BACKGROUND: Fracture healing has been linked to both the magnitude and distribution of mechanical stresses on the healing fracture tissues. However, direct clinical measurement of in vivo tibial axial loading is not possible. METHODS: Using computed tomography images, force plate data and recorded 3D interfragmentary micromovements, accurate 3D finite element models of a healing human tibial fracture were generated at 7, 10, and 16 weeks post-fracture and used to calculate longitudinal tibial forces and external fixator load-sharing during bilateral stance and walking. FINDINGS: Tibial load-sharing by the fixator decreased significantly as the fracture tissues developed even moderate stiffness, while tibial load increased steadily towards normal. INTERPRETATION: Quantitative assessment of the contribution of the external fixator is important as overloading of the callus due to insufficient support has been implicated in the retardation of the healing process.