SUMMATION OF ELEMENTARY PHONOMYOGRAMS DURING ISOMETRIC TWITCHES IN HUMANS

To study its summation principle, the phonomyogram (PMG) from the firs t interosseus dorsalis muscle was recorded in five subjects during sin gle twitches evoked by electrical stimulation over the motor point. By increasing the current pulse from threshold to maximal intensity, PMG amplitude increased linearly with motor unit recruitment. The twitch amplitude-intensity relationship was also linear. The PMG amplitude wa s therefore linearly related to the external force. For all these rela tionships highly significant correlation coefficients were found. Thes e relationships were interpreted as being a consequence of an orderly recruitment, although, contrary to what happens during voluntary contr action, the largest and strongest motor units were recruited before th e smallest and weakest ones during axon electrical stimulation. The PM G onset always preceded twitch onsets as indicated by latency measurem ents [mean 3.2 (SD 1.3) ms versus 11.5 (SD 3.9) ms, respectively]. Mor eover, PMG and twitch latencies may have been significantly reduced by recruitment, suggesting that orderly recruitment influenced both PMG amplitude and occurrence. These results were interpreted as being the result of the summation of elementary PMG from every contracting motor unit and the stiffness change of the muscle medium occurring with rec ruitment.