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

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The Journal of Physiology

Published online on

Abstract

•  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.