Thumb invariance during prehension movement: effects of object orientation

Area of Science:

• Biomechanics
• Human Motor Control
• Hand Function

Background:

• Understanding the distinct roles of the thumb and index finger in grasping is crucial for analyzing human prehension.
• Previous research suggests a stabilizing role for the thumb during reaching and grasping tasks.

Purpose of the Study:

• To investigate the specific contributions of the thumb and index finger during reaching to grasp a cylinder.
• To determine how object orientation influences the movement patterns of the thumb and index finger.

Main Methods:

• Defined a wrist-to-target axis for kinematic analysis.
• Quantified the perpendicular distances of the thumb and index finger from this axis across different object orientations.
• Analyzed hand pronation effects on finger positioning.

Main Results:

• The index finger consistently maintained a greater perpendicular distance from the axis than the thumb.
• Thumb's relative stability was confirmed, supporting its role as a movement guide.
• Index finger's perpendicular distance varied significantly with object orientation, notably increasing during pronation-required grasps.

Conclusions:

• The thumb acts as a stable guide during the reaching phase of grasping.
• The index finger dynamically adjusts its position to enable thumb stability and facilitate effective movement guidance.
• These findings elucidate the coordinated interplay between digits during object manipulation.