A body-powered prosthesis does not rely on an outside power source and is operated via a system of cables, harnesses, and sometimes, manual control. Exaggerated movements of the body are used to control the prosthesis and these movements are captured by harness systems and cables. These systems then operate the hand, or terminal, device. Examples of these exaggerated movements are:
- Bi scapular abduction/adduction
- Glen humeral flexion to operate terminal device and elbow
- Shoulder depression to lock and unlock elbow
To effectively control this type of prosthesis the user must possess significant strength and control over various body parts, including the shoulders, chest, and residual limb. There must be sufficient limb length, musculature, and range of motion.
The functional envelope refers to where the prosthesis must be placed, in reference to the user’s body. When wearing a body-powered prosthesis, the functional envelope is limited to directly in front of the user, from waist level to mouth level. The user can no longer maintain control of the prosthesis when attempting to operate the prosthesis out to the side, down by the feet, or above the head.
Fine motor skills can be challenges with this type of prosthesis, because the user has little control over grip strength or speed. The benefits of this type of system are durability, weight and cost.