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Research on whether muscular lower limb fatigue causes higher impact forces is equivocal. If lower limb fatigue increases vertical ground reaction forces during running, the athlete may be predisposed to injury. This information may help personal trainers make recommendations on whether it is best to run before or after activities that fatigue the lower limbs. PURPOSE: To determine if muscular lower limb fatigue affects vertical ground reaction forces during running. METHODS: Ten healthy, habitually active subjects aged 18-46y volunteered for the study. Each subject was instructed to jog at a medium fast constant speed approximately 6 m over two force plates (AMTI, 1500 Hz). GRFs were recorded during left foot contact, both before and after an exhaustive treadmill run determined by reaching an RPE of 15 on a 20-point scale and maintaining that pace for at least 30 seconds. Peak vertical impact forces for two to three trials were averaged for each condition and a dependent t-test (SPSS v.22) was used to analyze the mean difference in pre-and-post fatigue GRFs. Repeated measures ANOVA was used to graph within-subject differences due to the intervention. RESULTS: Mean GRF values for pre & post fatigue were 1518.95 +/- 406.23 N and 1503.39 +/- 401.34 N, respectively, which were not significantly different (p>0.05). CONCLUSION: No significant difference in peak vertical impact forces pre-and-post fatigue was observed. Future research should use a standardized fatigue protocol perhaps based on lactate levels or predicted max HR, and multiple trials across multiple days.

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