Consecutive SSCs increase the SSC effect in skinned rat muscle fibres.

Abstract

Muscle function is essential for generating force and movement, with stretch-shortening cycles (SSCs) playing a fundamental role in the economy of everyday locomotion. Compared with pure shortening contractions, the SSC effect is characterised by increased force, work, and power output during the SSC shortening phase. Few studies have investigated whether SSC effects transfer across consecutive SSCs. Therefore, we investigated SSC effects over three consecutive SSCs in skinned rat muscle fibres by analysing the isometric force immediately before stretch onset (F_onset), the peak force at the end of stretching (F_peak), and the minimum force at the end of shortening (F_min), along with mechanical (Work_SSC) and shortening work (Work_SHO) at different activation levels (20%, 60%, and 100%). Each SSC was followed by an isometric hold phase, allowing force to return to a steady state. Results indicated an increase in both F_peak (20.3%) and Work_SSC (60.9%) from SSC1 to SSC3 across all activation levels tested. At 20% and 60% activation, F_onset, F_min, and Work_SHO increased (range: 4.5-28.5%) from SSC1 to SSC3. However, at 100% activation, F_onset and Work_SHO remained unchanged, while F_min decreased (- 8.5%) from SSC1 to SSC3. These results suggest that the increase in SSC effects at submaximal activation may be primarily due to increased cross-bridge forces. The absence of increases in F_onset, F_min, and Work_SHO at 100% activation suggests that increases in F_peak and Work_SSC may not be attributed to increased cross-bridge force but could instead be caused by additional effects, possibly involving modulation of non-cross-bridge structures, likely titin, and their stiffness.