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Updated: Aug 16, 2026
07:25
An Objective and Child-friendly Assessment of Arm Function by Using a 3-D Sensor
Published on: February 12, 2018
Possible explanations for trajectory curvature in multijoint arm movements
1Department of Psychology, Faculty of Letters, Kyoto University, Japan. osu@erato.atr.co.jp
Summary
Human reaching movements are not always straight. This study suggests that planned movement paths are likely curved, challenging previous assumptions about motor control during reaching tasks.
Area of Science:
- Motor control
- Human movement science
- Biomechanics
Background:
- Human multijoint reaching movements are often assumed to follow straight paths.
- However, subtle curvatures are observed in certain workspace regions, questioning this assumption.
- Potential explanations include imperfect control, visual distortion, or arm dynamics interacting with straight trajectories.
Purpose of the Study:
- To investigate whether planned human reaching trajectories are inherently straight or curved.
- To test alternative explanations for observed movement curvatures.
- To determine the underlying nature of planned movement paths.
Main Methods:
- Participants were instructed to generate both straight and spontaneous movement paths.
- Movement paths were analyzed for straightness and curvature.
- Electromyography (EMG) was used to assess arm muscle activity and stiffness.
Main Results:
- Movements generated with instructions for straightness were significantly straighter than spontaneous movements.
- Spontaneous movements exhibited curvature even in the frontoparallel plane, where visual distortion is minimal.
- Straighter paths were achieved without increased arm stiffness, as indicated by EMG.
Conclusions:
- The observed curvatures are unlikely due to imperfect control, visual distortion, or arm dynamics.
- Findings suggest that planned human reaching trajectories are likely curved, not straight.
- This challenges the traditional view of straight-path planning in motor control.
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