Fast stretching of skeletal muscle fibres abolishes residual force enhancement.

The steady-state isometric force of a muscle after active stretching is greater than the steady-state force for a purely isometric contraction at the same length and activation level. The mechanisms underlying this property, termed residual force enhancement (rFE), remain unknown. When myofibrils are actively stretched while cross-bridge cycling is inhibited, rFE is substantially reduced, suggesting that cross-bridge cycling is essential to produce rFE. Our purpose was to further investigate the role of cross-bridge cycling in rFE by investigating whether fast stretching that causes cross-bridge slipping is associated with a loss of rFE. Skinned fibre bundles from rabbit psoas muscles were stretched slowly (0.08 µm s-1) or rapidly (800 µm s-1) while activated, from an average sarcomere length of 2.4 to 3.2 µm. Force was enhanced by 38±4% (mean±s.e.m) after the slow stretches but was not enhanced after the fast stretches, suggesting that proper cross-bridge cycling is required to produce rFE.