The tendon has to be seen functionally within the muscle-tendon-unit. The tendon has to transmit the force that is produced by the muscle, but acts also as a spring that stores energy. The tendon itself consists of three parts: the tendon-bone insertion, the mid-portion area, and the muscle-tendon junction. The biomechanical properties belong primarily on type I collagen, that degenerates in case of tendinopathy.