Dynamics of Step Length Adjustment During Running: A Comment on Patla, Robinson, Samways, and Armstrong (1989)

Warren, Young, and Lee (1986) found that treadmill runners control step length by regulating vertical impulse, which could be modulated by the optical time-to-contact variable tau. In contrast, Patla, Robinson, Samways, and Armstrong (1989) report a 40% contribution of horizontal impulse in overground running and a control strategy that depends on the timing of target cuing, undermining the tau-impulse theory. This discrepancy is due to differences in methodology, analysis, and interpretation. First, the cuing effect is exactly what the tau-impulse theory would predict. Second, new data on the dynamics of overground running show only a 20% contribution of horizontal impulse, which could be a biomechanical consequence of regulating vertical impulse. This is apparently due to important differences between treadmill and overground running. However, the data indicate that runners regulate a global impulse parameter that primarily affects vertical impulse, salvaging the tau-impulse theory.