The paper presents the prototype of a lower limb exoskeleton used during rehabilitation. Basic assumptions of the project are given. A numerical exoskeleton model was developed based on the finite element method. In order to limit the number of equations in the numeric task, separate models were developed to represent the main assemblies of the exoskeleton. Strength analysis is presented as an example of the ankle joint assembly. Internal loads have been studied in conditions of initiating contact of the patient's foot with the ground. The work contains strength parameters of the acrylonitrile butadiene styrene (ABS) with taken into account orthotropic properties. Parts of the exoskeleton which were made by 3D-printed technique were tested. The strength analysis was based on the results of measurements of the ground reactions recorded during the testing of the exoskeleton. All parameters which experimentally determined were implemented in the presented simulations. In addition, changes in the stresses during lower limb movement were investigated. The most important results of simulations are presented in a graphic form. The research method presented in this article can be used by mechanical device engineers who are not only designing technical equipment.