Hand synergies during reach-to-grasp

Area of Science:

• Neuroscience
• Biomechanics
• Motor Control

Background:

• The central nervous system (CNS) may employ synergies to simplify complex motor tasks like hand manipulation.
• Prior research indicated that static hand postures in mimed grasps can be represented by principal components, with higher-order components offering meaningful, albeit small, variance explanations.

Purpose of the Study:

• To investigate if the entire reach-to-grasp action can be described by a limited set of postural synergies.
• To determine if these identified synergies are consistent across various grasp types and object interactions.

Main Methods:

• Five adults performed five types of reach-to-grasp tasks with 16 objects of varying shapes and sizes.
• Three-dimensional hand and wrist kinematics were captured using a multi-camera system.
• Singular value decomposition (SVD) analyzed temporal marker data to extract common postural patterns (eigenpostures) and their time-varying weights.

Main Results:

• The primary eigenposture accounted for an average of 97.3% of hand shape variance, representing a basic open-and-close hand movement during reach.
• A secondary eigenposture (1.9% variance) refined thumb and finger movements, crucial for grasp preparation.
• Eigenpostures and their temporal dynamics showed similarity across participants and grasp types.

Conclusions:

• Reach-to-grasp movements are largely governed by a fundamental postural synergy, with additional refinements in finger and thumb positioning.
• This suggests a synergistic control strategy in the CNS for efficient and adaptable hand manipulation.
• The identified eigenpostures provide a reduced, yet informative, representation of hand movement during grasping actions.