Kinesthetic control of a multijoint movement sequence

The individual joint rotations of a movement sequence might be controlled either by a central motor plan or by motion-dependent (i.e., kinesthetic) sensory input. Most previous research has focused on how the nervous system uses central motor plans to control movement sequences. This study examined how the nervous system uses kinesthetic input to control a multijoint movement sequence. Human subjects were trained to extend the elbow horizontally at 22°/s and to open the hand as the elbow passed through a 2°-wide target zone. Different distances to the target zone were used to examine a wide range of movement times of the elbow to target zone (i.e., 150-1,500 ms). A hydraulic apparatus simulated a spring resistance to the elbow extension. In some trials, the spring constant was unexpectedly increased or decreased just before the subject initiated the elbow extension, causing the elbow to slow down or speed up. Because these changes in spring constant were randomly imposed and because no visual feedback was available, subjects had to use kinesthetic input to control this motor task. The experimental subjects employed two different strategies for the use of kinesthetic input to control this motor task. In the first strategy, the subjects used kinesthetic input related to the elbow rotation to detect and correct velocity errors caused by the changes in spring constant. The onset of error correction varied between 92 and 196 ms after the appearance of velocity errors. The proportion of the error corrected by the time the elbow reached the target zone varied between 31 and 78%, depending on the movement time to the target zone. However, because this correction for velocity errors was neither instantaneous nor complete, the changes in spring constant caused leads and lags in the time that the elbow reached the target zone. In the second strategy, subjects used kinesthetic input related to the elbow rotation to advance or delay the onset of the hand movement, thereby compensating for leads and lags in the arrival of the elbow at the target zone. These adjustments in the triggering time of the hand movement allowed subjects to open the hand while the elbow was in the target zone. This kinesthetic triggering mechanism was effective for elbow rotations reaching the target zone within 150-1,500 ms. These results suggest that, to fully understand how multijoint movement sequences are controlled by the nervous system, sensory mechanisms must be considered in addition to central mechanisms.