torques were recorded while the subject exerted a voluntary constant contraction which corresponded to 45% of the maximum integrated electromyographical activity of soleus muscle. The active torque-angle curve was obtained by subtracting, for the same angle, the passive from the total torques. It must be stressed that, when the muscle is maximally shortened, little or no torque was measured. For the angle corresponding to the top of the sub-maximal active curve (integrated electromyographic activity 45% of the maximum), the passive torque was about 1.75 Nm and showed no significant interindividual variations. This result contrasted with the marked interindividual shifts which affect both passive and active curves. This interdependence of passive and active curves in human soleus muscle is compatible with the results of a previous study in the cat showing the concomitant adaptations of sarcomere number and connective tissue length.