The contribution of motor commands to position sense differs between elbow and wrist

Knowing the position of our limbs is critical for accurate movement. Central motor command signals generated by the brain contribute to position sense at the human wrist, but this could not be demonstrated at the elbow. We tested whether this represents a fundamental difference between the two joints or whether it reflects the two different methods used to measure position sense. For both measurement methods, contraction of wrist muscles led to illusions that the wrist is displaced. No such illusions were detected at the elbow during muscle contraction. Thus, the contribution of centrally generated command signals to position sense differs between joints. Any contribution at the elbow joint is small and new methods will be needed to reveal it. Abstract Recent studies have suggested that centrally generated motor commands contribute to the perception of position and movement at the wrist, but not at the elbow. Because the wrist and elbow experiments used different methods, this study was designed to resolve the discrepancy. Two methods were used to test both the elbow and wrist (20 subjects each). For the wrist, subjects sat with their right arm strapped to a device that restricted movement to the wrist. Before each test, voluntary contraction of wrist flexor or extensor muscles controlled for muscle spindle thixotropy. After relaxation, the wrist was moved to a test angle. Position was indicated either with a pointer, or by matching with the contralateral wrist, under two conditions: when the reference wrist was relaxed or when its muscles were contracted isometrically (30% maximum). The elbow experiment used the same design to measure position sense in the passive elbow and with elbow muscles contracting (30% maximum). At the wrist when using a pointer, muscle contraction altered significantly the perceived wrist angle in the direction of contraction by 7 deg 3 deg, 12 deg (mean 95% confidence interval) with a flexor contraction and 8 deg 4 deg, 12 deg with an extensor contraction. Similarly, in the wrist matching task, there was a change of 13 deg 9 deg, 16 deg with a flexor contraction and 4 deg 1 deg, 8 deg with an extensor contraction. In contrast, contraction of elbow flexors or extensors did not alter significantly the perceived position of the elbow, compared with rest. The contribution of central commands to position sense differs between the elbow and the wrist. We are constantly aware of the position of our body in space because proprioceptors in our muscles and skin provide information about the position and movement of our limbs, as well as the forces involved in moving them. This information is not only important for self-awareness in our surroundings, it is also needed to control movements accurately. The key role played by proprioceptors in motor control is demonstrated in rare cases when proprioceptive afferents from muscle, joint and skin are lost and normal movement control is no longer possible (e.g. Cole, 1995; Hermsdorfer et al. 2008). Although the question of which signals contribute to the sense of joint position and movement has been debated many times (e.g. McCloskey, 1978, 1981; Gandevia, 1996), the general view over the last 30–40 years has been that the sense is derived from information provided by peripheral receptors. A key study by Goodwin et al. (1972) showed that muscle spindles were important contributors to limb position sense. Skin stretch receptors also contribute (Edin Collins et al. 2005). Joint receptors are unlikely to be important, except as limit detectors of movement (Burgess see also et al. will be the can in position sense. As as the arm muscles the command to the of the contraction the command as to alter the of In a task, such as the arm be to be for different limb under different can be to the experiments at the the an directly position signals at the wrist, but not at the in the are and used in motor Thus, about the position of the be muscles will have a motor command to and a motor command be in It could also be that the muscles of the information from central commands because muscles to at times muscle spindle information et al. see also Goodwin et al. This be used for wrist muscles elbow muscles as both a of joints. the flexors and extensors many joints and from an there be an the will have to the of the in the of a constantly be at the same position signals were from elbow We have that centrally generated command signals contribute to joint position sense at the wrist under in which the muscles are in a experiments at the elbow did not reveal an of position sense. This not that motor command signals are not used at the their is to be this for is as a it will a of different joints. the experiments have is an difference in the of central and peripheral proprioceptive information at two joints. have our of proprioceptive as movements muscle this has contributed to of the experiments were at of in experiments were at both of and This was by the and We are to for the