Interpreting muscle function from EMG: lessons learned from direct measurements of muscle force

Electromyography is often used to infer the pattern of production of force by skeletal muscles. The interpretation of muscle function from the electromyogram (EMG) is challenged by the fact that factors such as type of muscle fiber, muscle length, and muscle velocity can all influence the relationship between electrical and mechanical activity of a muscle. Simultaneous measurements of EMG, muscle force, and fascicle length in hindlimb muscles of wild turkeys allow us to probe the quantitative link between force and EMG. We examined two features of the force-EMG relationship. First, we measured the relaxation electromechanical delay (r-EMD) as the time from the end of the EMG signal to time of the end of force. This delay varied with locomotor speed in the lateral gastrocnemius (LG); it was longer at slow walking speeds than for running. This variation in r-EMD was not explained by differences in muscle length trajectory, as the magnitude of r-EMD was not correlated with the velocity of shortening of the muscle during relaxation. We speculate that the longer relaxation times at slow walking speeds compared with running may reflect the longer time course of relaxation in slower muscles fibers. We also examined the relationship between magnitude of force and EMG across a range of walking and running speeds. We analyzed the force-EMG relationship during the swing phase separately from the force-EMG relationship during stance phase. During stance, force amplitude (average force) was linearly related to mean EMG amplitude (average EMG). Forces during swing phase were lower than predicted from the stance phase force-EMG relationship. The different force-EMG relationships during the stance and swing phases may reflect the contribution of passive structures to the development of force, or a nonlinear force-EMG relationship at low levels of muscle activity. Together the results suggest that any inference of force from EMG must be done cautiously when a broad range of activities is considered.