Visual and cutaneous triggering of rapid step initiation.

The effects of a cutaneous imperative stimulus trigger cue (sural nerve stimulation) versus visual cuing of rapid step initiation were assessed in young, healthy subjects (n = 18). Two sets of experiments were conducted in which the vertical ground reaction force and EMG in tibialis anterior (TA) and gluteus medius (GM) were recorded in nine subjects and the vertical ground reaction force and center of pressure were recorded in a separate group of nine subjects. Subjects stood with one foot on a force platform with weight equally distributed and asked to take three steps as quickly as possible. A visual ready signal was followed at random times (0.5-2 s) by either a second visual go cue or stimulation of the sural nerve. Results revealed that sural cuing produced: (1) earlier onset times, greater vertical ground reaction forces and a greater rate of rise of force, (2) earlier onsets of TA and GM and greater mean EMG amplitudes in these muscles and (3) greater COP displacements in both the posterior and lateral direction. These results confirm previous reports on the functions of TA and GM in step initiation and further show that sural cuing enhances these EMG responses and the subsequent kinetic and kinematic changes associated with them.