The neuromechanical adaptations to Achilles tendinosis.

Human movement is initiated, controlled, and executed in a hierarchical system comprised of the nervous system, muscle, and tendon. If a component in the loop loses its integrity, the entire system must adapt to that deficiency. Achilles tendon, when degenerated, displays lower stiffness. This local mechanical deficit may be compensated by alteration of motor commands from the CNS. These modulations in motor commands from the CNS may result in altered activation of the agonist, synergist, and antagonist muscles. The purpose of this investigation was to study the effect of tendon degeneration on its mechanical properties, neuromechanical behaviour of surrounding musculature, and to explore the existence of the CNS modulation accompanying tendinosis. It is hypothesized that the degenerated tendon will lead to decreased tissue mechanical properties, protective muscle activation patterns, in addition to an up-regulated descending drive from the CNS. Strong evidence, presented in this study, suggests that tendinotic tendons are more compliant when compared to healthy tendons. This unilateral involvement affected the neuromuscular control on the involved side but not the non-involved side. The muscle tendon unit on the tendinotic side displays a reduced temporal efficiency, which results in altered CNS control. The altered CNS control is then expressed as an adapted muscle activation pattern in the lower leg. Taken together, this study illustrates the coordinated multi-level adaptations to a mechanical lesion in a tendon caused by pathology.