Individual muscle contributions to the in vivo achilles tendon force

OBJECTIVE: To ascertain the possibility of non-uniform stress within the achilles tendon due to individual force contributions of the triceps surae. DESIGN: Calculation of non-uniform stress through discrepancies in moments about the ankle joint. BACKGROUND: Non-uniform stress over the cross-sectional area have been implied in the etiology of achilles tendon injury and may influence functional aspects. However, this has not been empirically demonstrated. METHODS: In vivo achilles tendon forces were measured with an optic fibre technique during isometric plantarflexions at systematically varied knee angles and contraction intensities. A comparison to the plantar force measured underneath the metatarsal heads permitted the calculation of the achilles tendon contribution to the resultant plantarflexion moment. The achilles tendon force was further differentiated into gastrocnemius and soleus contributions. Individual muscle activation patterns were described. RESULTS: The average achilles tendon contribution to the resultant moment was 67.4%. Variations were found at different knee angles and contraction intensities. A force discrepancy of 967 N occurred between gastrocnemius and soleus over a gastrocnemius length change of 2.67 cm. This corresponded to a stress discrepancy of 21 N/mm(2) over the tendon cross-sectional area. Separate muscles showed individual activation patterns. CONCLUSIONS: Non-uniform stress in the achilles tendon can occur through modifications of individual muscle contributions. RELEVANCE: Non-uniform stress in the achilles tendon has been implied in its injury etiology. This study demonstrated such loading resulting from discrepancies in individual muscle forces.