Regional variation of tibialis anterior tendon mechanics is lost following denervation

Denervation or inactivity is known to decrease the mass and alter the phenotype of muscle. The mechanical response of tendon to inactivity that has been determined experimentally differs from what is reported by patients. We investigated the hypothesis that this difference was the result of artifacts of the testing process and did not represent what occurred in vivo. To test this hypothesis, a novel approach was used to determine the mechanical properties of the tibialis anterior (TA) tendon by optically measuring the end-to-end mechanical strains as well as the local strains at specific regions of excised TA tendon units. When the end-to-end strain of normal TA tendon is determined, stress-strain response curves show considerably more extensibility than when strain is measured across only the midsection of the tendon (mid-tendon). The strain experienced by the region close to the muscle (muscle tendon) is five times greater than the strain in either the mid-tendon or near the bone (bone-tendon). Five weeks of denervation decreased muscle mass by 67%; increased tendon mass by 10%; and changed the entire shape of the nonlinear response curve, including a loss in regional variation in strain, a 3.9-fold increase in end-to-end tangent modulus, and a 70% reduction in the toe region, as a result of a drastic reduction of the extensibility in the muscle-tendon region. The stress-strain response in the mid-tendon region of a normal TA tendon is therefore not indicative of its overall ability to deform in vivo as it transmits forces from muscle to bone.