Simulation of the surface EMG of an active muscle

A model of the motoneuronal (MN) pool-muscle system was developed. The model consists of four modules: (1) the input to the MN pool, (2) the MN pool, (3) the muscle and (4) the surface electromyogram (EMG). A control parameter activates the input fibers and determines the activity level of the muscle. A single compartment model with a homogenous membrane was used to model the MNs. The trajectory between spikes is determined by two voltage-gated K(+)- and two voltage-gated Ca(2+)-channels. The size of the MNs is adjusted by the size of the leakage conductance. The model muscle is of circular cross section and with parallel fibers. The motor unit (MU) territories are of circular shape and their area is proportional to the MU contraction force. Action potentials propagated along the muscle fibers are approximated by a dipole with a current source and current sink. The potential evoked by the dipole at the recording site is computed. The surface EMG is obtained by summing up (1) the potentials of all fibers of the MU and (2) the MU action potentials of all active MUs. Numerical results show that the MUs are recruited with increasing contraction force and that the active MUs modulate their firing frequency similar as in real muscles. The model will be used for investigation of the motor system in man.