Residual Force Enhancement is Attenuated by Shortening in a Magnitude-Dependent Manner

Abstract

The isometric force attained after active stretch is greater than that attained in a purely isometric contraction at the corresponding muscle length. This property is referred to as residual force enhancement (RFE). Although RFE is considered to contribute to the force/work enhancement in stretch-shortening cycles, it is unclear whether shortening that occurs after active stretch eliminates the influence of RFE. Therefore, we evaluated the influence of shortening on the magnitude of RFE. Skinned fibers of rabbit soleus (N = 45) were used for all experiments. The first test consisted of an isometric reference contraction at an average sarcomere length of 3.0 μm. The second test consisted of a typical way to induce RFE. This was achieved by active stretching of single fibres from an average sarcomere length of 2.7 to 3.0 μm. The third test consisted of a stretch shortening cycle. RFE was induced by active stretch from 3.0 to 3.3μm. Immediately after stretching, fibers were shortened to 3.0 μm (12.5% of the fiber length) in 0.5 ms. Force attained 15 s after the active stretch was used to evaluate the magnitude of RFE. Compared to the isometric reference condition, force was significantly greater in the normal RFE condition (108.4 ± 2.5%) and in the stretch-shortening cycle condition (RFE 103.5 ± 2.4%) (p< 0.001). This difference in the magnitude of RFE disappeared when the magnitude of shortening was decreased from 12.5% to 1% of the fiber length (p = 0.316). In conclusion, RFE is attenuated by shortening following active stretching. This attenuation effect depends on the magnitude of shortening and decreases with decreasing shortening magnitudes. The molecular events underlying these history-dependent force effects remain unknown.