THE INFLUENCE OF MIDSTANCE VERTICAL GROUND REACTION FORCE ON THE HUMAN WALK-RUN TRANSITION 519

Abstract

In a study on the trot-gallop transition of horses Farley & Taylor(1991) measured vertical ground reaction forces (VGRF) and found a critical level of force which seemed to be the controlling factor in the transition. The following experiment was planned to assess the roles of midstance VGRF and speed of locomotion in the human walk-run transition. If midstance VGRF has a greater role in the transition than speed, then an addition of weight (15% BW) to a subject should cause the preferred transition speed (PTS) to change while maintaining similar levels of midstance VGRF. Two sets of measurements were conducted on 10 subjects in an attempt to discern these roles. Using a treadmill instrumented to measure VGRF, a subject's PTS was determined by systematically increasing speed from 1 to 3 m/s in 0.2 m/s increments until the subject switched his/her preferred gait to running. Then midstance VGRF was measured while subjects walked and ran at speeds from 0.6 m/s below PTS to 0.6 m/s above PTS. At each speed the midstance VGRF was calculated as a mean over 5-10 strides. These two measurements were repeated under both weighted and unweighted conditions. With speed aligned at the PTS, midstance VGRF was plotted for each condition and averaged across subjects. As speed increased, midstance VGRF for both walking conditions decreased, then increased once above PTS. The variability of midstance VGRF significantly increased above the PTS (p<0.05). For both conditions the mean PTS was similar (2.07 vs. 2.11 m/s) and midstance VGRF at the PTS was only slightly greater under the weighted condition (p=0.67). While speed and midstance VGRF cannot be differentiated in their affect on the walk-run transition, the increase in midstance VGRF variability may indicate a perturbation potentially influencing the transition.