Residual Force Enhancement Is Attenuated in a Shortening Magnitude-dependent Manner.

INTRODUCTION: The isometric force attained after active stretch is greater than that attained in a purely isometric contraction. This property is referred to as residual force enhancement (RFE). Although RFE is thought to contribute to the enhanced force and power in stretch-shortening cycles (SSCs), it is unclear whether shortening that occurs after active stretch eliminates the RFE induced by active stretch. Therefore, we evaluated the influence of shortening on RFE.

METHODS: Skinned rabbit soleus fibers (N = 43) were used for all tests. Residual force enhancement was compared between the stretch-only condition and the SSC condition. In the SSC conditions, shortening magnitudes were either 1% or 12.5% of fiber length. The final muscle length where RFE was evaluated was 3 μm for all trials. In the SSCs with 12.5% shortening, the isometric force before and after the SSC was also compared.

RESULTS: Residual force enhancement was similar between the stretch only (7.9% ± 2.7%) and the SSC with 1% shortening condition (7.1% ± 2.9%) (P = 0.316), whereas RFE was smaller in the SSC with 12.5% shortening (3.5% ± 2.4%) than the stretch-only condition (8.4% ± 2.5%) (P < 0.001). The isometric forces after SSCs (0.437 ± 0.103 mN) were greater than those measured before the SSC (0.422 ± 0.104 mN) (P = 0.016).

CONCLUSIONS: Residual force enhancement was preserved when the shortening magnitude was small and was reduced when the shortening magnitude was large. Although RFE was attenuated by the 12.5% shortening, RFE was still observed, suggesting that RFE can contribute to SSCs.