Time-of-day, satellite cells, and velocity collectively influence ex vivo isovelocity force production in mouse...

Skeletal muscles are exquisitely designed to produce force that facilitate movement. Circadian "molecular clocks" residing in muscle play a role in regulating force production with muscle stem cell (satellite cells, SC) molecular clocks modulating isometric and eccentric force according to time-of-day. However, many tasks of daily living and exercise (i.e., walking/running) involve force and power produced during muscle shortening. Thus, the purpose of this study was to determine whether isovelocity forces and power are also modulated by SCs according to time-of-day. Using previously published samples (a mouse model capable of SC ablation), we evaluated isovelocity forces across a range of velocities (1-11 L_f/s) at two different times of day ZT1, ZT9 in the presence and absence of SCs. The main finding of this investigation was that isovelocity force production is regulated by a third-order interaction effect between time-of-day × SCs × velocity (p < 0.001). Additionally, a significant effect of time-of-day was observed for isovelocity force and power when comparing ZT1 vs. ZT9 SC⁺ mice whereas this effect was absent in SC^- animals. These results suggest SCs harbor a time-of-day and velocity dependent effect on isovelocity force production and power. Further work is required to elucidate the underlying mechanisms of this phenomenon.