Residual Force Enhancement Is Preserved for Conditions of Reduced Contractile Force

The isometric muscle 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). Because RFE is thought to be caused by a titin-based passive force, it should be preserved in reduced contractile force states. Therefore, we evaluated the magnitude of RFE in normal and reduced contractile force states.

Skinned fibers of rabbit psoas and soleus (N = 60) were used in all experiments. Reduced contractile force states were induced (i) by using a low Ca concentration (N = 30), (ii) by adding 20 mM butanedione monoxime (N = 15), and (iii) by lowering the pH level (N = 15). Force enhancement and reference isometric tests were conducted for each condition. In the force enhancement tests, fibers were actively stretched from an average sarcomere length of 2.4 to 3.0 μm. The isometric force attained 15 s after the end of stretching was used for analysis. In the isometric reference tests, fibers were activated isometrically at an average sarcomere length of 3.0 μm, and the force at steady state was used for analysis. The absolute and relative magnitudes of RFE were calculated.

The absolute RFE was the same for the normal and reduced contractile force states. Because the isometric reference force was smaller in the reduced contractile force states, the relative RFE was greater in the reduced contractile force than the normal states for all conditions.

RFE was preserved in the reduced contractile force states.