A muscle exercise and diagnostic system which includes a lever arm, a mounting arrangement for mounting the lever arm for rotation about a fixed axis, and a connecting arrangement for connecting a selected portion of the human body to the lever arm for rotation with the lever arm about a selected anatomical axis of rotation. The connecting arrangement establishes a fixed tangential mounting relation and a sliding radial mounting relation between the lever arm and the body portion. This permits free radial movement of the connecting arrangement relative to the fixed axis during an exercise motion. A velocity control arrangement is coupled to the lever arm for limiting the maximum permitted rotational velocity to a value predetermined in accordance with a preselected velocity control function which includes measured values of the distances from the point of attachment to the anatomical axis and to the fixed axis. An arrangement is provided for inputting these measured values to the velocity control arrangement. One such arrangement continuously registers with a potentiometer the distance from the connection point to the fixed axis and supplies that registered distance value to the velocity control computer. The measured value of distance from connection point to anatomical axis is input by a manual dial connected to a potentiometer.