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

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.

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.

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).

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.