INDIVIDUAL MUSCLE CONTRIBUTIONS TO THE IN-VIVO ACHILLES-TENDON FORCE

Objective. To ascertain the possibility of non-uniform stress within t he achilles tendon due to individual force contributions of the tricep s surae. Design. Calculation of non-uniform stress through discrepanci es in moments about the ankle joint. Background. Non-uniform stress ov er the cross-sectional area have been implied in the etiology of achil les tendon injury and may influence functional aspects. However, this has not been empirically demonstrated. Methods. In vivo achilles tendo n forces were measured with an optic fibre technique during isometric plantarflexions at systematically varied knee angles and contraction i ntensities. A comparison to the plantar force measured underneath the metatarsal heads permitted the calculation of the achilles tendon cont ribution to the resultant plantarflexion moment. The achilles tendon f orce was further differentiated into gastrocnemius and soleus contribu tions. Individual muscle activation patterns were described. Results. The average achilles tendon contribution to the resultant moment was 6 7.4%. Variations were found at different knee angles and contraction i ntensities. A force discrepancy of 967 N occurred between gastrocnemiu s and soleus over a gastrocnemius length change of 2.67 cm. This corre sponded to a stress discrepancy of 21 N/mm(2) over the tendon cross-se ctional area. Separate muscles showed individual activation patterns. Conclusions. Non-uniform stress in the achilles tendon can occur throu gh modifications of individual muscle contributions.