This paper introduce two main techniques used in developing rehabilitation engineering systems. Rehabilitation engineering system must support heavier loads and move at lower speed than other general robot systems. In this case, effect of gravity is dominant, so we define an anti -gravity index for comparing the performance of the system against gravity. This index made a choice of the frontal type of joint coordination. Secondly, advantage of differential gear is studied. In addition this paper reviews shortly the state of the art in rehabilitation engineering system for the disabled and the elderly people.