The study will be based on data from up to 15 trans-tibial prosthesis users. Participants will walk with their standard prosthesis, which will be equipped with a small sensor unit for the measurement of forces and moments during walking. The muscle activity of the thigh muscles will be measured using surface EMG sensors. All of the data collection will take place at the USR facilities (115 E Reindl Way, Milwaukee), where a multi camera motion analysis system is set up. Trials will require an overall time commitment of 5 hours at most, and will include normal walking, walking on carpet and gravel, walking up and down stairs, walking with fatigued thigh muscles.