Intramuscular aponeurosis modifies muscle fascicle behaviour at different muscle lengths and forces

Animal lower limb muscle-tendon units (MTUs) typically have intramuscular and extramuscular tendons, which are known as aponeuroses and tendons, respectively. Due to their similar passive material properties, both aponeuroses and tendons are often grouped into a single series elastic element that is thought to have a fixed stiffness at moderate to high muscle forces [1]. However, mounting evidence suggests that aponeurosis stiffness is greater in active vs. passive conditions [2], whereby active contraction stretches the aponeurosis in both width (i.e. transversely) and length (i.e. longitudinally) whereas passive stretch reduces aponeurosis width. Transverse aponeurosis strains therefore appear to modulate longitudinal aponeurosis stiffness [2]. Our research supports this notion as we have found that longer MTU lengths favour an apparently stiffer aponeurosis and reduced muscle fascicle strains because of greater increases in aponeurosis width, but similar absolute widths, compared with shorter MTU lengths [3].