Achilles tendon compliance: Implications for gait, injury and rehabilitation

Abstract

Tendons are inherently elastic materials: they strain in response to the stress that is applied to them. When muscles contract and produce force, this force is transferred to the skeleton via the tendon and hence causes the tendon to stretch and store elastic strain energy which can be used later for propulsion. The Achilles tendon is the best example of this, because its tendon is very long compared to the muscle fibres it attaches to. We have used B-mode ultrasound imaging in combination with motion analysis to examine the strain on the Achilles tendon during walking, running and hopping. We have found that the Achilles tendon is capable of strains much higher than that which was previously thought possible; approximately 5% during walking, 6-7% during running and up to 10% during hopping. It is likely that repetitive tasks that require such large strains will cause both heating and micro-damage, which makes the Achilles tendon susceptible to overuse injuries. We have also been able to examine the influence of using heel lifts to reduce Achilles tendon strain. Our results indicate that although the ankle is significantly more plantar flexed during walking and running with two different sized heel lifts (12 and 18 mm), there is no significant reduction in the Achilles tendon strain. This supports previous findings that indicate there is no reduction in the Achilles tendon force with heel lifts; therefore the benefits of such orthoses for degenerative disorders may be limited. Advances in ultrasound imaging techniques make it possible to better quantify the changes in mechanical properties and structure that occur as a result of therapeutical interventions.