Tension regulation during lengthening and shortening actions of the human soleus muscle

In the present study we investigated tension regulation in the human soleus (SOL) muscle during controlled lengthening and shortening actions. Eleven subjects performed plantar flexor efforts on an ankle torque motor through 30 degrees of ankle displacement (75 degrees-105 degrees internal ankle ang le) at lengthening and shortening velocities of 5, 15 and 30 degrees s(-1). To isolate the SOL from the remainder of the triceps surae, the subject's knee was flexed to 60 degrees during all trials. Voluntary plantar flexor e fforts were performed under two test conditions: (1) maximal voluntary acti vation (MVA) of the SOL, and (2) constant submaximal voluntary activation ( SVA) of the SOL. SVA trials were performed with direct visual feedback of t he SOL electromyogram (EMG) at a level resulting in a torque output of 30% of isometric maximum. Angle-specific (90 degrees ankle angle) torque and EM G of the SOL, medial gastrocnemius (MG) and tibialis anterior (TA) were rec orded. In seven subjects from the initial group, the test protocol was repe ated under submaximal percutaneous electrical activation (SEA) of SOL (to 3 0% isometric maximal effort). Lengthening torques were significantly greate r than shortening torques in all test conditions. Lengthening torques in MV A and SVA were independent of velocity and remained at the isometric level, whereas SEA torques were greater than isometric torques and increased at h igher lengthening velocities. Shortening torques were lower than the isomet ric level for all conditions. However, whereas SVA and SEA torques decrease d at higher velocities of shortening, MVA torques were independent of veloc ity. These results indicate velocity- and activation-type-specific tension regulation in the human SOL muscle.