Objectives: Determination of the phases in a stand-to-sit-to-stand task based on a derivative of the parameter with the lowest variance calculated based on the modified symmetry index. Material and Methods: The study group comprised 11 healthy women and 11 healthy men from the University of Physical Education in Warsaw. The examination of the kinematic and kinetic parameters of sitting down and standing up from a chair was carried out using the Vicon Mx system and Kistler force platforms. Results: The modified symmetry index was employed to select the parameters used to choose those with the lowest variance. Consequently, the phases in a stand-to-sit-to-stand task were determined. Conclusions: It was demonstrated that the task of sitting down is a reverse task with respect to standing up in terms of the symmetry of the parameters analyzed.