Coordination of force output on the basis of load anticipation : switching of force output in a muscle

Abstract

The present study was designed to investigate the coordination of force output on the basis of the anticipation. We measured fractionated reaction times to the regularly alternating serial visual stimuli. Subjects performed the right elbow flexion movements weighted by light and heavy loads alternately. We examined the responses caused by unexpected break of the regularity of the stimulus and the load (feinting stimulus). Premotor times and muscle activities on EMGs were discussed in terms of switching of force output in a muscle. The results may be summarized as follows: 1) Premotor time (PMT) to the regularly alternating stimulus (RAS) was shorter than that in simple reaction time. This fact suggests that the subjects anticipate the kind and timing of the next stimulus because of the alternative and periodic stimuli. 2) Responses to the feinting stimuli (FS) were characterized by biphasic burst patterns on both biceps and triceps EMGs. There was the first burst of activity followed by silent period, and then each muscle became active again. This biphasic pattern was observed in 20% of the responses to FS. 3) When the biphasic burst pattern was not observed on EMGs, PMT to FS was lengthened in comparison with that to RAS. 4) The first burst phase on EMG was regarded as the error response caused by unexpected stimulus, and the second burst phase as the corrected response. 5) The silent period results from a time lag between cancelation of the executing program and execution of new corrected program. 6) PMT for the heavy load was longer than that for the light load under the simple reaction condition. However, there was no effect of force output on PMTs to both RAS and FS.