When skeletal and cardiac muscle contract they generate movement through the actin-myosin ATPase reaction. However, the chemical reaction underlying movement remains unclear. Specifically, little is known about how the forces generated by muscle influence the chemistry of the force-generating reaction. We measured the effects of mechanical forces on the actin-myosin ATPase activity using a standard motility assay and analysis for skeletal and cardiac muscle myosin. A mechanical load was imposed on a sliding actin filaments using alpha-actinin, and the corresponding effects of load were determined using a novel experimental and theoretical protocol. Our results show that mechanical loads decrease both actin-myosin attachment and detachment kinetics during actin sliding in skeletal and cardiac muscle, providing further insight into the mechanics of muscle contraction.