Electromyographic activation and proportion of fast versus slow twitch muscle fibers: a genetic disposition for psychogenic muscle tension?

17 right-handed males volunteered for an experiment that compared task-related patterns of electromyographic (EMG) activation with data from muscle biopsy on proportion of slow-twitch ((ST) aerobic) to fast-twitch ((FT) anaerobic) muscle fibers. The biopsy was taken from the right-leg gastrocnemius muscle after EMG measurement from that area of the leg muscle. EMG was also recorded from the left forearm flexor carpi radialis area. Recordings were obtained from pre- and post-task resting periods and during 150 s of video-task performance when the right hand operated a joy-stick. The results showed a highly significant tonic EMG activation in the leg muscle of subjects with predominance of ST fibers, and this relationship generalized to the EMG from the 'passive' forearm. The proportion of ST to FT fibers is genetically defined and not altered by exercise. Therefore, our results lend support to a genetic differentiation between individuals with high vs. low probability of unintended build-up of muscle tension during perceptual-motor task performance.