Sex-based comparisons of muscle cellular adaptations after 10 weeks of progressive resistance training in middle-aged adults

Resistance training combined with adequate protein intake supports skeletal muscle strength and hypertrophy. These adapta-tions are supported by the action of muscle stem cells (MuSCs), which are regulated, in part, by fibro-adipogenic progenitors (FAPs) and circulating factors delivered through capillaries. It is unclear if middle-aged males and females have similar adapta-tions to resistance training at the cellular level. To address this gap, 27 (13 males, 14 females) middle-aged (40-64 yr) adults par-ticipated in 10 wk of whole body resistance training with dietary counseling. Muscle biopsies were collected from the vastus lateralis pre-and posttraining. Type II fiber cross-sectional area increased similarly with training in both sexes (P = 0.014). MuSC content was not altered with training; however, training increased PDGFRa + /CD90+ FAP content (P < 0.0001) and reduced PDGFRa + /CD90- FAP content (P = 0.044), independent of sex. The number of CD31+ capillaries per fiber also increased simi-larly in both sexes (P < 0.05). These results suggest that muscle fiber hypertrophy, stem/progenitor cell, and capillary adapta-tions are similar between middle-aged males and females in response to whole body resistance training.