The EMG-force relationships of skeletal muscle; dependence on contraction rate, and motor units control strategy

The dependence of the EMG vs force relationships on skeletal muscles contraction rate, and its motor units recruitment control strategy were studied. An electrical nerve stimulation technique capable of orderly excitation of motor units according to their size with simultaneous increase in their discharge rate was applied to isometric preparations of the M. gastrocnemius muscle of the cat. Muscle force and intramuscularly recorded EMG were normalized and plotted against each other. The method of least squares (p less than .05) was used to yield the best fit linear regression polynominials to the pooled data from several preparations. Strong dependency on the motor unit recruitment strategy was exhibited by the results. Linear EMG vs force relationships was evident when all the motor units were recruited to generate the initial 50% of the maximal force while discharge rate continued to increase up to the maximal force. Progressive increase in non-linearity was evident as recruitment of motor units generated 60% and up to 100% of the initial force concurrently with firing rate increase. Contraction rate (force generation rate) increase from 36% sec up to 360% sec resulted in nearly identical normalized EMG vs force relationships, although the maximal force increased by up to 22.5% as the contraction rate increased to 360% sec. The results of this study issue a clear warning against the direct use of EMG to predict muscle force in biomechanical and kinesiological research without correcting for the variables discussed in this paper.