Journal of Applied Physiology, Vol 59, Issue 3 821-825, Copyright © 1985 by American Physiological Society
Relation between electromyogram and force in fatigue
H. Kranz, J. F. Cassell and G. F. Inbar
The relationship between the surface electromyogram (SEMG) and force was
examined during maximal voluntary contraction (MVC). Isometric MVC of elbow
flexors were studied in 18 subjects who performed 27 trials, each
consisting of six MVCs lasting 45 s at intervals of 30 s. There was a
decrease in the median frequency (Fm) of the SEMG and of the compound
action potentials (CAP) during MVC. The CAPs demonstrated that the fall in
Fm was associated with a proportional increase in signal power, whereas CAP
amplitude did not decrease, indicating intact neuromuscular transmission.
The SEMG root-mean-square amplitude remained fairly constant, progressively
deviating from force with time of contraction (r = 0.40). When SEMG
amplitude was corrected for the Fm change, it tracked force more closely (r
= 0.68), indicating a fall in motoneuron drive during MVC. The corrected
SEMG was used to calculate the change in the generalized firing rate of
motoneurons. The firing rate decreased 60% in the first and sixth
contractions, tracked force closely, and corresponded to the firing rate
fall seen in late adaptation of motoneurons (r = 0.90, P less than 0.001).
