Muscle fiber typology is associated with the incidence of overreaching in response to overload training.

The aim of this study was to identify markers of training stress and characteristics of middle-distance runners related to the incidence of overreaching following overload training. Twenty-four highly trained middle-distance runners [ = 16 male, peak oxygen uptake (V̇o) = 73.3 (4.3) mL·kg·min; = 8 female, V̇o = 63.2 (3.4) mL·kg·min] completed 3 wk of normal training (NormTr), 3 wk of high-volume training (HVTr; a 10%, 20%, and 30% increase in training volume each successive week from NormTr), and 1 wk of taper (TapTr; 55% exponential reduction in training volume from HVTr ). Before and immediately after each training period, an incremental treadmill-running test was performed, while resting metabolic rate (RMR), subjective fatigue responses, and various resting blood biomarkers were assessed. Muscle fiber typology of the gastrocnemius was estimated by quantification of muscle carnosine using proton magnetic resonance spectroscopy and expressed as a -score relative to a nonathlete control group. Twelve runners were classified as functionally overreached (FOR) following HVTr [decreased running time to exhaustion (TTE)], whereas the other 12 were classified as acutely fatigued (AF; no decrease in running TTE). The FOR group did not demonstrate systematic alterations in RMR, resting blood biomarkers, or submaximal exercise responses, compared with the AF group. The gastrocnemius carnosine -score was significantly higher in the FOR group (-0.44 ± 0.57) than in the AF group (-1.25 ± 0.49, = 0.004,  = 1.53) and was also negatively correlated with changes in running TTE from pre- to post-HVTr ( = -0.55, = 0.005) and from pre-HVTr to post-TapTr ( = -0.64, = 0.008). Muscle fiber typology is related to the incidence of overreaching and performance supercompensation following increased training volume and a taper. Variability in the performance responses following an overload training period and subsequent taper was associated with the variation in the muscle fiber typology of the gastrocnemius. Runners with an estimated higher proportion of type I fibers (i.e., lower carnosine -score) were able to maintain performance in response to an overload training period and subsequently achieve a superior performance supercompensation. These findings show that muscle fiber typology contributes to the variability in performance responses following training.