Adaptive use of interaction torque during arm reaching movement from the optimal control viewpoint.

Van Hoan Vu,Brice Isableu,Bastien Berret

doi:10.1038/srep38845

Abstract

The study aimed at investigating the extent to which the brain adaptively exploits or compensates interaction torque (IT) during movement control in various velocity and load conditions. Participants performed arm pointing movements toward a horizontal plane without a prescribed reach endpoint at slow, neutral and rapid speeds and with/without load attached to the forearm. Experimental results indicated that IT overall contributed to net torque (NT) to assist the movement, and that such contribution increased with limb inertia and instructed speed and led to hand trajectory variations. We interpreted these results within the (inverse) optimal control framework, assuming that the empirical arm trajectories derive from the minimization of a certain, possibly composite, cost function. Results indicated that mixing kinematic, energetic and dynamic costs was necessary to replicate the participants’ adaptive behavior at both kinematic and dynamic levels. Furthermore, the larger contribution of IT to NT was associated with an overall decrease of the kinematic cost contribution and an increase of its dynamic/energetic counterparts. Altogether, these results suggest that the adaptive use of IT might be tightly linked to the optimization of a composite cost which implicitly favors more the kinematic or kinetic aspects of movement depending on load and speed.

Highlights

Considered unconstrained 3D arm pointing movements to a horizontal planar target
Kinematic costs are necessarily associated with a “compensation” strategy because they ignore interaction torque (IT) during motor planning while energetic and dynamic costs may fall within the “exploitation” category as they take into account IT at the planning stage
The experimental results showed that IT partly contributed to net torque (NT) thereby assisting the movement and that such contribution increased with limb inertia and movement speed

Summary

Introduction

Considered unconstrained 3D arm pointing movements to a horizontal planar target. This laboratory task can be thought as similar to pushing a door to open it or to the cup of tea example given above; in these tasks, there is no unique final hand position for their achievement[19,20]. If IT is compensated, such modifications should have non-significant effect on the arm trajectories, except that the MTs should be adjusted to cancel the larger ITs. Third, we aimed at accounting for the empirical pointing strategies via inverse optimal control with composite cost functions[21,22,23]. We hypothesized that any adaptive compensation/exploitation trade-off of IT could be associated to the minimization of composite cost mixing kinematic, dynamic or energetic quantities

Objectives

Methods

Results

Conclusion