Wakeling et al. 30 suggested the findings are the result of a change in the motor unit recruitment pattern during sustained submaximal activity, specifically that runners may increase recruitment of fast-twitch fibers and de-recruit slow-twitch muscle fibers

in a time-dependent manner in order to generate the power output necessary to maintain a constant speed. In the present study, each trial in which the runner changed foot-strike pattern from an FFS in the pre-run condition to an MFS in the post-run condition (P3 in the minimalist shoe type; P1 and P4 in the traditional shoe type) was accompanied by a trend toward an increase in median frequency of the medial selleck compound gastrocnemius after the 50-km run. This finding was consistent with the fatigue pattern demonstrated in the

experiment of Wakeling et al., 30 suggesting that a similar change in motor unit recruitment pattern may be contributing to the change in median frequency observed in the present study. Thus, muscle fatigue of the gastrocnemius, as well greater muscle damage, as observed as significantly greater CPK values among non-RFS runners than RFS runners after a 161-km ultramarathon, may contribute to the change of foot-strike pattern in long-distance runners. However, further investigation CDK inhibitor is warranted to support this theory, as well as alternative explanations that may contribute to the findings in the present study. Our final hypothesis, i.e., that step rate would increase and step length would decrease in the post-run condition in both shoe types was consistent with our findings, as well as with previous studies, namely a high-intensity, relatively short distance fatigue protocol,10 a marathon distance,17 and ultramarathon distances.11 and 14 As expected, the completion of a 50-km run resulted in a significant increase

in RPE between pre- and post-run conditions, consistent with a previous ultramarathon distance study of Martin et al.31 Of the four runners that subjectively identified post-run gastrocnemius fatigue (1 runner in both shoe types, 1 runner in only the minimalist shoe type, and 1 runner in only the traditional shoe type), two runners through demonstrated an increased median frequency and altered the initial contact area from lateral forefoot to lateral midfoot. The other two runners that subjectively identified post-run gastrocnemius fatigue demonstrated a decreased median frequency and did not alter initial contact area between lateral forefoot and lateral midfoot. In addition, heart rate elevations were observed consistently between pre- and post-run conditions, within expectations for an endurance-type event. Heart rate was between 116 and 150, or 59%–77% of estimated maximum heart rate, as determined by 220 − age. Of note, each runner experienced a reduction in body mass over the 50-km run for each trial, between 0.4 kg (0.6%) and 3.6 kg (4.