The aging Achilles tendon: model-predicted changes in calf muscle neuromechanics

Forward propulsion depends on the forces generated by the triceps surae muscles and transmitted through the muscles' subtendons, which merge and twist to form the Achilles tendon (AT). As people age, the AT may undergo structural changes that could alter the subtendons' ability to transmit forces or function with some independence; prominent changes include increased tendon compliance and a proliferation of interfascicular adhesions compared to younger tendon. However, the effects of age-related changes on the subtendons are difficult to isolate in vivo. Here, we used a Hill-type musculoskeletal model of the triceps surae muscle-subtendon units to simulate the effects of age-related changes on gastrocnemius (GAS) and soleus (SOL) muscle contractile dynamics across a range of physiological force levels during fixed-end contractions. We simulated individual and dual muscle excitations with altered tendon compliance (ε_o = 3 %, 6 %, 9 %) and inclusion of a shared tendon. Consistent with fundamental muscle mechanics, compared to stiffer tendons, increased tendon compliance elicited more than three times the GAS and SOL fiber shortening and greater muscle excitation - effects that increased with requisite force demand. However, our model results also suggest combinatory effects of increased tendon compliance and interfascicle adhesions in the aging AT that deleteriously amplify redistribution from the GAS to the SOL which may be functionally detrimental during gait.