Effect of Running Speed on Achilles Tendon Injury Potential

Abstract

: Previous literature has suggested that increasing running speed is associated with a reduction in cumulative load per distance traveled and thereby a decrease in injury risk. However, the tissue damage incurred by a bout of loading is a function of its stress-life (S-N) behavior which describes the exponential relationship between loading magnitude and cycles to failure. Current cumulative loading measures do not consider the S-N relationship which may in fact lead to erroneous conclusions about tissue damage and injury risk. PURPOSE: The purpose of this study was to examine the influence of running speed on an S-N weighted impulse measure at the Achilles tendon. METHODS: Ten participants ran overground at 2.5, 3.5, and 4.5 m/s while force and motion capture data were recorded. An inverse dynamics analysis was used to calculate joint moments and AT force was calculated from the ankle joint moment and the AT moment arm which was a function of ankle joint angle. AT impulse per step was calculated as the time integral of the AT force curve. A weighted impulse measure was quantified where AT force was raised to the power of 9. This value was derived from in vitro tendon testing and describes the slope of the S-N tendon curve. Impulse and weighted impulse per km were calculated as the respective impulse per step measures multiplied by the number of steps necessary to run 1km. A Friedman test examined the main effect of speed followed by Bonferroni adjusted pair-wise comparisons. RESULTS: A significant main effect of speed was observed for impulse per step (p=0.003), impulse per km (p<0.001) and weighted impulse per km (p<0.001). In general, the impulse per step and impulse per km decreased with running speed (p≤0.013); however, no significant differences were observed between 3.5 and 2.5 m/s (p≥0.047). On the other hand, the weighted impulse per km increased with running speed (p=0.005). CONCLUSION: Using a traditional measure of cumulative impulse suggests that running faster may decrease the risk of Achilles tendon injury. This counterintuitive conclusion is not reached when using a weighted impulse measure that considers the S-N behavior of the tendon.