Signals of motor command bias joint position sense in the presence of feedback from proprioceptors.

Joint position sense is believed to be mediated by muscle afferent signals. Because a "phantom" hand produced by a sensory and motor nerve block appears to move in the direction of voluntary effort, signals of "motor command" or "effort" can influence perceived joint position. To determine whether this occurs when sensory signals are available, three studies assessed position sense when motor command and afferent signals were available, but joint movement was prevented. First, the hand was positioned to stop movement at the proximal joint of the middle finger, and movement at the distal joint was impossible because the muscles had been "disengaged". Voluntary efforts produced illusory position changes in the direction of the effort (12.6 +/- 2.0 degrees distal joint; 12.3 +/- 2.3 degrees proximal joint for efforts at 30% maximum; means +/- SD). Second, when subjects attempted to move the index finger under isometric conditions, the index finger appeared to move 7.4 +/- 1.2 degrees in the direction of efforts. These illusions graded with the level of effort (10 or 30% maximum) and far exceeded any real joint movement. Finally, because changes in muscle afferent feedback might have accompanied the voluntary efforts, all forearm and hand muscles were completely paralyzed by locally infused rocuronium. During paralysis, passive wrist position was signaled accurately, but, during attempted efforts (30% maximum), perceived wrist position changed by 9.7 +/- 4.9 degrees . Before paralysis, isometric efforts changed it by 6.7 +/- 3.6 degrees . Thus all studies concur: when joint movement is prevented, signals of motor command contribute to joint position sense.