The relationship between electromyogram and muscle force.

Some guidelines are given for the interpretation of the electromyogram (EMG). In static isometric contractions there is, usually a linear, relationship between muscle force and smoothed rectified EMG (SRE). It should be considered, however, that most of the time several muscles are active simultaneously around a joint. This finding implies that the relationship between the SRE of a single muscle and the total joint moment needs not be linear. During movements, the force-length-velocity relationship and the elastic properties of muscle should be taken into account. Also, EMGs during concentric movements are larger than those measured during isometric actions. A further consideration in relating force to EMG is that the force signal has a much lower frequency content than the rectified EMG. Finally, the limited speed of the activation and deactivation process results in a 50 to 200 ms delay of the muscle force relative to the EMG.