Contractile properties differ between skeletal, cardiac and smooth muscles as well as between various skeletal muscle fiber types. This functional diversity is thought to be mainly related to different speeds of myosin head pulling cycles, with the molecular mechanism of force generation being essentially the same. In this study, force-generating attachments of myosin heads were investigated by applying small perturbations of myosin head pulling cycles in stepwise stretch experiments on skeletal muscle fibers of different type. Slow fibers (frog tonic and rat slow-twitch) exhibited only a 'slow-type' of myosin head attachment over the entire activation range, while fast fibers (frog and rat fast-twitch) displayed a 'slow-type' of myosin head attachment at low levels of activation, and an up to 30-times faster type at high levels of activation. These observations indicate that there are qualitative differences between the mechanisms of myosin head attachment in slow and fast vertebrate skeletal muscle fibers. (C) 2009 Elsevier Inc. All rights reserved.