Abstract
The steady-state isometric force following active muscle shortening or lengthening is smaller (force depression; FD) or greater (residual force enhancement; RFE) than a purely isometric contraction at the corresponding length. The mechanisms behind these phenomena remain not fully understood, with few studies investigating the effects of FD and RFE in stretch-shortening cycles (SSC). The purpose of this study was to investigate the influence of RFE and peak force at the end of the stretch phase on the steady-state isometric force following shortening. Isometric thumb adduction force measurements were preceded by an isometric, a shortening contraction to induce FD, and SSCs at different stretch speeds (15°/s, 60°/s, and 120°/s). The different peak force values at the end of stretch and the different amounts of work performed during shortening did not influence the steady-state isometric force at the end of the SSC. We conclude that the FD following SSC depends exclusively on the amount of RFE established in the initial stretch phase in situations where the timing and contractile conditions of the shortening phase are kept constant .
Highlights
Residual force enhancement is defined as an increase in the steady-state isometric force following active stretching compared to the corresponding force of a purely isometric contraction at the same muscle length[1,2,3]
These observations led to the conclusion that during a stretch-shortening cycles (SSC) shortening-induced force depression does not depend on the work performed during shortening or the force prior to shortening, as has been assumed to date for pure shortening contractions, but may depend exclusively on the amount of force enhancement produced in the initial stretch phase; that is, force depression is offset by the force enhancement achieved in the stretch phase
The present study was designed to investigate the influence of residual force enhancement (RFE) and peak force at the end of stretching on the steady-state isometric force following shortening preceded by different stretch speeds
Summary
Residual force enhancement is defined as an increase in the steady-state isometric force following active stretching compared to the corresponding force of a purely isometric contraction at the same muscle length[1,2,3]. Force depression is defined as a decrease in the steady-state isometric force following active shortening compared to the purely isometric force at the corresponding muscle length[1,14,15]. Marechal and Plaghki (1979) proposed that FD is associated with a reduction of the proportion of attached cross-bridges because of a stress-induced deformation of the actin filaments entering the myofilament overlap zone in the shortening phase This theory is consistent with the reduction in muscle stiffness observed in the FD state compared to purely isometric reference contractions[18], and the maintenance of ATP consumption per unit of force in the FD state[19]. Fortuna et al showed that stretch induced RFE preceding shortening affects the steady-state isometric force (FD) in a time- and speed-dependent manner[21]. These observations led to the conclusion that during a SSC shortening-induced force depression does not depend on the work performed during shortening or the force prior to shortening, as has been assumed to date for pure shortening contractions, but may depend exclusively on the amount of force enhancement produced in the initial stretch phase; that is, force depression is offset by the force enhancement achieved in the stretch phase
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