Force and surface mechanomyogram relationship in cat gastrocnemius

The aim of this study was to compare the force (F) and the muscle transverse diameter changes during electrical stimulation of the motor nerve. In four cats the exposed motor nerves of the medial gastrocnemius were stimulated as follows: (a) eight separate trials at fixed firing rates (FR) from 5 to 50Hz (9s duration, supramaximal amplitude); (b) 5 to 50Hz linear sweep in 2.5, 5, 7.5 and 10s (supramaximal amplitude, separate trials); (c) four separate trials at 40Hz, the motor units (MUs) being orderly recruited in 2.5, 5, 7.5 and 10s. The muscle surface displacement was detected by a laser distance sensor pointed at the muscle surface. The resulting electrical signal was termed surface mechanomyogram (MMG). In stimulation patterns (a) and (b) the average F and MMG increased with FR. With respect to their values at 50Hz the amplitude of the unfused signal oscillations at 5Hz was much larger in MMG than in force. The signal rising phase was always earlier in MMG than in F. In (c) trials, F increased less in the first than in the second half of the recruiting time. MMG had an opposite behaviour. The results indicate that the force and the lateral displacement are not linearly related. The different behaviour of F and MMG, from low to high level of the MUs' pool activation, suggests that the force generation and the muscle dimensional change processes are influenced by different components of the muscle mechanical model.