Mechanics of aponeurosis and its role in muscle contraction: current insights and future directions

Aponeuroses are collagen-rich connective tissues that link muscle fascicles to tendon, shaping muscle architecture and contributing to force transmission within the muscle–tendon unit (MTU). Despite their importance, their influence on muscle mechanics and MTU function remains incompletely understood. This review synthesizes current knowledge of aponeurosis properties and mechanical behaviour, identifies critical gaps in understanding, and outlines key directions for future research needed to clarify their functional significance in vivo. Aponeuroses share many structural and material properties with tendon, including hierarchical collagen organization and nonlinear, anisotropic, viscoelastic behaviour under tensile loading. Unlike tendon, which primarily experiences uniaxial forces, aponeuroses are attached to contracting muscle and are therefore subjected to multidirectional, spatially heterogeneous loading. Muscle fibre shortening, rotation, and radial expansion impose complex combinations of longitudinal and transverse forces that are transmitted through deformable connective tissue layers at the muscle-aponeurosis interface, resulting in deformations that vary with muscle force, length, and activation. These observations demonstrate that tendon-like material properties alone do not capture the complex mechanical behaviour of aponeuroses in vivo. Further investigation is needed to understand aponeurosis behaviour during locomotion, variation across muscles and species of different sizes, and changes with sex, aging, and disease. Advancing the field will require coordinated experimental and 3D modelling approaches capable of capturing heterogeneous deformations and realistic material behaviour. Filling these knowledge gaps will provide a more complete understanding of how aponeuroses contribute to muscle and locomotor performance.