The stretch-shortening cycle (SSC) revisited: residual force enhancement contributes to increased performance during fast SSCs of human m. adductor pollicis

Abstract

The stretch-shortening cycle (SSC) occurs in most everyday movements, and is thought to provoke a performance enhancement of the musculoskeletal system. However, mechanisms of this performance enhancement remain a matter of debate. One proposed mechanism is associated with a stretch-induced increase in steady-state force, referred to as residual force enhancement (RFE). As yet, direct evidence relating RFE to increased force/work during SSCs is missing. Therefore, forces of electrically stimulated m. adductor pollicis (n = 14 subjects) were measured during and after pure stretch, pure shortening, and stretch-shortening contractions with varying shortening amplitudes. Active stretch (30°, ω = 161 ± 6°s(-1)) caused significant RFE (16%, P < 0.01), whereas active shortening (10°, 20°, and 30°; ω = 103 ± 3°s(-1), 152 ± 5°s(-1), and 170 ± 5°s(-1)) resulted in significant force depression (9-15%, P < 0.01). In contrast, after SSCs (that is when active stretch preceded active shortening) no force depression was found. Indeed for our specific case in which the shortening amplitude was only 1/3 of the lengthening amplitude, there was a remnant RFE (10%, P < 0.01) following the active shortening. This result indicates that the RFE generated during lengthening affected force depression when active lengthening was followed by active shortening. As conventional explanations, such as the storage and release of elastic energy, cannot explain the enhanced steady-state force after SSCs, it appears that the stretch-induced RFE is not immediately abolished during shortening and contributes to the increased force and work during SSCs.

Keywords: Concentric; eccentric; electrical stimulation; force depression; force enhancement; force redevelopment; muscle; potentiation; thumb.

Figure 1

Experimental protocol. After subject preparation, assessment of voluntary maximum force (MVC) at 20° thumb angle, and adjustments of the stimulation intensity (50–60% of MVC), the experiment was started using a pseudorandomized block design. First block: 3 pure shortening contractions beginning at 30° with shortening amplitudes of 10° (SHO-10), 20° (SHO-20) and 30° (SHO-30). Second block: 3 SSCs and corresponding isometric reference contractions at 0°, 10°, and 20°, pairwise randomized. SSCs started at 0° with 30° lengthening. In SSC-30/30, 30° lengthening was followed by 30° shortening ending at 0°. In SSC-30/20 and SSC-30/10, the 30° stretch was followed by 20° and 10° shortening, ending at 10° and 20°, respectively. Third block: pure stretch contraction (STR-30) from 0° to 30° always performed after the isometric reference contraction at 30°.

Figure 2

Typical force-time (n = 1, filtered with lowpass 10 Hz) and angle-time (reduced schematic illustration) traces of pure shortening (light blue/gray), stretch-shortening (dark blue/gray) and isometric references (dotted black lines) at final thumb angle position 0° (A, a), 10° (B, b), and 20° (C, c). Pure shortening contractions always started at a 30° thumb angle, stretch-shortening always started from 0° with 30° lengthening immediately followed by 30° (a), 20° (b), or 10° (c) of shortening. Steady-state muscle forces were measured between 2.5–3s after end of shortening and at the corresponding times for the isometric reference contractions (vertical lines). Exemplarily highlighted the force-time and angle-time traces of stretch-shortening (dark blue) versus pure shortening (light blue) with shortening amplitude of 30°.

Figure 3

Typical data (n = 1, filtered with lowpass 10 Hz) of force redevelopment after pure shortening (SHO-30, SHO-20, SHO-10) and stretch-shortening (SSC-30/30, SSC-30/20, SSC-30/10) contractions and corresponding double exponential fit (black lines, R² > 0.99). Force is normalized to the amount of force redevelopment, meaning 0 represents the force at the end of shortening and 1 corresponds to steady-state force 2.5–3 sec after shortening. SSC contractions show higher rates of fast redevelopment (k_f) and reduced half-value periods.

Figure 4

Mean forces and standard deviations, normalized to the forces obtained during purely isometric reference contractions at corresponding thumb angles, 2.5–3 sec after pure shortening (light blue), pure stretch (gray) and stretch-shortening (dark blue) contractions. Pure shortening always started from 30° thumb angle with amplitudes of 30° (SHO-30), 20° (SHO-20) and 10° (SHO-10). Pure stretch and stretch-shortening tests always started from a 0° thumb angle with lengthening of 30°. After lengthening, stretch-shortening trials were immediately followed by 30° (SSC-30/30), 20° (SSC-30/20) or 10° (SSC-30/10) shortening. All forces are significantly different to isometric reference forces (dashed line), except SSC-30/30 and SSC-30/20 (‘ns’). Brackets and asterisks (*) mark significant (P < 0.05) differences in forces after stretch-shortening compared to forces after pure shortening.

Figure 5

Zoomed-in typical force-time (n = 1, filtered with lowpass 10 Hz) and angle-time (reduced schematic illustration) traces of pure shortening (light blue), stretch-shortening (dark blue) and isometric reference contractions (dotted black line) at the final thumb angle position of 20°. Note, following muscle stretching, forces in the stretch-shortening cycles always exceed the forces of the pure shortening contractions and force redevelops to a level that is significantly greater than the isometric reference forces after the pure shortening contractions.