Architectural features of multiarticular muscles

Abstract

Most of the available information on neuromuscular organization and musculoskeletal mechanics comes from a few muscles that physiologists choose to study because they are relatively simple. They usually cross a single hinge joint, have a single band of innervation, and are composed of muscle fibres with similar lengths, all acting in parallel. In most species, such muscles are in the minority. In order to develop quantitative models of complete musculoskeletal systems, it is necessary also to study the less simple muscles. This has revealed several types of architecture that were somewhat unexpected and may have important functional consequences. Limb muscles that span more than one joint are often quite long and must withstand large length changes; their parallel fascicles often contain multiple, relatively short fibres interdigitated in-series. Axial muscles often span many joints and are composed of in-series compartments. A variety of neuromuscular architectures seems to have evolved to deal with problems of mechanical instability arising from imbalances of force between fibres, motor units and compartments. These may be prone to particular failure modes in response to athletic strain, dystrophic diseases, and functional electrical stimulation.