The control of timing and amplitude of EMG activity in landing movements in humans

The control of self-initiated falls from different heights was studied. The objective of the study was to investigate in a quantitative manner the mod ulation of EMG timing (i.e. onset from take-off and duration from onset to touch-down) and amplitude (before and after foot contact) as a function of fall height. The muscles studied were m. soleus and m. tibialis anterior. K inematic (ankle joint angle) and kinetic (ground reaction force) variables were also measured. Six subjects took part in the experiments that consiste d of ten landings from each of five heights (0.2, 0.4, 0.6, 0.8 and 1 m) on to a force platform. We found a consistent pattern of co-contraction before and after touchdown across the fall heights studied. In both muscles, the onset of pre-landing EMG activity occurred at a longer latency following ta ke-off when landing from greater heights. The absolute EMG duration was aff ected to a lesser extent by increasing fall height. These findings suggest that the onset of muscle activity of the muscles studied prior to foot cont act is timed relative to the expected time of foot contact. Pre- and post-l anding EMG amplitude tended to increase with height. Despite a doubling in the magnitude of ground reaction force, the amplitude of ankle joint rotati on caused by the impact remained constant across heights. These findings su ggest that the observed pattern of co-contraction is responsible for increa sing ankle joint stiffness as fall height is increased. The attainment of a n appropriate level of EMG amplitude seems to be controlled by (a) timing m uscle activation at a latency timed from the expected instant of foot conta ct and (b) varying the rate at which EMG builds up.