Muscle is the prime mover of animal locomotion, but muscle force is not usually equal to output force. Instead, the instantaneous ratio between output and input force (the mechanical advantage, MA) is modulated by intervening skeletal elements. Traditional functional analysis, focussing on the instantaneous action of gearing, suggests that smaller MA favours speed, while larger MA favours force. However, mounting evidence suggests that this perspective is too simplistic when considering the outcome of a dynamic muscle contraction. We approach this problem systematically by introducing an analytical framework to study the effect of gearing. This framework reveals two mechanisms by which the output speed of a contraction is affected by gearing. First, gearing can limit muscle work output, because it is bound not only by muscle displacement, but also by muscle shortening velocity. Second, it controls how each unit of muscle work is partitioned into system energies (e.g. kinetic energy vs heat). Consequently, smaller gear ratios do not always result in larger output speeds. No single optimal gear ratio exists for any organism, but rather the optimum depends on internal muscle properties and contraction dynamics, as well as external forces. The analytical framework we propose provides a blueprint to determine optimality conditions across muscle physiology, anatomy, mechanical environment and size, and so to investigate form-function relationships in musculoskeletal gearing.