A study of motor unit activation pattern by an all poles model of the muscular sound

Two types of motor units (MUs) can be found in the muscle, type I and type II, activated at low and high levels of contraction, respectively. The firing frequencies are roughly in the range of 8-18 Hz for type I and in the range of 15-30 Hz for type II MUs. An all-poles model is used to describe the dynamics of the sound myogram (SMG), which is related to the mechanical activity of the excited muscle fibers. The SMG was recorded during isometric contractions lasting 4 s at 20% and 80% of the maximal voluntary contraction. The results show that at low contraction level a second-order autoregressive model, AR(2), is the optimal model, whereas at high contraction level the optimal model order is AR(4). The poles computed from the models reflect the activation patterns of the MUs.<>