Muscular co-operation during joint stabilisation, as reflected by EMG.

The experiment that was carried out consisted of subjects pushing an external object (a heavy pendulum) using stable and unstable handles of increasing mobility. Using this protocol it was possible to distinguish between the motor and stabilising functions of the muscles of the upper extremity. The motor functions were realised by the extensors of the upper extremity, whereas stabilising functions were effected by the muscles spanning the wrist joint. The experiment involved synchronised measurements of the electromyographic (EMG) activity of the muscles in question together with several mechanical quantities revealed against the external object: force, velocity and power. As a result, the instantaneous and global EMG contributions of the extensor and stabilising muscles were determined. It was found that it is the equilibrium state of the object being set in motion and not its mobility (expressed in terms of the number of degrees of freedom) that influences the forces produced by individual muscles. We also suggest that the realisation of stabilising functions by skeletal muscles is a necessary condition of performing any voluntary and co-ordinated movement.