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Increasing plantarflexion of the ankle decreases the stress on the Achilles tendon during ambulation in an ankle immobiliser, but how much stress is optimal in the various phases after an Achilles injury?
The atrophying effects of immobilisation and the strengthening effects of training and exercise on all musculoskeletal tissues (bone, cartilage, ligament, muscle, and tendon) are well known, and therefore much effort has been expended during recent decades on studies to clarify whether the old concept of post-traumatic immobilisation could be safely replaced by rehabilitation programmes using early controlled mobilisation. The controlled experimental and clinical trials have yielded convincing evidence that early controlled mobilisation is superior to immobilisation for musculoskeletal soft tissue injuries.1 2 This holds true not only in primary treatment of acute injuries, but also in their postoperative management. The superiority of early controlled mobilisation is especially apparent in producing quicker recovery and return to full activity, without jeopardising the long term rehabilitation.
Although the above general message is clear, many interesting problems with respect to the content and timing of controlled rehabilitation need further clarification. For example, clear answers are needed on the effects of various joint positions on the mechanical stress of injured and repaired tendons and ligaments. This study addresses one of these problems: using surface electromyography and isometric torque measurements, the authors determined the relative stresses on the Achilles tendon during weightbearing with immobilisation in varying degrees of ankle plantarflexion. As the result of carefully conducted measurements with 10 adult subjects, they calculated that the Achilles forces averaged about 550 N during normal walking, 370 N during walking with the ankle immobilised in neutral position, 280 N during walking with the ankle immobilised with a 0.5 inch heel lift, and 190 N when walking with the ankle immobilised with a 1 inch heel lift. This result—that is, immobilisation with increasing plantarflexion of the ankle decreases the EMG activity and force of the calf muscles and thus the resultant Achilles tendon strain during walking—is not surprising as it follows common sense and the basic rules of musculoskeletal biomechanics. It should be of use if, after an Achilles tendon injury, reduction of stress is desired while progressing with the patient's weightbearing.
What remains a challenge is the question about the limits of desirable and undesirable stresses on the Achilles tendon during various phases of healing. As complete immobility with zero stress leads to tissue atrophy and adhesions whereas too much stress too early is likely to lengthen the healing tissue and jeopardise the repair, we need to know much more about the optimal tendon strains and rehabilitation after an Achilles tendon injury.
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