Load-displacement properties of the human triceps surae aponeurosis in vivo.

1. The present investigation measured the load-displacement and stress-strain characteristics of the proximal and distal human triceps surae aponeurosis and tendon in vivo during graded voluntary 10 s isometric plantarflexion efforts in five subjects. 2. During the contractions synchronous real-time ultrasonography of aponeurosis displacement, electromyography of the gastrocnemius, soleus and dorsiflexor muscles, and joint angular rotation were obtained. Tendon cross-sectional area and moment arm were obtained from magnetic resonance (MR) images. Force and electromyography data from dorsiflexion efforts were used to examine the effect of coactivation. 3. Tendon force was calculated from the joint moments and tendon moment arm, and stress was obtained by dividing force by cross-sectional area. Aponeurosis and tendon strain were obtained from the displacements normalised to tendon length. 4. Tendon force was 3171 +/- 201 N, which corresponded to 2.6 % less than the estimated force when coactivation was accounted for (3255 +/- 206 N). Aponeurosis displacement (13.9- 12.9 mm) decreased 30 % (9.6-10.7 mm) when accounting for joint angular rotation (3.6 deg). Coactivation and angular rotation-corrected stiffness yielded a quadratic relationship, R 2 = 0.98 +/- 0.01, which was similar for the proximal (467 N mm(-1)) and distal (494 N mm(-1)) aponeurosis and tendon. Maximal strain and stress were 4.4-5.6 % and 41.6 +/- 3.9 MPa, respectively, which resulted in a Young's modulus of 1048-1474 MPa. 5. The mechanical properties of the human triceps surae aponeurosis and tendon in vivo were for the first time examined. The stiffness and Young's modulus exceeded those previously reported for the tibialis anterior tendon in vivo, but were similar to those obtained for various isolated mammalian and human tendons.