A gravity-balance device, which may be an orthotic support, has a support, a lever pivotally mounted on the support, a spring extending from the support to the lever, and a cam mounted on one of the lever and the support. The cam is positioned so that the spring wraps round the cam as the lever pivots, altering the effective point of action of the spring. A thing being supported can hang from the lever. The thing may be a sling supporting a human arm parallel to the lever, so that the spring compensates for the weight of the arm as the arm is raised and lowered. The cam may be a smooth curve, a polygon, or a row of pins at the corners of an imaginary polygon.
