Extending Energy Optimization in Goal-Directed Aiming from Movement Kinematics to Joint Angles

ABSTRACT

Energy optimization in goal-directed aiming has been demonstrated as an undershoot bias in primary movement endpoint locations, especially in conditions where corrections to target overshoots must be made against gravity. Two-component models of upper limb movement have not yet considered how joint angles are organized to deal with the energy constraints associated with moving the upper limb in goal-directed aiming tasks. To address this limitation, participants performed aiming movements to targets in the up and down directions with the index finger and two types of rod extensions attached to the index finger. The rod extensions were expected to invoke different energy optimizing strategies in the up and down directions by allowing the distal joints the opportunity to contribute to end effector displacement. Primary movements undershot the farthest target to a greater extent in the downward direction compared to the upward direction, showing that movement kinematics optimize energy expenditure in consideration of the effects of gravity. As rod length increased, shoulder elevation was optimized in movements to the far-up target and elbow flexion was optimally minimized in movements to the far-down target. The results suggest energy optimization in the control of joint angles independent of the force of gravity.

Keywords:

• energy optimization
• goal-directed aiming
• joint angles
• tool use

ACKNOWLEDGMENTS

The authors would like to thank Emyl Smit for his technical assistance.

FUNDING

The authors would like to thank the Natural Sciences and Engineering Research Council of Canada for a grant obtained by Digby Elliott and James Lyons (NSERC).

Notes

1. The velocity profile in the primary direction of movement was used in the calculation of reversals. This is because the cumulative displacement profile involves the squaring of numbers and thus, cannot achieve negative values.

2. In two-way interactions, Bonferroni-adjusted pairwise comparisons were made for the simple main effects of one independent variable at each level of the other independent variable. When performed on SPSS, the p values for comparisons are adjusted by multiplying the unadjusted p value by the number of contrasts being made. Thus, significant mean differences in consideration of this adjustment are represented as p ≤ .05.

3. Lyons et al. (2006) also included forward and backward movements performed with the aiming apparatus positioned in a horizontal orientation. However, these movements were performed in a separate block of trials than the vertical movements.

4. The same direction by target interaction, F(2, 20) = 27.81, p < .001, η_G² = .18, and post hoc significant differences as shown in the Results section are also shown in an analysis of only the finger trials.