How does the nervous system control muscles? Muscles have two primary features for neural control: 1) Division into multiple motor units, which are groups of fibres with shared properties, and 2) Rate-coded excitation in which the muscle is activated with repeated spikes from motor neurones. Both of these features are traditionally neglected in standard muscle models. We created a mathematical framework for modelling a force-control task by controlling instantaneous firing rates and motor unit recruitment.

A salient feature of muscle behaviour is the Force-Velocity relationship (FV), meaning that muscles become weaker (less force) at higher speeds. When muscle forces combine to move a joint, the FV properties of the individual muscles add. We created a way to visualise a joint-level FV relationship in musculoskeletal simulations. We found that this visualisation helps us understand the complex mechanics of human reaching. We also found that co-contracting muscles (e.g. biceps and triceps acting in opposition) help the arm resist mechanical disturbances.