Effects of postural instability on the coordination between posture and arm reaching

Area of Science:

• Biomechanics
• Motor Control
• Human Movement Science

Background:

• Reaching movements from a standing posture rely on complex coordination of hand motion and body posture.
• Kinematic redundancy, the abundance of joint configurations for a given movement, is crucial for ensuring stability.
• Increased postural demands can potentially compromise the stability of reaching movements.

Purpose of the Study:

• To investigate how postural instability affects the utilization of kinematic redundancy.
• To examine the stabilization of finger and center-of-mass trajectories during reaching from standing under varying postural stability conditions.

Main Methods:

• Sixteen healthy adults performed reaching tasks under stable and unstable base-of-support conditions.
• Three-dimensional motion capture recorded marker positions at 100 Hz.
• Uncontrolled manifold (UCM) analysis quantified the use of kinematic redundancy for finger (ΔV_EP) and center-of-mass (ΔV_COM) stabilization.

Main Results:

• Finger trajectory stabilization (ΔV_EP) showed a dynamic pattern, decreasing mid-movement and increasing towards the end.
• Postural instability significantly reduced finger trajectory stabilization (ΔV_EP) in the latter half of the movement and at movement offset.
• Center-of-mass stabilization (ΔV_COM) remained largely unaffected by postural instability.

Conclusions:

• Postural instability appears to reduce the capacity to employ kinematic redundancy for stabilizing reaching movements.
• The central nervous system may prioritize maintaining postural stability over precise limb movement control when postural challenges arise.
• Findings suggest a trade-off between postural and focal movement control under destabilized conditions.