The effect of muscular fatigue on stride rate was observed only during the first minute of running following quadriceps fatigue. There was no effect of muscular fatigue (either quadricep or hamstring) on maximal knee flexion during the support or swing phase of running. One possible explanation for the minimal muscular fatigue effect is that the fatigue protocol was not intense enough to elicit a change in running mechanics. On average, 11.7 ±2.42 repetitions were completed during the fatigue protocol for quadriceps, vs 10.8 ±1.56 repetitions for hamstrings. It is possible that a more intense fatigue protocol utilizing both concentric and eccentric exercises may elicit different results. However, the subjects were highly motivated athletes, and each related that the exercise was intense. The increased stride rate during the first minute of running following quadriceps fatigue is evidence that the system made subtle adjustments to running mechanics to compensate for the fatigued quadriceps. During running, the quadriceps function in part to absorb the energy of ground impact through eccentric activity through midsupport (Winter, 1983). An explanation for the increased stride rate may be that adjustments were made to reduce the demand on the quadriceps. Derrick, Hamill, and Caldwell (1996) reported that the energy absorbed by the hip, knee and ankle during non-fatiged running was dependent on stride rate. Subjects ran at a constant speed, and changed stride rate to higher or lower rates compared to the preferred stride rate. It was reported that the level of impact was lower and the energy absorbed by the lower extremity was lower during the higher stride rates. The energy absorbed by each joint was dependent on stride rate relative to the preferred stride rate. At stride rates greater than preferred, most of the energy was absorbed by the knee and ankle. At stride rates lower than the preferred, most of the energy was absorbed by the knee (Derrick, et al., 1996). During non-fatigued running, Hamill, Derrick, and Holt (1995) reported that the metabolic cost of running at a particular speed was minimized when subjects ran at a preferred stride rate. Interestingly, the shock attenuation was not minimized during running at the preferred stride rate. That is, the system optimized the metabolic cost but not the mechanical cost of running. The increased stride rate during QF compared to NF running may be evidence that the system compensated for muscular fatigue by optimizing the mechanical cost of running but not the metabolic cost. This hypothesis has yet to be tested, but would appear to be tenable. Further research into the effects of muscular fatigue on running mechanics may lend insight as to how the system optimizes performance.
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