Investigations of the time series structure of the EMG during sustained isometric contractions of back muscles.

The experiment aimed at elucidating electromyographic signs of fatigue of lumbar muscles. On two days, 4 subjects had to hold 4 different target forces to fatigue. Bipolar surface EMG's were derived at the L3 level. The time series structure of the EMG's was characterized by means of autoregressive models. The r.m.s.- value did not reveal uniform time-dependent changes, except for an increase during the highest force level. The variance explained by a linear stochastic process systematically increased during the sustained contractions. The mean power frequencies exhibited similar changes.