Effects of muscle mass on muscle force predictions in human movement

Abstract

Muscle mass significantly influences skeletal muscle behaviour, potentially explaining why traditional massless Hill-type models struggle to predict the forces generated by larger muscles during dynamic, submaximal contractions. However, the applicability of mass-enhanced Hill-type models in human locomotion remains unexplored. Here, we compared the predicted force from a one-dimensional (1D) mass-enhanced Hill-type muscle model with a traditional 1D massless Hill-type muscle model across a range of experimentally measured human movements. Kinematic and electromyographic data were collected from 20 participants performing locomotor tasks and supplemented with existing cycling data. Muscle size was geometrically scaled by factors from 0.1 to 10, which causes lengths to be scaled proportionally, cross-sectional area and peak isometric force F0 with the square, and mass with the cube of the factor. Muscle tissue mass (inertia) and movement speed/frequency increased the differences between mass-enhanced and massless predictions of force and power. At high cadence and the largest scale, the normalized root-mean-square difference (RMSD) between force traces reached 7% of F0 (averaged across muscles). However, the differences between the models were minimal (less than 1%) at the human-sized scale 1. Given that real muscle also deforms in three dimensions, we still do not know the extent to which this extra dimensionality affects muscle forces for these human movements.