EMG patterns and forces developed during step-down.

These experiments describe electromyographic activity and forces developed during step-down from several heights. The external environment of the experimental subjects was varied by blindfold or Achilles' tendon vibration during descent. EMG activity of the step-down limb, vertical force of impact and knee angle were measured. The results show that the triceps surae muscle contracts prior to the time of impact of the foot with the substrate if information about the step height is supplied to the subject and if the sensory system is not disturbed by vibration. In the presence of vibration or if the step height is unknown, the triceps surae does not contract prior to foot contact. This produces a large force transient which must be absorbed by the skeletal structure, rather than being damped by a lengthening contraction of the triceps surae. The data suggest that the forces developed during ordinary step-down may be the cause of bone and injury in people with impaired vision or in patients with damaged nervous systems who exhibit large responses to vibration.