Change of required foot impact position during running to exhaustion on a treadmill

In healthy human subjects running on a treadmill, measurements were made to determine how rapidly and precisely they were able to adjust the impact position of their feet using visual clues. Foot impact position was measured with force transducers at each corner of the treadmill. Required longitudinal positions were indicated by three lines of light ("target-lines"), projected transversely across the treadmill plate. Test runs were conducted until exhaustion at speeds exceeding the aerobic threshold (mean endurance time about 9 min; mean belt speed 12 km/h, mean inclination 6.9%). During running, required foot positions were, at irregular intervals and during periods of 7 steps each, temporarily switched from the most posterior target-line (TL3) to one of the two more anterior ones (TL2 or TL1). The first change of foot impact position took place within about 175-225 ms after a switch from TL3 to TL2/TL1; this initial small change was in a direction opposite to that required. The total adjustment of foot impact position, as seen during 7 consecutive test steps, took place during at least 2-3 steps (0.8-1.2 s). Towards the end of the test run, with exhaustion approaching, the adjustment process became somewhat slower and showed an increasing lengthwise deficit. Our results demonstrate the existence of prolonged multi-step processes of motor adjustment during treadmill running plus a sensitivity of the foot placement process to time-on-task (when approaching exhaustion?).