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.

Coordinated Turn-and-Reach Movements. I. Anticipatory Compensation for Self-Generated Coriolis and Interaction Torques

“…It has been proposed that internal representations of limb dynamics within the CNS are used in the predictive compensation of forces arising from intersegmental dynamics (Beer et al 2000;Krakauer et al 1999;Sainburg and Kalakanis 2000;Sainburg et al 1993Sainburg et al , 1999Topka et al 1998). In a study of mechanisms underlying the control of interaction torques during reaching movements, Sainburg et al (1999) found that once subjects adapted to reaching with a load that altered their effective forearm inertia, they produced large errors in subsequent reaches to the same target with a new load configuration.…”

“…In a study of mechanisms underlying the control of interaction torques during reaching movements, Sainburg et al (1999) found that once subjects adapted to reaching with a load that altered their effective forearm inertia, they produced large errors in subsequent reaches to the same target with a new load configuration. Open-looped forward simulations using the muscle torques calculated from the adapted trials predicted the initial errors in movement direction.…”

Conclusions on motor control depend on the type of model used to represent the periphery

“…Not all researchers use a model that includes an elaborate description of the intrinsic muscle properties (Sainburg et al 1999;Scheidt et al 2005;Shadmehr and Mussaivaldi 1994). This raises the question to what extent conclusions on motor control depend on the level of simplification with which the dynamics of the periphery are modeled.…”

“…The simplest way to model the periphery is by neglecting all neural and muscle-tendon properties, resulting in a torque-driven model: motor commands prescribe directly the joint torques over time (e.g., Kawato 1990;Sainburg et al 1999). The simplest way to take into account the viscoelastic properties of the muscles and tendons is to describe the periphery as a second-order time-invariant linear massspring-damper system (e.g., Scheidt et al 2005;Shadmehr and Mussaivaldi 1994;Thoroughman and Shadmehr 1999).…”

Inter-Joint Coordination Deficits Revealed in the Decomposition of Endpoint Jerk During Goal-Directed Arm Movement After Stroke

“…14 Internal forces include those produced by "interaction forces" (i.e., interaction torques) imposed on each limb segment by motion of the segments attached to it. 14 It has been shown previously that shoulder and elbow torques are tightly coordinated in healthy planar reaching movements 15,16,17 and 3D arm movements 18 such that interaction torques are wellcompensated. 19 Failure of the nervous system to properly compensate for multi-joint interaction torques can lead to increased movement curvature and the desynchronization of motions at the shoulder and elbow joints.…”