Eccentric Overload during Resistance Exercise: A Stimulus for Enhanced Satellite Cell Activation.

Satellite cells (SC) are of importance for muscular adaptation to various forms of exercise. A single bout of high-force eccentric exercise has been shown to induce SC activation and, for electrically stimulated exercise, SC differentiation. PURPOSE This study aimed to assess if one bout of concentric/eccentric exercise with damaging eccentric overload (CON/ECC+) provides a sufficient stimulus to induce SC activation, proliferation and differentiation. METHODS Biopsies from the vastus lateralis muscle of recreationally active males were obtained in the rested condition and again from the contralateral leg seven days after exhaustive concentric/eccentric (CON/ECC, n = 15) or CON/ECC+ (n = 15) leg extension exercise and in a non-exercising control group (CG, n = 10). Total SC number (Pax7+), activated (Pax7+/MyoD+), and differentiating (myogenin+) SCs, fiber type distribution, and myofibers expressing neonatal myosin heavy chain (MHCneo) were determined immunohistochemically. Creatine kinase (CK) and myoglobin were measured in venous blood. Isokinetic strength tests were repeatedly conducted. RESULTS Significant increases in CK and myoglobin (p = 0.001) indicated myofiber damage while maximal strength was not impaired. Only after CON/ECC+, SC content (p = 0.019) and SC related to type II fibers (p = 0.011) were significantly increased. A significant increase in the proportion of activated SCs occurred after CON/ECC+ only (p = 0.003), the increase being significantly (p < 0.05) different from the changes after CON/ECC and in CG. The number of differentiating SC and MHCneo remained unchanged. CONCLUSION Eccentric overload during leg extension exercise induced significant SC activation, increases in SC content and in SC number related to type II myofibers. However, there were no signs of increased SC differentiation or formation of new myofibers.