Muscles are potent sensory organs. In Sherrington’s view, the sense they provide – proprioception – was private to the motor system and used for the purpose of controlling movement and posture. We know since the report of Goodwin et al. (1972) to the society that the signals from muscles also reach perceptual levels. We have learnt since then that much happens distal to the muscle, within the muscle and sensory receptors, and centrally to create an inconstant the relationship between actual force and movement and their representation within the CNS. How well muscle receptors report movement depends on how the movement is transferred to muscles. Both dead-band and elastic effects in tendon influence the transfer with perceptual and motor consequences (Refshauge et al., 1998; Chew et al., 2008). As many muscles cross two joints, the ambiguity in the response of stretch receptors alters the perception at the individual joints (Sturnieks et al., 2007). Contraction levels alter the recruitment and discharge levels of muscle spindles. This improves responsiveness at low contraction levels but this falls away at higher levels, creating a sensory-contractile interaction in weak muscles performing equivalent tasks to normal muscles (Butler et al., 2008). It is generally believed that the signal used to judge heaviness and applied force arises centrally through a corollary discharge from the motor command that drives the muscle (Gandevia & McCloskey, 1977). In a deafferented subject, this central mechanism is isolated and shown to have a gain of ~unity (i.e. fatigue to 50% makes an object feels twice as heavy). However in subjects with normal sensation the gain is much less. Golgi tendon organ firing adapts rapidly and to greater extents with high force. This property was exploited to show that the dominant signal driving perception arises in the periphery, not centrally, and the force reported by muscular receptors depends on the recent contractile history of the muscle. However, prolonged curarisation results in objects feeling lighter, an effect that is explained by central adaptation. With these levels of somotosensory inconstancy the accuracy of motor function observed requires a process that continuously calibrates sensory signals to the sensation of achieving the task.