The achilles tendon is one of the strongest tendons in the human body, and a healthy tendon will only rupture under very severe conditions. Experiments and statistics show a high probability of tendon rupture in the case of a person trying to make a quick start and thereby toppling over in the ankle joint. Toppling over means a quick overturning in the lower ankle joint as it may occur when starting out of the sagittal plane. Based on a verified model of the achilles tendon, of M. Gastrocnemius and M. Soleus (Hatze, H. (1980) IEEE Trans. Automatic Control 25, 375-385; Hoy et al. (1990) J. Biomechanics 23, 1185-1198.), the stress in different tendon fibres during such a movement is investigated. The results of this simulation show that the expected extreme stresses in the lateral tendon fibres are a possible starting point for a rupture.