 Surgical repair of Achilles tendon ruptures is known to significantly reduce the risk of re-rupture and to accelerate the time to return to activity compared with non-surgical treatments 1, 2. Although sutured, Achilles tendon rupture requires an extended rehabilitation period following surgery to function normally again. However, the currently available information on this post-operative treatment suggest that the currently available rehabilitation guidelines, which includes early weight bearing, does not accomplish optimal recovery of muscle-tendon function. It has been demonstrated that reduced capacity to perform heel-rises, diminished range of motion of the ankle joint, and reduced calf muscle mass is correlated with a delay in return to activity, and all of these factors could be related to elongation of the healing tendon 3. Importantly, it appears that preventing tendon elongation during rehabilitation improves the clinical outcome, but the actual mechanism for the elongation and thus how to prevent it remains unknown 4. With newly developed techniques in our laboratory we will determine the mechanical properties of human whole Achilles tendon, in vivo, which makes it possible to explore how tendons respond to the regimes following a suture repair and rehabilitation regime. 