Modelling muscle forces: From scaled fibres to physiological task-groups

Muscles contain a collection of motor units that can have different mechanical properties. It should be expected that the mechanical output from a muscle partly depends on the particular motor units that are activated for a given task. However, muscle models used for biomechanical simulations typically assume that a whole muscle can be represented by a scaled-up muscle fibre. Some models allow the intrinsic properties of the muscle to adapt to the activation levels, but even these may miss important features of the muscle contractions. This paper will discuss how the activity from different types of motor units may be detected during a contraction, and will demonstrate how muscle models that incorporate distinct units result in improved predictions of muscle force.