The variability of the biomechanical parameters is considered a predictor of falling risk. The causes of this variability are intrinsic (normal variations, presence of a pathology and aging effect) or extrinsic (the evaluating methodology, variation of environmental conditions, etc.). The free moment represents the vertical moment applied in the center of pressure and is considered to be sensitive to the movement and the imbalances of the whole body in the transversal plane. Based on a case study, a knee surgical intervention, the article presents a method for determining the variability of the free moment using the coefficient of variation for different time intervals of the stance phase. The main results of our study indicate a different magnitude of the variability of the free moment for one lower limb compared with the other one, the variability of the free moment for the operated knee being smaller, with values between 15, 8–65, 6%. The smaller values of the variability for the operated knee joint reflect a more challenging situation from a functional point of view, generated by this joint. Evaluating the variability of the free moment in time could be a useful tool for the progress of the rehabilitation period.