We're all taught that tendons connect muscles to bones, but like most generalities this statement, while true, doesn't quite capture the complexity involved in the musculoskeletal system. For example, many limb muscles have broad collagenous sheets, or aponeuroses, that lie in between their fibers and the more rope-like tendons that transmit forces to the skeleton. These aponeuroses can be quite large, enveloping much of the distal muscle belly, and yet we have little sense of their purpose. Manny Azizi and Tom Roberts of Brown University wondered whether muscle shape changes during contraction might deform aponeuroses, altering their stiffness and adding some functional versatility to the system. The key to their idea is that unlike tendons, which are almost exclusively stretched along their long axis, aponeuroses likely deform in a more complicated fashion and in so doing might perform a previously unappreciated role when muscles are active.
To test their idea, Azizi and Roberts used an in situ preparation of the turkey lateral gastrocnemius (a major ankle extensor), which possesses a large superficial aponeurosis. Using a variety of techniques, they were able to stimulate the muscle to contract, as well as measure and control its force production and length changes. They attached 15-20 small steel markers in an array throughout the aponeurosis so that its deformations could be tracked in the longitudinal and transverse dimensions during stimulated contractions. An experiment consisted of subjecting the muscle to 10 contractions over a broad range of forces, all the while using 3D video fluoroscopy to measure marker movements in the aponeurosis. Following these active contractions, the muscle was driven through a series of passive sinusoidal length changes while the aponeurosis deformations were again tracked.
During active contractions, as muscles generated force and began to shorten, the aponeuroses stretched longitudinally. The longitudinal strains were relatively small and linearly related to the muscle's force production, as one might expect of tissue connected directly to a contracting element. Perhaps more surprising was the fact that the aponeuroses were also stretched in the transverse direction, and these strains were always quite high. Moreover, this transverse stretching altered the mechanical properties of the aponeurosis: the greater the transverse strain, the higher its longitudinal stiffness. The simultaneous stretching of the aponeurosis in orthogonal directions was clearly a function of the muscle actively contracting because during passive length changes in unstimulated muscles, longitudinal aponeurotic strains were out of phase with transverse strains (i.e. stretching in one direction led to contraction in the other).
So why does an actively contracting muscle stretch its aponeurosis transversely (i.e. orthogonal to the muscle force's line of action)? As the muscle's fibers shorten longitudinally, it must expand transversely to maintain a constant volume, and because the aponeurosis is so intimately connected to the muscle belly, it too expands and thus stretches transversely.
Why should I care? Well, as aponeurotic tissue is stretched transversely, its longitudinal stiffness is increased. Recall that aponeuroses act as liaisons between muscles and their tendons, and if their stiffness can be modulated, so too can their effectiveness at transmitting forces from muscles to bones and storing elastic energy. Effective (and variable) force transmission and elastic energy storage potential seem important considerations when studying or modeling the musculoskeletal system in the context of locomotion. Given their size and abundance in limb musculature, it isn't necessarily surprising that aponeuroses have functional relevance, but it is very nice to begin to understand what this relevance might be.
