Spontanous rupture at the distal part of the gastrocnemius tendon (GT) is the second most common non-traumatic tendon injury in dogs, whereas the other strands of the common calcaneal tendon do not seem to have a predisposition to rupture. In order to discover why we investigated the common calcaneal tendons of 63 dogs microscopically and biomechanically. Both the gastrocnemius and superficial digital flexor tendon (SFT) had multiple low vascularized fibrocartilaginous areas within their distal course as opposed to regular parallel fibered areas in the proximal tendon areas. Biomechanical testing revealed that the distal sections in both tendons show a 50% and 70% lower tensile strength (F(max)/kg BW) than the proximal sections (p<0.01), respectively. On the contrary, tensile load (F(max)/mm(2)) only differed minimally between proximal and distal sections in both tendons (8% and 9%, respectively), whereas the tensile load of the distal gastrocnemius tendon is 35% lower than of the distal superficial flexor tendon (p<0.01). To the authors' knowledge, this is the first study to experimentally show that there are different biomechanical properties within the same tendon. The maximum load to failure is lower in the GT compared to the SFT within the same dog which explains its higher incidence of rupture in the field. The avascular fibrocartilaginous structure in the distal gastrocnemius tendon seems to play a further role in the pathogenesis of spontaneous rupture.