BACKGROUND: Muscle-tendon stiffness has been previously studied in association with muscle performance, joint stability, and risks of injury. Given the role of leg stiffness for force absorption during running, it is possible that muscle fatigue could induce changes in leg stiffness thereby reflecting a change in injury risk potential. The purpose of this study was to examine changes in single (SL) and double (DL) leg vertical stiffness during a nine-mile training run.METHODS: Eight healthy runners (4 females), void of current lower extremity injuries, had recently completed a half/full marathon, and were currently training one distance run (≥9mi) per week completed a 9-mile run. The run was split into three 3-mile loops. SL and DL vertical leg stiffness was assessed at baseline, and after competing miles 3, 6, and 9 by having participants hop in place at self-selected frequencies while vertical ground reaction forces (vGRF) were recorded. Three outcome measures were computed from the vGRF, vertical leg stiffness, hop frequency, and vertical total body center of mass (vTBCM) displacement. Separate hop mode (SL, DL) by time analyses of variance were conducted. Significant time effects were examined using post hoc trend analyses.RESULTS: Relative to baseline, the training run prompted a significant 16.5% (2.2±3.0 kN/m) increase in vertical leg stiffness (P=.045) with a concurrent significant 18.9% (-.036±.025m) decrease in vTBCM displacement (P=.014). Only linear trends were statistically significant for both vertical leg stiffness (P=.046, d=.683) and vTBCM displacement (P=.009, d=1.1). While the training run induced significant increases in hop frequency (DL:.51±.34Hz, SL:.25±.35Hz), with both DL (P=.003, d=.1.4) and SL (P=.038, d=.80) showing significant linear increases, the effect on DL hop frequency was significantly greater than SL (P=.020, d=1.0). While DL vertical leg stiffness was significantly greater than SL (P=.002, d=.92), vTBCM displacement (P=.002, d=.64) was significantly greater for SL than DL. CONCLUSIONS: These study results demonstrate that the increase in vertical leg stiffness induced with the training run can be explained by an increase in hop frequency and concurrent decrease in vTBCM displacement. It is likely that the increased stiffness is secondary to changes associated with muscle fatigue that occurs during a long-distance run. Future studies should examine which muscles are responsible for the stiffness changes identified in the current study.

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