DOES ELASTIC ENERGY ENHANCE WORK AND EFFICIENCY IN THE STRETCH-SHORTENING CYCLE

This target article addresses the role of storage end reutilization of elastic energy in stretch-shortening cycles. It is argued that for di screte movements such as the vertical jump, elastic energy does not ex plain the work enhancement due to the prestretch. This enhancement see ms to occur because the prestretch allows muscles to develop a high le vel of active state and force before starting to shorten. For cyclic m ovements in which stretch-shortening cycles occur repetitively, some a uthors have claimed that elastic energy enhances mechanical efficiency . In the current article it is demonstrated that this claim is often b ased on disputable concepts such as the efficiency of positive work or absolute work, and it is argued that elastic energy cannot affect mec hanical efficiency simply because this energy is not related to the co nversion of metabolic energy into mechanical energy. A comparison of w ork and efficiency measures obtained at different levels of organizati on reveals that there is in fact no decisive evidence to either suppor t or reject the claim that the stretch-shortening cycle enhances muscl e efficiency. These explorations lead to the conclusion that the body of knowledge about the mechanics and energetics of the stretch-shorten ing cycle is in fact quite lean. A major challenge is to bridge the ga p between knowledge obtained at different levels of organization, with the ultimate purpose of understanding how the intrinsic properties of muscles manifest themselves under in-vivo-like conditions and how the y are exploited in whole-body activities such as running. To achieve t his purpose, a close cooperation is required between muscle physiologi sts and human movement scientists performing inverse and forward dynam ic simulation studies of whole-body exercises.