To achieve optimal flexibility in biological internal fracture fixation two questions require clarification: which biomechanical parameter controls healing and what are the boundary conditions thereof? Fracture movement interacts with callus and local stress and strain are influencing the reaction of the tissue cells. A linear gradient of strain was created inside a sheep tibia osteotomy using an active external fixator. The effect of different amounts of strain applied at 10 stimulation cycles/day on the occurrence of callus and on enabling osseous connection of the fragments was evaluated using micro-radiology to determine the amount of calcified new bone formation and its quality of gap bridging. A strong relation between level of strain and amount of callus was observed. Depending on the strain level different pattern of connections were seen. At the lowest investigated gap strain level of about 7% direct connection of the fragments within the gap occurred. Beyond 13% the callus only connected indirectly outside the gap. At over 36% callus did not connect the fragments anymore comparable to a situation in hypertrophic non-unions. The observed strong relation between interfragmentary strain and reduced osseous bridging may support the hypothesis that the elongation at rupture of connecting tissue plays an important role defining the upper limit for solid bridging. In planning fracture treatment, the amount of fracture mobility resulting in interfragmentary strain may play a crucial role to achieve solid healing.