Findings from animal experiments are sometimes contradictory to the idea that the tendon structure is a simple elastic spring in series with muscle fibers, and suggest influence of muscle contraction on the tendon mechanical properties. The purpose of the present study was to investigate the influence of muscle contraction levels on the force-length relationship of the human Achilles tendon during lengthening of the triceps surae muscle-tendon unit. For seven subjects, ankle dorsiflexion was performed without (passive condition) and with contraction of plantar flexor muscles (eccentric conditions, at 3 contraction levels) on an isokinetic dynamometer. Deformation of the Achilles tendon during each trial was measured using ultrasonography. The Achilles tendon force corresponding to the tendon elongation of 10 mm in the passive condition was significantly smaller than those in the eccentric conditions (p < 0.05 or p < 0.01). Within the eccentric conditions, the Achilles tendon force corresponding to the tendon elongation of 10 mm was significantly greater in the maximal contraction level than those in submaximal eccentric conditions (p < 0.05 or p < 0.01). In addition, the tendon stiffness was greater in higher contraction levels (p < 0.05 or p < 0.01). Present results suggest that the human tendon structure is not a simple elastic spring in series with muscle fibers. (c) 2011 Elsevier Ltd. All rights reserved.