Abstract: A system for detecting a limit of acceptable rotation for a joint is disclosed. The system includes a support that supports the joint and keeps its pivot point substantially stationary with respect thereto. The system also includes an extensible leg connected to the joint. A contoured member is mounted adjacent the extensible leg and has a surface shaped as a portion of a sphere. The contoured member is located so the pivot point of the joint substantially coincides with the center of the sphere and the periphery of the contoured member corresponds to the limit of acceptable rotation for the joint. Thus, as the leg pivots, the contoured member also moves a similar amount. A detector is used to detect the periphery of the contoured member surface and to assist in preventing further rotation of the joint in that direction.