Meeting Abstract
Senescence in animals decreases locomotor ability. It is well known that there is an increased amount of collagen held together by stronger covalent bonds within the skeletal muscles that power locomotion. Changes in intramuscular collagen content are also accompanied by changes in the mechanical properties of tendons and aponeuroses acting in series with muscles. However, it is unclear how such changes affect the performance of the muscle-tendon unit (MTU). To understand how changes in mechanical properties of isolated components affect an integrated MTU, we compared the Young’s modulus and resilience of all the component tissues that make up the medial gastrocnemius MTU in mature (4 mos) and aged (33 mos) rats. We find a 1.4-2 fold increase in the Young’s modulus of all the tissues examined: whole muscles, muscle fiber bundles, tendons and aponeuroses. No significant effect of aging was observed on tissue resilience. Aponeuroses, given their sheet-like structure were tested biaxially. With aging, the stiffness of the aponeurosis along the line of action of the muscle increases significantly with increasing orthogonal strain. The same was not true for young animals or for tissues tested along the transverse direction. Additionally, we show significant architectural differences in old MTUs: muscles become smaller, longer and thinner, with lower pennation angles, while tendon cross sectional area remains unchanged. Our results indicate that due to both biochemical and architectural changes aged muscles became stiffer and had decreased capacity for force production while operating in series with elastic elements that had increased stiffness. These results suggest that age-related structural changes may limit the capacity of muscles to utilize elastic energy storage and change where muscles operate on the force-length curve in vivo. Supported by NIH AR055295.
