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Women and men present different metabolic responses to exercise, yet whether this phenomenon results from differences in fiber type (FT) composition or other sex-specific factors remains unclear. Therefore, our aim was to examine the effects of sex and FT independently on AMP-activated protein kinase (AMPK), acetyl-CoA carboxylase (ACC), Tre-2/BUB2/CDC1 domain family (TBC1D)1, and TBC1D4 in response to acute exercise. Segregated pools of myosin heavy chain (MHC) I and MHC IIa fibers were prepared from vastus lateralis biopsies of young trained men and women at rest and during recovery (0 min, 45 min, 90 min, or 180 min) from high-intensity interval exercise (6 × 1.5 min at 95% maximum oxygen uptake). In resting MHC I vs. IIa fibers, AMPKα2, AMPKγ3, and TBC1D1 were higher and TBC1D4 expression was lower in both sexes, along with higher phospho (p)-TBC1D1Ser660and lower p-TBC1D4Thr642. Women expressed higher ACC than men in MHC IIa fibers and higher AMPKβ1, AMPKβ2, TBC1D1, and TBC1D4 in both FTs. Immediately after exercise, p-AMPKαThr172increased only in MHC IIa fibers, whereas p-ACCSer221increased in both FTs, with no change in p-TBC1D1Ser660or p-TBC1D4Thr642. During recovery, delayed responses were observed for p-AMPKαThr172in MHC I (45 min), p-TBC1D4Thr642in both FTs (45 min), and p-TBC1D1Ser660(180 min). FT-specific phosphorylation responses to exercise were similar between men and women. Data indicate that sex and FT independently influence expression of AMPK and its substrates. Thus failing to account for sex or FT may reduce accuracy and precision of metabolic protein measurements and conceal key findings.NEW & NOTEWORTHY This investigation is the first to compare muscle fiber type (FT)-specific analysis of proteins between the sexes, providing comprehensive data on AMP-activated protein kinase (AMPK), acetyl-CoA carboxylase (ACC), Tre-2/BUB2/CDC1 domain family (TBC1D)1, and TBC1D4 before and in the hours following high-intensity interval exercise (HIIT). Expression and phosphorylation of specific AMPK isoforms, ACC, TBC1D1, and TBC1D4 were shown to be FT dependent, sex dependent, or both, and TBC1D1 showed an unexpected delay in FT-dependent phosphorylation in the time period following HIIT.

Human skeletal muscle fibers exist across a continuum of slow → fast-twitch. The amount of each fibertype(FT) influences muscle performance but remains largely unexplored in elite athletes, particularly from strength/power sports. To address this nescience,vastus lateralis(VL) biopsies were performed on World/Olympic (female, n=6, “WCF”) and National-caliber (female, n=9, “NCF”; and male, n=6, “NCM”) American weightlifters. Participant accolades included 3 Olympic Games, 19 World Championships, 25 National records, and >170 National/International medals. Samples were analyzed for myosin heavy chain (MHC) content via SDS-PAGE using two distinct techniques: single fiber (SF) distribution (%) and homogenate (HG) composition. These athletes displayed the highest MHC IIa concentrations ever reported in healthyVL(23±9% I, 5±3% I/IIa, 67±13% IIa, and 6±10% IIa/IIx), with WCF expressing a notable 71±17% (NCF=67±8%, NCM=63±16%). The heavyweights accounted for 91% of the MHC IIa/IIx fibers. When compared to SF, HG overestimated MHC I (23±9 vs. 31±9%) and IIx (0±0 vs. 3±6%) by misclassifying I/IIa fibers as I and IIa/IIx fibers as IIx. These findings suggest athlete caliber (World vs. National), training experience, and body mass determine FT% more than sex and refutes the common pronouncement that women possess more slow and fewer fast-twitch fibers than men. Our results also show the abundance of pure MHC IIa and rarity of IIx in elite strength/power-trained athletes, indicate a potential link between MHC IIa/IIx frequency and body mass, and question the fidelity of HG as a measure of FT% distribution. The extreme fast-twitch abundance partially explains how elite weightlifters generate high forces in rapid time-frames. These data highlight the need for more cellular and molecular muscle research on elite anaerobic athletes.