The difference in physiological responses to ambulation effort, of above-knee (AK) amputees, using a prosthesis with open or locked knee mechanism was investigated at the Orthopaedic Rehabilitation Department of the Loewenstein Hospital. Of seventeen patients who were examined, fourteen were vascular patients over the age of fifty, and three were young traumatic amputees. Subjects performed an unrestrained walking test, using their prosthesis with an open-knee, and then with a locked knee joint mechanism. Heart rate and walking speed were measured, while oxygen consumption was evaluated only in the young patients. Results show that the use of a locked-knee joint resulted in a lower increase in heart rate in both groups. Comfortable walking velocity, as chosen by the subject himself, was higher with a locked-knee joint in the aged group and with an open-knee joint in the younger amputees. Oxygen consumption as measured for the younger patients was higher with an open-knee joint. In conclusion, a locked knee is recommended for the older amputees as it enables a higher walking velocity and lower effort. The preference of an open-knee in the young amputees is due to their better physical condition which enables a higher walking speed although energy expenditure and heart rate are higher.