DIRECT DYNAMICS SIMULATION OF THE IMPACT PHASE IN HEEL-TOE RUNNING

The influence of muscle activation, position and velocities of body se gments at touchdown and surface properties on impact forces during hee l-toe running was investigated using a direct dynamics simulation tech nique. The runner was represented by a two-dimensional four- (rigid bo dy) segment musculo-skeletal model. Incorporated into the muscle model were activation dynamics, force-length and force-velocity characteris tics of seven major muscle groups of the lower extremities: mm. glutei , hamstrings, m. rectus femoris, mm. vasti, m. gastrocnemius, m. soleu s and m. tibialis anterior. The vertical force-deformation characteris tics of heel, shoe and ground were modeled by a non-linear visco-elast ic element. The maximum of a typical simulated impact force was 1.6 ti mes body weight. The influence of muscle activation was examined by ge nerating muscle stimulation combinations which produce the same (exper imentally determined) resultant joint moments at heelstrike. Simulated impact peak forces with these different combinations of muscle stimul ation levels varied less than 10%. Without this restriction on initial joint moments, muscle activation had potentially a much larger effect on impact force. Impact peak force was to a great extent influenced b y plantar flexion (85 N per degree of change in foot angle) and vertic al velocity of the heel (212 N per 0.1 m s(-1) change in velocity) at touchdown. Initial knee flexion (68 N per degree of change in leg angl e) also played a role in the absorption of impact. Increased surface s tiffness resulted in higher impact peak forces (60 N mm(-1) decrease i n deformation). It was concluded that changes in initial kinematic con ditions are primarily responsible for variations in impact forces, and that muscular co-contraction does not play a major role.