The purpose of this study was to evaluate the relationship between force and stiffness following stretch of activated fibers, while simultaneously changing contractility by interfering with the cross-bridge kinetics and muscle activation. Single fibers dissected from lumbrical muscles of frogs were placed at a length 20% longer than the plateau of the force-length relationship, activated, and stretched by 5% and 10% of fiber length (speed: 40% fiber length/s). Experiments were conducted with maximal and sub-maximal stimulation in Ringer solution, and with the addition of 2 mM and 5 mM of the myosin inhibitor 2,3-Butanodione monoxime (BDM) to the solution. The steady-state force following stretch of an activated fiber was higher than the isometric force produced at the corresponding length in all conditions investigated. Lowering the frequency of stimulation decreased the force and stiffness during isometric contractions, but did not change force enhancement and stiffness enhancement following stretch. Administration of BDM decreased the force and stiffness during isometric contractions, but increased the force enhancement and stiffness enhancement following stretch. The relationship between force enhancement and stiffness suggests that the increase in force following stretch may be caused by an increase in the proportion of cross-bridges attached to actin. Since BDM places cross-bridges in a weakly-bound, pre powerstroke state, our results further suggest that force enhancement is partially associated with a recruitment of weakly-bound cross-bridges into a strongly-bound state.