Background High-impact loads have been linked with running injuries. Fatigue has been proposed to increase impact loads, but this relationship has not been rigorously examined, including the associated role of muscle strength, power, and endurance. Purpose This study aimed to investigate the effect of fatigue on impact loading in runners and the role of muscle function in mediating changes in impact loading with fatigue. Methods Twenty-eight trained endurance runners performed a fixed-intensity time to exhaustion test at 85% of VO2max. Tibial accelerations were measured using leg-mounted inertial measurement units and sampled every minute until volitional exhaustion. Tests of lower limb muscle strength, power, and endurance included maximal isometric strength (soleus, knee extensors, and knee flexors), single leg hop for distance, and the one leg rise test. Changes in peak tibial acceleration (PTA, g) were compared between time points throughout the run (0%, 25%, 50%, 75%, and 100%). Associations between the change in PTA and lower limb muscle function tests were assessed (Spearman`s rho [rs]). Results PTA increased over the duration of the fatiguing run. Compared with baseline (0%) (mean ± SD, 9.1g ± 1.6g), there was a significant increase at 75% (9.9g ± 1.7g, P = 0.001) and 100% (10.1g ± 1.8g, P < 0.001), with no change at 25% (9.6g ± 1.6g, P = 0.142) or 50% (9.7g ± 1.7g, P = 0.053). Relationships between change in PTA and muscle function tests were weak and not statistically significant (rs = -0.153 to 0.142, all P > 0.05). Conclusions Peak axial tibial acceleration increased throughout a fixed-intensity run to exhaustion. The change in PTA was not related to performance in lower limb muscle function tests.
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