Regulation of impact forces during treadmill running

The impact maximum of the ground reaction force (GRF) has been discussed extensively in the context of running injuries while numerous footwear-related papers aimed at modifying this parameter mainly by shoe construction. Several mechanisms have been proposed but to date the regulation of impact forces in running is not completely understood. In this paper findings on impact regulation will be reviewed and a statistical approach will be used to analyze variations in contact mechanics in 10 runners. Subjects were tested during running at 4 m/s on an instrumented treadmill. Sagittal plane kinematics, rearfoot eversion, tibial acceleration, pressure under the heel inside the shoe and vertical GRF were collected simultaneously over 19 +/- 2 steps. Linear correlations were used to investigate relationships between single variables. A general regression model across the entire data set was employed and variable selection by backward elimination revealed the factors subject, minimum knee angle, rearfoot inversion at touch-down (TD) and the vertical velocity of the foot at TD as variables with the strongest contribution to the fit of the model. These four kinematic variables were re-entered in intra-individual regressions revealing an average R-2 of 0.926 and an average power of the model of over 80%. The results support previous studies that suggest kinematic parameters to be linked to impact regulation. However, each subject appears to make use of individual combinations of mechanical impact modulation factors while some variables showed inverse relationships compared to results from intervention studies. Muscular activation remains as a possible factor in the regulation of touch-down mechanics in running and may be linked to running injuries.