Anthropomimetic Soft Robotic Forearm with Independently Articulated Carpal Bones Enabling Human-Like Adaptive Stiffness Modulability

2026-09-24

AI Summary

This study addresses the unclear morphological mechanisms underlying adaptive stiffness regulation in the human wrist. We developed an anatomically accurate biomimetic soft forearm incorporating independent carpal bones, integrating multi-degree-of-freedom actuation with stiffness ellipse analysis to systematically characterize joint stiffness under diverse muscle activation patterns. Our findings demonstrate that carpal morphology predominantly governs stiffness modulation and elucidate the critical role of proximal carpal coupling in determining stiffness along the low dart-thrower’s motion (DTM) direction, successfully replicating human wrist stiffness characteristics. This work reveals the biomechanical principles governing stiffness regulation and establishes a novel paradigm for designing anthropomorphic dexterous robotic wrists.

Abstract

The human wrist exhibits adaptive stiffness modulability: joint stiffness anisotropy can be actively regulated through muscle co-contraction. This functionality is essential for stable manipulation, yet the underlying morphological factors remain unclear. To identify these factors, we developed an anatomically accurate anthropomimetic soft robotic forearm comprising eight independently movable carpal bones interconnected by ligaments, 22 actuated muscles, and compliant fingertips. We measured wrist joint stiffness under four muscle activation patterns across three skeletal configurations: anatomically normal carpal bones, a fused proximal carpal row, and a geometric ellipsoidal skeleton. The stiffness ellipse exhibited low stiffness along the dart-throwing motion (DTM) direction when finger muscles were activated, but high stiffness along the same direction when wrist and finger muscles were activated simultaneously. These results agree with previously reported human measurements, demonstrating that precise anatomical replication reproduces human-like stiffness modulability. Fusing the proximal carpal row eliminated the low DTM-direction stiffness under finger muscle activation, while the geometric ellipsoidal skeleton showed poor stiffness ellipse reorientation across all conditions. Carpal bone motion analysis revealed significantly opposing coupling patterns between wrist and finger muscles at the proximal carpal row, accompanied by a consistent but non-significant trend at the midcarpal joint, providing a mechanical explanation for this modulation. These findings demonstrate that carpal bone morphology plays a dominant role in human wrist stiffness modulation and provide design principles for humanoid robot wrists.

Problem

• wrist stiffness modulation
• carpal bone morphology
• anthropomimetic robotics
• adaptive stiffness
• joint stiffness anisotropy

Innovation

• anthropomimetic soft robotics
• adaptive stiffness modulation
• carpal bone morphology
• wrist biomechanics
• humanoid robot design