Contractile, elastic, and neural mechanisms of muscular performance enhancement during stretch-shortening-cycles
The stretch-shortening cycle (SSC) refers to the muscle action when an active muscle stretch is immediately followed by active muscle shortening. This combination of eccentric and concentric contractions is the most important type of muscle action during human locomotion including patterns such as walking, running or jumping. Two special features characterize SSCs: First, during the concentric push-off phase of a SSC, force, work, and power production are increased up to 50% compared to a purely concentric contraction without preceding eccentric stretch. Second, this increase in performance during SSCs is accompanied by an increased efficiency. Despite clear evidence concerning the increase in performance and efficiency in various experimental human and animal studies, the underlying mechanisms remain a matter of debate. This is because none of the currently accepted mechanisms can entirely explain the increase in performance and efficiency during SSCs.
The aim of this research project is to provide a holistic analysis of contractile, biomechanical, and neuromechanical factors contributing to the increased performance during SSCs. In this context, one main focus lies on a widely neglected and/or controversial mechanism, which is thought to sit within the contractile element itself and which is associated with the giant muscle protein titin. For this purpose, closely intertwined experiments on skinned fibers of rat m. soleus as well as on human m. triceps surae will be conducted. The in vitro experiments on isolated rat muscle fibers directly address the supposed mechanism within the contractile element. Further, by inserting an artificial elastic structure in series to the muscle fibers, the contribution of series-elasticity to increased performance during SSCs will be investigated. The experiments on human m. triceps surae are designed to transfer the findings from the fiber experiments to the level of the in vivo muscle-tendon-complex. Additionally, the experiments are supposed to clarify the contribution of each mechanism to the increase in performance during SSCs under physiological conditions. Therefor ultrasound allows to analyze the interaction between the contractile and series elastic elements. Further, variations in muscle activation (voluntary vs. electrical stimulation) and the recording of evoked potentials after stimulation of the motor cortex, the corticospinal tract and the motor nerve help to assess how neuromuscular aspects contribute to the increased performance during SSCs.
Overall, this research project increases knowledge on basic muscle function and helps to better understand SSCs as one of the most important type of muscle action of everyday human locomotion. This is not only interesting in terms of basic research on muscle function but also has the potential to contribute to the development of efficient humanoid motors as used in medical devices, robotics and prosthetics.