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The aim of this study was to determine if muscle oxidative capacity is influenced by alterations in training volume in middle-distance runners.Twenty-four highly-trained middle-distance runners (n=16 male; VO2peak=73.3(4.3) ml·kg·min-1; n=8 female, VO2peak=63.2(3.4) ml·kg·min-1) completed 3 weeks of normal training (NormTr), 3 weeks of high-volume training (HVTr; a 10, 20 and 30% increase in training volume during each successive week from NormTr), and a 1-week taper (TapTr; 55% exponential reduction in training volume from HVTr week 3). Before, and immediately after each training period, the rate of recovery of muscle oxygen consumption (mVO2) of the gastrocnemius medalis was measured using near-infrared spectroscopy, with the rate constant indicating muscle oxidative capacity. Time to exhaustion (TTE) and VO2peak were determined during a maximal incremental treadmill test.Twelve subjects were classified as being functionally overreached (FOR) following HVTr (decreased running TTE and high perceived fatigue), whereas the other twelve subjects were classified as acutely fatigued (AF; no decrease in running TTE). The AF group demonstrated a significant increase in muscle oxidative capacity following HVTr (rate constant: 15.1% (SD; 9.7%) min-1; p=0.009), with no further improvement following TapTr, while there was no change in muscle oxidative capacity for FOR at any time point (p>0.05). Compared to the FOR group, the AF group had substantially larger improvements in TTE from pre-HVTr to post-TapTr (FOR: 8.8% (3.7%); AF: 3.2% (3.0%); p=0.04).The present study was able to demonstrate that muscle oxidative capacity was increased in response to a period of HVTr, but only in runners who did not develop FOR. Furthermore, runners who did not develop FOR had substantially larger performance improvements following a taper period.