1. Recent studies have shown that the CNS uses proprioceptive information to coordinate multijoint movement sequences: proprioceptive input related to the kinematics of one joint rotation in a movement sequence can be used to trigger a subsequent joint rotation. In this paper we adopt a broad definition of 'proprioception,' which includes all somatosensory information related to joint posture and kinematics. This paper addresses how the CNS uses proprioceptive information related to the velocity and position of joints to coordinate multijoint movement sequences. 2. Normal human subjects sat at an experimental apparatus and performed a movement sequence with the right arm without visual feedback. The apparatus passively rotated the right elbow horizontally in the extension direction with either a constant velocity trajectory or an unpredictable velocity trajectory. The subjects' task was to open briskly the right hand when the elbow passed through a prescribed target position, similar to backhand throwing in the horizontal plane. The randomization of elbow velocities and the absence of visual information was used to discourage subjects from using any information other than proprioceptive input to perform the task. 3. Our results indicate that the CNS is able to extract the necessary kinematic information from proprioceptive input to trigger the hand opening at the correct elbow position. We estimated the minimal sensory conduction and processing delay to be 150 ms, and on the basis of this estimate, we predicted the expected performance with different degrees of reduced proprioceptive information. These predictions were compared with the subjects' actual performances, revealing that the CNS was using proprioceptive input related to joint velocity in this motor task. To determine whether position information was also being used, we examined the subjects' performances with unpredictable velocity trajectories. The results from experiments with unpredictable velocity trajectories indicate that the CNS extracts proprioceptive information related to both the velocity and the angular position of the joint to trigger the hand movement in this movement sequence. 4. To determine the generality of proprioceptive triggering in movement sequences, we estimated the minimal movement duration with which proprioceptive information can be used as well as the amount of learning required to use proprioceptive input to perform the task. The temporal limits for proprioceptive processing in this movement task were established by determining the minimal movement time during which the task could be performed. When the elbow reached the target position in <210 ms, subjects were no longer able to perform the task according to the instructions provided. 5. In an analysis of motor learning, we tracked the subjects' performances over 3 days. Accuracy was extremely high from the outset, and the only significant effect of practice was on the variable spatial error, a measure of the variability of the elbow position at which the subject opened the hand. Variable spatial error decreased over the 1st 30-40 trials in the 1st day of practice and remained at that level; however, variable error decreased only in the trials performed at the fastest of 7 velocities of elbow rotation. 6. We conclude that the CNS extracts and uses proprioceptive information related to both the velocity and the angular position of the joint in this movement sequence. Several heuristic models are presented to suggest possible strategies with which the CNS might process proprioceptive information to coordinate movement sequences. Our analyses of minimal movement times and motor learning indicate that proprioceptive triggering is potentially useful for movement sequences lasting at least 210 ms and that it does not have to be learned to be used during novel movement tasks.
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