A modified mechanical overload model for inducing hypertrophy in slow- and fast-twitch skeletal muscles in mice

Mechanical loading induces skeletal muscle hypertrophy, however the underlying molecular mechanisms remain incompletely understood. In rodents, synergist ablation (SA) is widely used to induce hypertrophy, yet most studies focus on fast-twitch muscles, such as the plantaris (PL), because traditional ablation causes severe inflammation in slow-twitch muscles like the soleus (SOL), complicating data interpretation. Recent studies have highlighted the distinct responses of slow- and fast-twitch muscles to mechanical overload. In this study, we developed a refined rodent model of mechanical overload-induced hypertrophy targeting both slow (SOL) and fast (PL) muscles. We compared this modified partial synergist ablation (PSA) method with conventional SA and tenotomy (TT) regarding muscle hypertrophy, fiber type composition, inflammation, and regeneration. C57BL6/J male mice (16 weeks old, n = 18) were assigned to one of three surgical groups: SA, TT, and PSA. In all groups, the relative SOL weight in overloaded legs significantly increased compared to the contralateral control legs (SA: 137%; TT: 118%; PSA: 120%). Similarly, PL weight increased (SA: 145%; TT: 134%; PSA: 115%). Notably, inflammation and muscle regeneration were observed in the SOL of SA and TT groups but not in PSA group. The ratio of central nuclei to subsarcolemmal nuclei among EdU⁺/PCM1⁺ nuclei were much higher in SOL compared to those in PL among three models, suggesting distinct modality of hypertrophy in slow- and fast-muscles. These findings indicate that PSA provides a minimal inflammatory and effective approach for inducing hypertrophy in both fast and slow muscles, making it a valuable model for studying muscle adaptation.