Normal force distribution and posture of a hand pressing on a flat surface

Area of Science:

• Biomechanics
• Human factors engineering
• Ergonomics

Background:

• Understanding hand strength is crucial for predicting hand postures and finger loads during object interaction.
• Previous research has not fully explored the combined effects of hand posture and surface orientation on hand force exertion.

Purpose of the Study:

• To analyze the influence of hand posture (whole hand vs. fingertips) and surface orientation on hand force production.
• To investigate joint moments and loads across different hand postures and surface angles.

Main Methods:

• Twelve participants (6 males, 6 females) performed pushing exertions at varying force levels (10% to 100% Maximum Voluntary Contraction - MVC) against a plate at four angles (-45°, 0°, 45°, 90°).
• Hand postures included whole hand and fingertip-only contact.
• Inter-digit joint angles were recorded to estimate joint moments and map hand/finger motion.

Main Results:

• Participants exerted approximately twice the force with a whole hand compared to using only fingertips.
• Force distribution showed higher contribution from the palm (72-75%) than the thumb (11-13%) at 0°, 45°, and 90° angles.
• Males generated significantly higher maximum forces than females (49-62% higher) for most angles, except -45°.
• Thumb joint loads were consistently higher than other fingers, contributing 12% to total force during whole-hand exertions.
• Joint moments were 30% higher in fingertip conditions versus whole-hand conditions.
• Metacarpophalangeal (MCP) joint moments were greater than Distal Interphalangeal (DIP) and Proximal Interphalangeal (PIP) joint moments.

Conclusions:

• Hand posture and surface orientation are critical determinants of hand force output and distribution.
• Sex differences in maximal force production exist, influenced by surface angle.
• Thumb and specific finger joints (MCP) experience higher loads, particularly under fingertip conditions, highlighting potential areas for ergonomic consideration and injury prevention.