Ground reaction forces are often used as a primary descriptive component in the analysis of the support phase of running. The purpose of this study was to investigate the changes in ground reaction forces during the support phase of the running stride at different running speeds.

Ten skilled distance runners, who ran with a heel-toe footfall pattern at all running speeds, served as subjects. The experimental set-up consisted of a Kistler force platform interfaced to a Tektronix 4051 Graphics Calculator. Each subject performed 10 successful trials at each of 4 running speeds ranging from a sprint pace (7 m/sec) to a jogging pace (4 m/sec). Running speed was monitored by a photoelectric timing system. Subjects were allowed adequate practice to ensure consistent running speed and a "normal" footfall pattern on the force platform. Adequate time was allowed between trials and conditions to minimize any possible fatigue effects.

Data processing consisted of the evaluation of 19 variables for each of the 40 10-trial conditions for both relative and absolute curves. Consequently, 10 vertical, 5 fore/aft and 4 medial/lateral variables were used in the analysis. The relative time curves were generated using a cubic spline technique which transformed the absolute time events curves into 200 equally spaced data points to assist in making speed comparisons. Separate multivariate analyses of variance with planned orthogonal comparisons were carried out on the 3 relative and 3 absolute data sets.

Statistically significant differences were found in the relative and absolute ground reaction force curves at the different running speeds (p 7lt;​ 0.01). While events in the ground reaction force curves occurred at the same time relatively, there were magnitude changes in the forces and impulses of the curves as a direct result of the running speed. The results suggested that, although the same skill was being performed at each running speed, differences in the ground reaction force curves exist.