ELECTROGRAPHIC EVALUATION OF MECHANICAL RESPONSE IN MAMMALIAN SKELETAL MUSCLE IN DIFFERENT CONDITIONS

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Abstract
The relation of EMG amplitude to the mechanical response of the tibialis anticus m. of cats under Dial anesthesia was studied under a variety of conditions. If constancy of the number of responding motor units is assured by the use of supramaximal indirect stimulation, marked increases in the mechanical response can be obtained by increasing the load, initial length, or stimulus frequency while the EMG amplitude and integrated electrical output remain constant. Under cortical stimulation, similar procedures result in parallel increases in the electrical and mechanical responses. Increasing the intensity of either cortical or indirect sub-maximal stimulation results in parallel increases in EMG and mechanical response. Fatigue during either cortical or indirect tetanization is characterized by parallel declines in EMG and mechanical response. It appears that EMG amplitude is based largely upon the number of responding motor units. Pro-prioceptive reflex facilitation apparently plays an important role in determining the extent of recruitment and therefore the amplitude of the EMG. It is concluded that under physiol. conditions allowing normal quantitative control of muscle innerva-tion by the degree of cortical excitation and proprioceptive reflex activity, EMG amplitude is a reliable indicator of the intensity of the mechanical response.

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