Eccentric training induces greater hypertrophy while causing more muscle damage than concentric training. This study examined the effects of small-range eccentric contractions (SR-ECCs) and large-range eccentric contractions (LR-ECCs) on muscle morphology, contractility, and damage in rats. Thirty male Fischer 344 rats were divided into five groups: small-range ECC single-bout (SR-ECC_SB, n = 4), large-range ECC single-bout (LR-ECC_SB, n = 4), SR-ECC intervention (SR-ECC_Intv, n = 7), LR-ECC intervention (LR-ECC_Intv, n = 8), and control (Cont, n = 7). These groups underwent transcutaneous electrical stimulation involving 80 ECCs twice a week for four weeks. The results indicated that the LR-ECC_SB group had more Evans blue dye-positive fibers than other groups. The SR-ECC_Intv group showed no increase in the mean myofiber cross-sectional area. However, Pax7+ and Ki67+ cells significantly increased in both ECC_Intv groups compared to the Cont group, and the connective tissue area was significantly greater in the LR-ECC_Intv than in others. Muscle force was lower in both ECC_Intv groups compared to the Cont group. These findings suggest that SR-ECC intervention may induce a smaller increase in the number of fibers with a large myofiber cross-sectional area and satellite cell proliferation with less muscle damage and myofibrosis compared to LR-ECCs.

Electrical stimulation-induced muscle contraction (ESMC) has demonstrated various physiological benefits, but its effects on the secretion of undercarboxylated osteocalcin (ucOC), a bone-derived cytokine, remain unclear. This study explored the relationship between ESMC, bone strain, and ucOC secretion through two experiments. In the first, young male Fischer 344 rats were divided into three groups: low-frequency ES (LF, 10 Hz), high-frequency ES (HF, 100 Hz), and control (CON). Acute 30-min transcutaneous ES was applied, and both bone strain and ucOC levels were measured. In the second experiment, rats underwent LF or HF long-term ES (two sessions per week for 4 weeks), with ucOC and insulin levels monitored. Results revealed a significant peak in ucOC at 6 h post-acute LF-ESMC. Despite HF-ESMC generating greater bone strain, LF-ESMC, with smaller but repetitive bone strain, proved more effective in stimulating ucOC secretion. In the long-term study, both ESMC groups exhibited early increases in ucOC, with a positive correlation to insulin levels. In conclusion, bone strain induced by ES-mediated muscle contraction promotes ucOC secretion, with both the magnitude and frequency of strain playing critical roles.