Modelling Skeletal Muscle Motor Unit Recruitment Contributions To Contractile Function: Part 1 — Velocity, Force and Power

Abstract

There is no current method that can directly measure in-vivo human motor unit recruitment and their individual incremental contributions to muscle contractile velocity, force and power. The purpose of this research was to 1) acquire previously published data of single fibre contractile velocity, force and power for the different skeletal muscle fibre types, corrected for muscle temperature, 2) develop a computational model of motor unit recruitment spanning the 5 fibre type categories (types I, I-IIa, IIa, IIab, and IIb) and four different slow to fast twitch proportions (80-20,60-40,40-60,20-80% ST-FT, respectively), and 3) use the model to compute changes in motor unit contributions to contractile velocity, force and power. The order of motor unit recruitment was based upon motor unit size and ranged from 85 (type-I) to 207 (type IIb) fibres⋅unit-1. The total number of motor units across the four categories were 3,582, 3,308, 3,041, and 2,757, respectively. Data for 20 vs 100% recruitment for contractile velocity of the 80-20% ST-FT were 0.055 vs 0.09 m⋅s-1, respectively, and for 20-80% ST-FT were 0.0589 vs 0.1569 m⋅s-1, respectively. Contractile force data were 28.065 vs 202.01 N, and 28.065 vs 248.14 N, respectively. Contractile power data were 1.545 vs 18.136 Watts, and 1.421 vs 38.957 Watts, respectively. The model succeeded in transferring data from single muscle fibre to motor unit and whole muscle contraction kinematics. Such modelling has future applications to the energetics of muscle contraction at the motor unit and muscle fibre level, and for guiding robotic replication of human muscle contractile function.