Force and surface mechanomyogram relationship in cat gastrocnemius

The aim of this study was to compare the force (F) and the muscle transvers e diameter changes during electrical stimulation of the motor nerve. In fou r cats the exposed motor nerves of the medial grastrocnemius were stimulate d as follows: (a) eight separate trials at fixed firing rates (FR) from 5 t o 50 Hz (9 s duration, supramaximal amplitude); (b) 5 to 50 Hz linear sweep in 2.5, 5, 7.5 and 10 s (supramaximal amplitude, separate trials); (c) fou r separate trials at 40 Hz, the motor units (MUs) being orderly recruited i n 2.5, 5, 7.5 and 10 s. The muscle surface displacement was detected by a l aser distance sensor pointed at the muscle surface. The resulting electrica l signal was termed surface mechanomyogram (MMG). In stimulation patterns ( a) and (b) the average F and MMG increased with FR. With respect to their v alues at 50 Hz the amplitude of the unfused signal oscillations at 5 Hz was much larger in MMG than in force. The signal rising phase was always earli er in MMG than in F. In (c) trials, F increased less in the first than in t he second half of the recruiting time. MMG had an opposite behaviour. The r esults indicate that the force and the lateral displacement are not linearl y 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. (C) 1999 Elsevier Science Ltd. All rights reserve d.