Solid ankle-foot orthoses (AFOs) are designed to immobilise the ankle but numerous studies have measured a considerable ankle range of motion (ROM) in AFO users. Measurement of ankle kinematics may be affected by soft-tissue artefact (STA) of the knee marker, deformation of the AFO or tibial movement within the AFO. A new model based on the Conventional Gait Model (CGM) was developed to calculate these effects. Although movement of the AFO within the shoe should not affect the measured ankle joint angle the model also allows an estimation of this movement. Seven children (13 limbs) with spastic diplegic cerebral palsy were assessed to present the benefits of the new model compared to the CGM. STA of the knee marker was estimated to result in a 1.5° overestimation of total ankle ROM (from 8.2° to 9.7°). STA error was strongly related to angle of knee flexion (r=0.82) with an average maximum error of 3.8°. AFO deformation contributed approximately two thirds of the ankle ROM (6.0±4.3°) with the remaining third from tibial movement relative to the AFO (2.8±0.9°). Movement of the AFO within the footwear was very small (1.8±0.8°). A strong positive relationship (r=0.9) was found between body mass (kg) and AFO deformation which was statistically significant (p<0.001). This is the first model to attempt to quantify different contributions to ankle dorsiflexion measured during gait analysis of people wearing AFOs.