The present study aimed to investigate changes in elasticity, blood volume, and oxygen saturation of tendon caused by repeat contractions of different durations. Eight male subjects performed endurance tests, which consisted of isometric plantar flexion with two different durations: a shorter contraction (SC) and a longer contraction (LC). During endurance tests and a recovery period, we measured the total hemoglobin (THb) and oxygen saturation (StO(2)) of the medial gastrocnemius muscle and Achilles tendon using near infrared spectroscopy and red laser lights. In addition, elongation of the Achilles tendon during the endurance test was measured via ultrasonography. Compared to the resting level, the THb and StO(2) of the tendon increased significantly after SC, but not after LC. On the other hand, Achilles tendon became more compliant after LC, but not after SC. These results suggested that duration of contraction affected not only elasticity, but also blood volume and oxygen saturation of tendons.