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 μm, and stretched by approximately 5–15 per cent SL. When stretch was performed at 1 μm s⁻¹ SL⁻¹, the transition between the two phases occurred at a critical stretch (SL_c) 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 μm 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 SL_c to 14.6±0.6 nm hs⁻¹ (n=23), suggesting that pre-power stroke cross bridges are largely responsible for stretch forces.