Skeletal muscles are the motors of locomotion. Due to their complex structure, they can withstand impacts during contraction. Running fast causes impacts when the legs hit the ground, to which the muscle responds by oscillating in a damped manner (wobbling mass behaviour). The wobbling mass behaviour is strongly dependent on the animal mass and the muscle inertia and they influence the ground reaction force, the joint load and moments, and may also influence the force generation at the
sarcomere level. We aim to understand how the muscle reacts locally and globally under real leg impact acceleration scenarios across animal species, from a body mass of about 0.1kg to 500kg.
Previously, our group had developed a new setup that significantly reduced the limitations of in vivo measurements, emulating legged impact situations and directly measuring muscle oscillations in isolated muscles. We showed ground-breaking insight into the strains and damping properties of the contractile unit (the sarcomere) in legged impact situations. Later, we were able to decompose the time domain signal of wobbling mass dynamics into frequency components (eigenfrequencies) to study what makes up the time domain signal to increase our understanding of the delicate interplay between muscle contraction and muscle inertia properties in wobbling mass dynamics.
Yet, our earlier findings were limited to studying the working conditions of rat (body mass ≈0.4kg) M. gastrocnemius in legged impact situations. To expedite our knowledge regarding muscular design, we will determine tendon properties from highly dynamic impact situations and include these new findings in a complex numerical suspended spring-mass model. Compared to our previously published spring-mass model, the enhanced model must include both distal and proximal tendon stiffness asymmetries and increase the number of lumped masses. With this, we will explain the state-of-art experimental findings of wobbling mass eigenfrequencies. By working out scaling allometries of all muscle tissues contributing to muscle eigenfrequencies, we will estimate eigenfrequencies in larger animals, compare our findings to the literature, and examine their functional relevance. Furthermore, we will combine non-linear force-strain-rate characteristics of sarcomeres with the gained scaling allometry knowledge to estimate sarcomere dynamics and damping properties in legged impact situations across species.
This project will contribute to better understanding the relationship between skeletal muscle structure and function in daily use, skeletal muscle damage, and muscle regeneration by correlating typical external load conditions during legged impacts with internal dynamic processes in the muscle tissue across species of different sizes (from small to large mammals). Furthermore, an improved design understanding of the universal biological actuator ‘muscle’ will contribute to developing biomimetic robotics actuators and prostheses.