Force depression following a stretch-shortening cycle is independent of stretch peak force and work performed during shortening

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 .

Figure 1

Experimental design of the study. Following participant preparation, a maximum voluntary contraction (MVC) was performed at a 30° thumb abduction angle and the stimulation intensity was then adjusted to produce 50–60% of the MVC force. Next, a force depression (FD) test at 60°/s shortening velocity and a corresponding isometric reference contraction at 0° were performed. Stretch-shortening cycles with varying stretch velocities (15°/s, 60°/s, 120°/s) and a given shortening velocity (60°/s) and shortening magnitude (30°) followed in a randomized order. Then, an isometric reference contraction at 30°, followed by the randomized RFE tests at varying stretch velocities (15°/s, 60°/s, 120°/s) were performed. Last, an isometric reference contraction at 0° was performed to assess muscle fatigue.

Figure 2

Thumb adduction force (top) and metacarpophalangeal joint angle (bottom) as a function of time for an isometric reference contraction (solid line, Ref_Isometric), a pure shortening-induced force depression (dashed line, FD_60°/s) and a stretch-shortening-cycle at 120°/s (dotted line, SSC_120°/s). The force before stretching (b-STR; •), the maximum force at the end of the stretch (e-STR; ▲), the minimum force at end of shortening (e-SHO; ■) and the average isometric steady-state force (500 ms) prior to muscle deactivation were assessed.

Figure 3

Mean (±SE) values of peak force at the end of the stretch for the SSCs performed at different stretching speeds (SSC_15°/s, SSC_60°/s, and SSC_120°/s). There was a significant increase in peak force for increasing stretching speeds (p < 0.001).

Figure 4

Mean (±SE) values of minimum force (N) for the pure FD test and the SSC tests performed at different stretching speeds (SSC_15°/s, SSC_60°/s, and SSC_120°/s). There was a significant difference in the minimum force for pure FD compared to all SSCs, but there was no significant difference across the SSCs. (*Compared to SSCs; p < 0.001).

Figure 5

Mean (±SE) values of work performed during shortening for the pure FD test and the SSC tests performed at different stretching speeds (SSC_15°/s, SSC_60°/s, and SSC_120°/s). The work performed during shortening for all SSC was significantly greater compared to that measured for the pure FD test. (*Compared to all SSC; p < 0.001).

Figure 6

Mean (±SE) values of residual force enhancement for the varying stretching speeds (RFE_15°/s, RFE_60°/s and RFE_120°/s) normalized to the values of the isometric reference contraction at the corresponding thumb angle. Residual force enhancement values were similar (p > 0.05) across the different stretch speeds (17.7% ± 1.7%, 16.2% ± 2.2%, 17.9% ± 3.1% for RFE_15°/s, RFE_60°/s, and RFE_120°/s, respectively).

Figure 7

Mean (±SE) values of force depression for pure FD tests (FD_60°/s) and for SSC tests performed at 15°/s, 60°/s and 120°/s (SSC_15°/s, SSC_60°/s, SSC_120°/s, respectively) normalized to the values of the isometric reference contraction at the corresponding thumb angle. Force depression without prior stretching was 25.9% ± 2.5% and was significantly greater compared to all SSCs (18.8% ± 2.0%, 17.4% ± 1.9%, 18.1% ± 2.4% for SSC_15°/s, SSC_60°/s, and SSC_120°/s, respectively). However, no significant difference was found between SSCs. (*Compared to all SSC; p < 0.05).