Intersegmental dynamics are controlled by sequential anticipatory, error correction, and postural mechanisms

Abstract

The purpose of this study is to examine the mechanisms underlying control of intersegmental dynamics during reaching movements. Two experiments were conducted to determine the relative contributions of anticipatory and somatosensory feedback mechanisms in controlling intersegmental dynamics and whether adaptation to novel intersegmental dynamics generalizes across a range of movement directions. The mechanisms used to control interaction torques were examined by altering the inertial load of the forearm. Movements were restricted to the shoulder and elbow and supported on a horizontal plane by a frictionless air-jet system. Subjects made rapid out-and-back movements over a target line presented on a computer screen. The screen cursor disappeared at movement onset, and hand paths were displayed after each movement. After subjects adapted to a novel inertial configuration, the position of an attached mass was changed on pseudorandom trials. During these "surprise" trials, movements were initiated with the torque patterns appropriate to the previously learned inertial condition. As a result, characteristic errors in initial movement direction were predicted by an open-looped forward simulation. After these errors occurred, feedback mediated changes in torque emerged that, surprisingly, further decreased the accuracy of movement reversals. Nevertheless at the end of movement, the hand consistently returned to the starting position. It is plausible that the final position was determined completely by feedback-mediated changes in torque. In a second experiment, adaptation to a novel inertial load during movements made in a single direction showed limited transfer across a range of directions. These findings support and extend those of previous reports, which indicated combined anticipatory and postural mechanisms to coordinate rapid reaching movements. The current results indicate a three-stage control system that sequentially links anticipatory, error correction, and postural mechanisms to control intersegmental dynamics. Our results, showing limited generalization across directions, are consistent with previous reports examining adaptation to externally applied forces and extend those findings to indicate that the nervous system uses sensory information to recalibrate internal representations of the musculoskeletal apparatus itself.

FIG. 1.

Experimental set-up: $X$ and $Y$ represent axes of coordinate system originating at shoulder. Shoulder and elbow angles were measures as $\theta$ and $\varphi$, respectively. After each trial, the hand path was displayed on the computer screen, each circle representing the hand position every 25 ms.

FIG. 2.

Hand path (left) and tangential velocity (right) profile for a typical trial. Trials were segmented into 3 temporal phases: outward, inward, and reversal. The portion of the hand path occurring within the reversal phase is shown in bold.

FIG. 3.

Typical hand paths during adaptation to the medial load. Gray circles indicate the “start” location.

FIG. 4.

A: typical hand paths after adaptation to the medial load (left), for a surprise trial (center), and after adaptation to the lateral load (right). B: elbow and shoulder joint angles corresponding to the hand paths in A. Time of direction reversal at each joint is marked by arrows. Interjoint coupling time, between these reversals, is denoted by $\Delta t$. Cross hairs represent the joint angle measured at $V{\max }_1$. Dashed lines allow comparison of these angles across the trials.

FIG. 5.

Mean and SE of elbow (top) and shoulder (center) excursions measured at $V{\max }_1$ (left) and of the interjoint coupling interval (bottom), measured as the time between the direction reversals at each joint ($\Delta t$ in Fig. 4B). Individual mean and SEs for each subject are shown separately and marked with the subjects’ initials in the legend. Grouped mean and SE ofthese average values is indicated by the bars.

FIG. 6.

Elbow (top) and shoulder (bottom) joint torques from the trials shown in Figs. 4 and 5. Interaction, net, and muscle torques are shown separately at each joint. Reversal phase is indicated by the gray rectangle drawn across each set of torque profiles.

FIG. 7.

A: actual compared with simulated hand paths. Simulated surprise trial (right) was calculated using the muscle torques from the adapted medial load trial (left). An actual surprise trial is overlaid on the simulated trial (right). B: measured initial direction and reversal errors for actual and simulated trials. Individual mean and SEs are shown separately and marked with the subjects’ initials in the legend, while grouped means and SEs are represented by the bars.

FIG. 8.

A: averaged hand paths from a single subject who trained with the 126° target. All 10 trials from each condition shown were synchronized to peak tangential hand velocity $(V{\max }_1)$. Solid lines, averaged lateral mass trials from the lateral mass training session; dotted lines, averaged lateral mass trials from the the medial mass training session. SE bars for the $x$ and $y$ dimensions are shown every 100 ms. B: difference in mean error between medial and lateral training sessions averaged across all subjects. Black, data from subjects who trained with the 126° target; gray, data from subjects who trained to the 90° target. Data from trials performed with the medial load are indicated with circles, whereas data from lateral load trials are indicated with squares.