A ground angle estimation technique for use on ankle-foot-orthosis AFO, during gait is proposed. Strain gauge sensors were mounted on a foot orthosis in order to give information about strain in the sagittal plane. The ankle angle of the orthosis was fixed. Strain characteristics were therefore changed when walking on slopes. It was investigated if strain information could be used for detection of inclination and estimation of inclination angle. With a Fourier series representation of the strain during a gait cycle, ground angle at different walking speeds and inclinations could be estimated with similar accuracy as previous studies using kinematically based estimators. This indicates that embedded strain sensors can be used for online control of future orthoses with inclination adaptation.