Muscle force, work, and power output during active muscle shortening are increased immediately following active muscle lengthening. This increase in performance is known as the stretch-shortening cycle- (SSC) effect. Mechanisms of the SSC-Effect include neuromuscular pre-activation, stretch-reflex contributions, and recoil of elastic energy stored in tendons (Fig. 1). In addition, actual experimental studies demonstrated that the SSC-effect is present in the sarcomere itself. More recently, it has been suggested that cross-bridge kinetics and non-cross-bridge viscoelastic elements (e.g., titin) contribute to the SSC-effect. As cross-bridges and non-cross-bridge structures are characterized by a velocity dependence, the impact of stretch-shortening velocity on the SSC-effect is of particular interest.
In a project funded by the German Research Foundation (DFG), the team around Prof. Siebert, an associate member of the Cluster of Excellence SimTech from the Institute for Sports and Movement Science at the University of Stuttgart, in cooperation with Prof. Hahn (University of Bochum) and Prof. Seiberl (Bundeswehr University Munich), has now been able to show that the SSC-effect in skinned fibers increases with SSC velocity. For this purpose, Dr. André Tomalka conducted in vitro isovelocity ramp experiments using single skinned fibers of rat soleus muscles. By using a cross-bridge inhibitor (Blebbistatin), active cross-bridge contributions could be isolated from passive non-cross-bridge contributions.
The experiments yielded the following main results: Energy recovery is enhanced with cross-bridge-inhibition and increased with increasing velocity. This enhancement can be explained by the viscoelastic properties of the non-cross-bridge structure titin. Thus, our experimental findings suggest that the energy stored in titin during the eccentric phase contributes to the SSC-effect in a velocity-dependent manner. Consequently, we show that the SSC-effect is velocity-dependent – since power output increases with increasing SSC-velocity.
This SSC-effect study promotes a basic understanding of human locomotion since SSCs are part of the most basic, everyday type of muscle contraction. The separation of cross-bridge and non-cross-bridge structures is of primary importance to give a more detailed understanding of the potential involvement of viscoelastic elements, such as titin, working as an energy-storing spring during lengthening contractions and SSCs. This information is required for the improvement of muscle models as well as for improved predictions by multi-body models.
Power Amplification Increases With Contraction Velocity During Stretch-Shortening Cycles of Skinned Muscle Fibers