Performance increase in the stretch-shortening cycle

The stretch-shortening cycle (DVZ) describes the muscular functioning when an activated muscle is eccentrically stretched and shortens again immediately afterwards. This combined sequence of eccentric and concentric muscle activity is the most common form of muscular action during human locomotion and is a component of basic movement patterns such as walking, running or jumping. In this context, the DVZ is characterized by two peculiarities: First, muscle strength, work, and power are increased by up to 50% during the concentric propulsive phase of the DVZ compared to purely concentric muscle actions. Second, this increased power is accompanied by increased efficiency. Although evidence of force increase and efficiency in the DVZ has already been provided in a large number of experiments in humans and animals, the underlying mechanisms are still controversially discussed today. This is because the previously accepted mechanisms cannot fully elucidate the increased efficiency in the DVZ. The aim of this research project is a holistic analysis of contractile as well as bio- and neuromechanical factors that contribute to muscular performance enhancement in the DVZ. In particular, the focus is on a hitherto neglected mechanism that is suspected in the contractile element and is associated with the muscle protein titin. To this end, closely coordinated experiments are being performed on both isolated rat soleus muscle fibers and human triceps surae muscle fibers. The in vitro experiments on rat muscle directly address the performance enhancement hypothesized in the contractile element. In addition, the contribution of series elasticity in the DVZ is analyzed by inserting an artificial elastic structure in series with the contractile element. On the one hand, the experimental studies on the human triceps surae muscle are used to transfer the in vitro results to the structural level of the in vivo muscle-tendon complex. On the other hand, the contribution of the different mechanisms to the performance enhancement in the DVZ under physiological conditions. Ultrasound is used to analyze the interaction of contractile element and passive series elasticity for this purpose. In addition, the variation of muscle activation (voluntary vs. electrical stimulation) as well as the recording of evoked potentials of motor cortex, spinal pathway and motor nerve help to assess to what extent neuromuscular components play a role in the performance enhancement in the DVZ.

This research project contributes to a better understanding of the DVZ as the most important everyday contraction form of the musculature. This not only promotes the fundamental understanding of human locomotion, but can furthermore find application in the field of medical technology, robotics and prosthetics, for example, for efficient humanoid drives.

Literature:

• Holzer, D., Millard, M., Hahn, D., Siebert, T., Schwirtz, A., Seiberl, W., 2023. Tendon compliance and preload must be considered when determining the in vivo force–velocity relationship from the torque–angular velocity relation. Sci Rep 13. [link]
• Tomalka, A, Weidner S, Hahn D, Seiberl W, Siebert T., 2021. Power amplification increases with increasing contraction velocity during stretch-shortening cycles of skinned muscles fibers. Front. Physiol., doi: 10.3389/fphys.2021.644981 [link]
• Seiberl, W., Hahn, D., Power, G.A., Fletcher, J.R., Siebert, T., 2021. Editorial: The Stretch-shortening Cycle of Active Muscle and Muscle-tendon Complex: What, Why and How It Increases Muscle Performance? Front Physiol.doi:10.3389/fphys.2021.693141 [link] 
• Tomalka, A, Weidner S, Hahn D, Seiberl W, Siebert T., 2020. Cross-bridges and sarcomeric non-cross-bridge structures contribute to increased work in stretch-shortening cycles. Front. Physiol., doi: 10.3389/fphys.2020.00921 [link]
• Holzer, D., Paternoster, F.K., Hahn, D., Siebert, T., Seiberl, W., 2020. Considerations on the human Achilles tendon moment arm for in vivo triceps surae muscle-tendon unit force estimates. Sci Rep 10, 19559. [link]
• Held, S, Siebert, T., Donath, L., 2020. Changes in mechanical power output in rowing by varying stroke rate and gearing.  European Journal of Sport Science. doi: 10.1080/17461391.2019.1628308.  [link]
• Held, S., Siebert, T., Donath, L., 2020. 10% Higher Rowing Power Outputs After Flexion-Extension-Cycle compared to an Isolated Concentric Contraction in Sub-Elite Rowers. Front Physiol 11.  [link]
• Held, S., Siebert, T., Donath, L., 2020. Electromyographic activity of the vastus medialis and gastrocnemius implicates a slow stretch-shortening cycle during rowing in the field. Sci Rep 10, 9451. [link]