Repeated stretch-shortening cycles enhance force and work despite brief deactivation between cycles

Abstract

Stretch-shortening cycles (SSCs) enhance force and mechanical work compared with pure shortening contractions and are fundamental to locomotion. Although SSC mechanics are well studied, it remains unclear whether mechanical cycle-to-cycle carryover effects persist across repeated SSCs despite brief deactivation or inhibition of cross-bridge (XB) force production. To address this, single-skinned fibres from rat extensor digitorum longus muscles (n=21) underwent three consecutive SSCs at a slow velocity (1% maximal shortening velocity). Fibres were activated isometrically for 20s before SSC1, whereas SSC2 and SSC3 were each preceded by 4s of deactivation followed by 20s of isometric activation. Experiments were performed under control conditions and with Blebbistatin to inhibit XB force production. Repeated SSCs produced progressive performance enhancements under both conditions despite brief deactivation. From SSC1 to SSC3, peak force (Fpeak) increased by 14.6% and total SSC work (WorkSSC) by 50.1% under control conditions, and by 37.5% and 67.6%, respectively, with Blebbistatin. Increased short-range stiffness under control conditions suggests XB-mediated stretch resistance, whereas persistent increases during XB inhibition indicate contributions from non-XB structures, particularly titin. These findings support a sarcomeric memory effect that enhances stretch resistance and work during repeated SSCs.