Investigating the Correlation between Force Output, Strains, and Pressure for Active Skeletal Muscle Contractions

TL;DR

This study uses numerical simulations to analyze how muscle force correlates with strains and pressure during contractions, revealing strains as reliable proxies for force estimation in muscles.

Contribution

It demonstrates that principal and volumetric strains strongly correlate with muscle force, providing a basis for non-invasive force estimation methods.

Findings

Pressure does not correlate well with force at high activation levels.

Strong correlation between force and strains at all muscle locations.

Pressure relaxation affects pressure-force correlation away from muscle center.

Abstract

Measuring the forces of individual muscles in a muscle group around a joint is non-trivial, and researchers have suggested using surrogates for individual muscle forces instead. Traditionally, experimentalists have shown that the force output of the skeletal muscle tissue can be correlated to the intra-muscular pressure (IMP) generated by the muscle belly. However, IMP proves difficult to measure in vivo, due to variations from sensor placement and invasiveness of the procedure. Numerical biomechanical simulations offer a tool to analyze muscle contractions, enabling new insights into the correlations among non-invasive experimentally measurable quantities such as strains, and the force output. In this work, we investigate the correlations between the muscle force output, the principal, shear and volumetric strains experienced by the muscle, as well as the pressure developed within the muscle belly as the tissue undergoes isometric contractions with varying activation profiles and magnitudes. It is observed that pressure does not correlate well with force output under higher sub-maximal and maximal activation levels, especially at locations away from the center of the muscle belly due to pressure relaxation effects. This study reveals strong correlations between force output and the strains at all locations of the belly, irrespective of the type of activation considered. This observation offers evidence for further in vivo studies using experimentally measurable principal and volumetric strains in the muscle belly as proxies for the force generation by the individual muscle and consequently enables the estimation on the contribution of various muscle groups to the total force.