Pre-power stroke cross bridges contribute to force during stretch of skeletal muscle myofibrils

When activated skeletal muscle is stretched, force increases in two phases. This study tested the hypothesis that the increase in stretch force during the first phase is produced by pre-power stroke cross bridges. Myofibrils were activated in sarcomere lengths (SLs) between 2.2 and 2.5 mu m, and stretched by approximately 5-15 per cent SL. When stretch was performed at 1 mu m s(-1) SL-1, the transition between the two phases occurred at a critical stretch (SLc) of 8.4 +/- 0.85 nm half-sarcomere (hs)(-1) and the force (critical force; F-c) was 1.62 +/- 0.24 times the isometric force (n = 23). At stretches performed at a similar velocity (1 mu m s(-1) SL-1), 2,3-butanedione monoxime (BDM; 1 mM) that biases cross bridges into pre-power stroke states decreased the isometric force to 21.45 +/- 9.22 per cent, but increased the relative F-c to 2.35 +/- 0.34 times the isometric force and increased the SLc to 14.6 +/- 0.6 nm hs(-1) (n = 23), suggesting that pre-power stroke cross bridges are largely responsible for stretch forces.