Transients of the force and surface mechanomyogram during cat gastrocnemius tetanic stimulation

The aim of the study was to investigate the time relationship between force and muscle surface displacement, detected as the surface mechanomyogram (MMG) by a laser distance sensor, in the transient phases of a tetanic stimulation. For this purpose the motor nerve of the exposed medial gastrocnemius of four cats was supramaximally stimulated at 30, 40 and 50 Hz for 9 s. Force was detected by a transducer connected at the distal tendon while MMG was measured after pointing the laser beam at the muscle belly. We found that the MMG always anticipated and trailed the force changes during the on- and off-phase of the tetani, respectively. Independently of the stimulation rate, the half-times of the two signals were: on-phase, about 76 ms for force and 33 ms for MMG; off-phase, about 83 ms for force and 132 ms for MMG. There are two main comments to make about these results. First, during the on-phase the shortening of the contractile elements results at first in a muscle geometry change with low output force. After this, when the slack of the elastic-connective tissue has been taken up, the tension is efficiently transmitted to the tendon. Second, the different force and MMG dynamics in the on- and off-phases determine a counter-clockwise hysteresis with more force produced at a given muscle surface displacement during relaxation. To explain the results, the possible specific roles of some components of the muscle mechanical model, muscle mechanical properties and intra-muscular phenomena taking place during contraction have been discussed.