Assessing gait kinematics and finding a mathematical formulation of it, is at great interest in biomechanical field. While the vast majority of the studies are targeting the healthy subjects in order to describe methods of processing the kinematical data, the purpose of this paper is to explore the use of Fourier transform on joint angles of a subject with mild ankle instability. The raw were recorded using an ultrasound-based motion analysis system during gait movement. The angular time series of hip, knee and ankle in sagittal plane were used for applying of Fourier approximation. The results of the approximation were compared with the subject’s measurements using statistical analysis in order to observe how well the approximation fit the original data and also to identify how the ankle instability can be framed by the approximation. Analyzing the results, we can conclude that the Fourier approximation can successfully be applied on a subject with gait alteration, and also that the method can be extended to a group of patients.