The effects of isometric and isotonic force production on perceived static joint position were investigated in 12 adult subjects. The joint examined was the proximal interphalangeal joint of one index finger, and its perceived position was determined from matching movements of the equivalent joint on the other hand. When the perturbed joint was free to move, even when supporting substantial loads, the position was accurately estimated; however, when the subject was required to exert substantial isometric force against the device imposing the joint movement, significant errors occurred: these errors were in the direction of the increasing force. Similar effects were evident during increasing isometric flexion force in anaesthetized fingers. It is suggested that force-related afferent discharge from muscle, presumably originating in tendon organ receptors, contributes to static joint position sense. This force-feedback may allow the nervous system to accommodate for the effects of changing fusimotor bias, but it also appears to induce errors when afferent information of force and length provide potentially conflicting information.