Interaction between pre-activity and stretch reflex in human triceps brachii during landing from forward falls.

1. Electromyographic (e.m.g.) profiles of proximal arm muscles were studied in human subjects falling forward onto a platform. 2. The stretching of the triceps lasted 200-300 msec for deep falls, and immediately after impact angular velocities of the elbow joint up to 1000 degrees sec-1 were reached. 3. For angles of fall between 50 and 90 degrees, more than half of the subjects exhibited marked short-latency e.m.g. responses of the triceps brachii. Such responses began 20-30 msec after touchdown, arising from a more or less plateau-like activity which started about 130 msec before impact. In some cases distinct later responses were found, the second peak having a latency of 60-80 msec after touchdown. 4. The early e.m.g. response even appeared when the subject was blindfolded and when the depth of the fall was randomly varied. 5. It is concluded that both the pre-existing activity and the spinal stretch reflex contribute significantly to the over-all activity of the triceps during stretch after impact.