PURPOSE: The objectives were to determine the satellite cell (SC) response among endurance-trained cyclists (n=8; VO2max: 63.1 ± 8.4 mL/Kg/min)to a period of intensified training (ICT) (10 days) and 10 days of recovery (RVT), with and without protein supplementation. METHODS: Muscle biopsies were obtained from the vastus lateralis prior to- and immediately following ICT and RVT. Fluorescent microscopy was used to analyze SCs and myosin heavy chain I and IIa (MHC I and IIa). Data were analyzed using magnitude-based inferences. RESULTS: MHC I SCs were exceptionally abundant at baseline (38 ± 20 SCs/100 fibers). MHC I SC count decreased (unclear) from PreCHO to ICTCHO and then likely increased by 60 ± 64 percent following RecCHO , with no other differences in SC content regardless of nutrition or training phase. Myonuclear content of MHC I fibers most likely increased by 16 ± 6 percent from baseline to ICTCHO and likely remained higher (17 ± 15 percent) than baseline following recovery. Likewise, MHC IIa myonuclear content likely increased 14 ± 14 percent from PreCHO to RecCHO. Though there were no changes in fiber size (cross sectional area) under CHO conditions, MHC I fiber size very likely increased by 14 ± 8 percent and MHC IIa fiber size likely increased by 16 ± 19 percent with PRO supplementation. CONCLUSIONS: Trained endurance cyclists possess a relatively large pool of SCs that appeared to facilitate measurable myonuclear accretion with heavy training under carbohydrate conditions. . Also, based on the muscle fiber hypertrophy and lack of other apparent physiological changes, PRO supplementation appeared to benefit skeletal muscle. These data strengthen the growing body of evidence demonstrating the non-hypertrophic role of SCs in skeletal muscle
