Strain in shock-loaded skeletal muscle and the time scale of muscular wobbling mass dynamics

Kasper B Christensen,Michael Günther,Tobias Siebert,Syn Schmitt

doi:10.1038/s41598-017-13630-7

Abstract

In terrestrial locomotion, muscles undergo damped oscillations in response to limb impacts with the ground. Muscles are also actuators that generate mechanical power to allow locomotion. The corresponding elementary contractile process is the work stroke of an actin-myosin cross-bridge, which may be forcibly detached by superposed oscillations. By experimentally emulating rat leg impacts, we found that full activity and non-fatigue must meet to possibly prevent forcible cross-bridge detachment. Because submaximal muscle force represents the ordinary locomotor condition, our results show that forcible, eccentric cross-bridge detachment is a common, physiological process even during isometric muscle contractions. We also calculated the stiffnesses of the whole muscle-tendon complex and the fibre material separately, as well as Young’s modulus of the latter: 1.8 MPa and 0.75 MPa for fresh, fully active and passive fibres, respectively. Our inferred Young’s modulus of the tendon-aponeurosis complex suggests that stiffness in series to the fibre material is determined by the elastic properties of the aponeurosis region, rather than the tendon material. Knowing these stiffnesses and the muscle mass, the complex’ eigenfrequency for responses to impacts can be quantified, as well as the size-dependency of this time scale of muscular wobbling mass dynamics.

Highlights

A common working condition of skeletal muscles during animal locomotion is active contraction during acceleration through space
The muscle-tendon complex (MTC) was fixed to the frame with small bone tissue pieces of the calcaneus and femur left at the end of the distal and proximal tendons, respectively, and subsequently stretched to its optimal length, which was inferred from literature[6]
All dropping trials were done with fully active muscles, except for the last trial, which was performed with the stimulation switched off.The muscle force was measured by a force transducer serving as a rigid connector between the suspending clamp of the upper tendon and the frame

Summary

Introduction

A common working condition of skeletal muscles during animal locomotion is active contraction during acceleration through space. If an animal’s locomotion requires repulsion from a solid substrate like soil or wood with the distal ends of the limbs contacting the substrate surface at finite impact velocities, shock-wave-like accelerations are induced to the bones. These shock waves are transmitted to the limb muscles[1,2,3] via their suspensions and contact areas to the bones and adjacent muscles. There is no knowledge about the mechanical characteristics of cross-bridges loaded during this fundamentally physiological impact situation so far In this situation, wave propagation is a typical response phenomenon of condensed matter, that is observable by time-varying strain. We restricted the experimental condition to muscles that were oriented vertically, and we solely analysed strains in the longitudinal fibre direction

Methods

Results

Conclusion