Elastic properties of human Achilles tendon are correlated to muscle strength

1 Laboratory of Sports Sciences, Department of Life Sciences, Graduate School of Arts and Sciences, The University of Tokyo, Meguro, Tokyo; 2 Japan Society for the Promotion of Science, Chiyoda, Tokyo; 3 Institute of Physical Education, Keio University, Yokohama, Kanagawa; and 4 Department of Sports Sciences, Waseda University, Tokorozawa, Saitama, Japan Submitted 18 June 2004 ; accepted in final form 21 March 2005 The purpose of this study was to investigate whether the mechanical properties of the Achilles tendon were correlated to muscle strength in the triceps surae in humans. Twenty-four men and twelve women exerted maximal voluntary isometric plantar flexion (MVIP) torque. The elongation ( X) and strain of the Achilles tendon ( ), the proximal part of which is the composite of the gastrocnemius tendon and the soleus aponeurosis, at MVIP were determined from the displacement of the distal myotendinous junction of the medial gastrocnemius using ultrasonography. The Achilles tendon force at MVIP (F) was calculated from the MVIP torque and the Achilles tendon moment arm. There were no significant differences in either the F- X or F- relationships between men and women. X and were 9.8 ± 2.6 mm and 5.3 ± 1.6%, respectively, and were positively correlated to F ( r = 0.39, P < 0.05; r = 0.39, P < 0.05), which meant that subjects with greater muscle strength could store more elastic energy in the tendon. The regression y -intercepts for the F- X ( P < 0.01) and F- ( P < 0.05) relationship were significantly positive. These results might indicate that the Achilles tendon was stiffer in subjects with greater muscle strength, which may play a role in reducing the probability of tendon strain injuries. It was suggested that the Achilles tendon of subjects with greater muscle strength did not impair the potential for storing elastic energy in tendons and may be able to deliver the greater force supplied from a stronger muscle more efficiently. Furthermore, the difference in the Achilles tendon mechanical properties between men and women seemed to be correlated to the difference in muscle strength rather than gender. tendon elongation; tendon strain; ultrasonography; stiffness Address for reprint requests and other correspondence: T. Muraoka, Dept. of Sports Sciences, Waseda Univ., 2-579-15 Mikajima, Tokorozawa, Saitama, 359-1192 Japan (E-mail: muraoka{at}aoni.waseda.jp )