Force and surface mechanomyogram frequency responses in cat gastrocnemius

Muscle surface displacement is a mechanical event taking place simultaneous ly with the tension generation at the tendon. The two phenomena can be stud ied by the surface mechanomyogram signal(MMG) (produced by a laser distance sensor) and the force signal (from a load cell). The aim of this paper was to provide data on the reliability of the laser detected MMC in muscle mec hanics research. To this purpose it was verified if the laser detected MMG was suitable to estimate a frequency response in the cat medial gastrocnemi us and its frequency response was compared with the one retrieved by the fo rce signal at the tendon level. The force and MMG from the exposed medial g astrocnemius of four cats were analysed. The frequency response was investi gated by sinusoidally changing the number of orderly recruited motor units, in different trials, in the 0.4-6 Hz range. It resulted that it was possib le to model the force and MMG frequency response by a critically damped sec ond-order system with two real double poles and a pure time delay. On the a verage, the poles were at 1.83 Hz (with 22.6 ms delay) and at 2.75 Hz (with 38 ms delay) for force and MMG, respectively. It can be concluded that MMG appears to be a reliable tool to investigate the muscle frequency response during stimulated isometric contraction. Even though not statistically sig nificant, the differences in the second-order system parameters suggest tha t different components of the muscle mechanical model may specifically affe ct the force or MMG. (C) 2000 Elsevier Science Ltd. All rights reserved.