The interaction of biomechanical demands and the speed-accuracy trade-off for the control of multi-directional, three-dimensional targeted reaching movements

The performance of targeted reaching movements can be influenced by speed-accuracy demands such as the distance to the target and biomechanical demands based on target direction. Therefore, task difficulty for whole-arm reaching movements may be influenced by both speed-accuracy and biomechanical demands. This study aimed to examine the interaction of biomechanical factors such as directional inertia and joint movement demands based on variations in target direction and speed-accuracy demands based on variations in target distance on three-dimensional reaches that required endpoint accuracy. Ten right-handed, young adults (5 female, 27.1 ± 3.4 yrs) completed targeted reaches to inter-target combinations in a circular array (Center-Out & Out-Center, 10 cm reach distance, index of difficulty (ID) = 2.78; Long/Diameter, 20 cm reach distance, ID = 3.78) with both the dominant and non-dominant arms in a single session. As predicted by Fitts' Law, movement time increased with increased target distance in both arms (p < 0.05). However, movement times for reaches with the same ID varied with reach direction in both arms (p ≤ 0.05). Directional effects were also found for peak acceleration and joint excursion (p < 0.01) in a manner suggesting that both inertia and joint coordination demands influenced movement time for reaches with the same speed-accuracy demands. In the non-dominant left arm, movement time varied based on start position (p < 0.05), suggesting that the initial shoulder-elbow configuration influenced reach times. Future studies should consider the effects of inertia and limb coordination demands in addition to ID when incorporating and comparing multi-directional reaches in experimental designs.