DISCRETE MECHANICAL PROPERTIES OF HUMAN, IN VIVO, ACHILLES TENDON AND APONEUROSIS

Abstract

PURPOSE: To examine the load-deformation and strain characteristics of the separate free Achilles tendon and aponeurosis, in vivo, during graded voluntary 10-s isometric plantarflexion efforts. METHODS: Two separate trials with synchronous real-time ultrasonography for displacement of the marked free tendon or the tendon-aponeurosis complex, electromyography of the gastrocnemius, soleus, and dorsiflexor muscles, and joint angular rotation were obtained during 10-s ramp isometric contractions in 5 subjects. MR images provided segment lengths and cross-sectional area. A thin needle was inserted into the free tendon to provide a marker. Tendon-aponeurosis complex displacement less that of the free tendon yielded separate aponeurosis displacement. RESULTS: Tendon force during the free tendon trial was 2641 ± 306 N, and was 87 ± 8 % of that during the trial for tendon-aponeurosis complex. The segment lengths and deformations were 74.0 ± 0.8mm and 5.8 ± 0.8 mm for the free tendon, and 14.5 ± 1.3mm and 2.1 ± 0.6 mm for the aponeurosis, respectively. The mechanical stiffness was 759 ± 132 N/mm for the free tendon and 2615 ± 428 N/mm for the aponeurosis, P < 0.05. The strain was 8.0 ± 1.2 % for the free tendon and 1.4 ± 0.4 % for the aponeurosis, P < 0.01. Stored energy normalized to segment length was 89 ± 25 J/m for the free tendon and 14 ± 3.7 J/m for the aponeurosis, P < 0.05. The cross-sectional area of the Achilles tendon was 73 ± 4 mm2, which yielded a stress of 36.5 ± 4.6 MPa with a modulus of 788 ± 181 MPa for the free tendon. CONCLUSIONS: The mechanical and strain properties of the human, in vivo, free Achilles tendon and aponeurosis were for the first time examined. The data showed that free tendon has i) lower mechanical stiffness, ii) greater strain, and iii) more stored energy per unit length compared to that of the aponeurosis. These observations suggest that the aponeurosis and free tendon have discrete properties with different functional roles during loading.