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

Yoshinobu Obata; Yinlai Jiang; Hiroshi Yokoi; Shunta Togo

cs.ROSep 24, 2026

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

Authors: Yoshinobu Obata, Yinlai Jiang, Hiroshi Yokoi, Shunta Togo

Organizations: Department of Mechanical and Intelligent Systems Engineering, The University of Electro-Communications, Tokyo, Japan · Center for Neuroscience and Biomedical Engineering, The University of Electro-Communications, Tokyo, Japan

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.

Figures & tables

Abbreviation	Definition
AP	Adductor pollicis
APB	Abductor pollicis brevis
APL	Abductor pollicis longus
BB	Biceps brachii
DRUJ	Distal radioulnar joint
DTM	Dart-throwing motion

Table 1

Component	Specification
Actuators	22 $\times$ Dynamixel XL330-M288-T
Wrist muscles	4 (FCR, FCU, ECRL/ECRB, ECU)
Forearm rotation muscles	4 (SP, BB, PQ, PT)
Finger and thumb muscles	14 (FDP $\times$ 4, FPL, EIP, EDM,
	14 ( ED, EPB, EPL, APL, APB, OP, AP)
Degree of freedom	3 (Wrist) and 13 (Fingers)

TABLE 1 : Anthropomimetic robotic wrist specifications

FIG. 2 : Developed anthropomimetic soft robotic forearm. (a) Assembled system comprising 28 bones actuated by 22 servo motors. (b) Fabricated anthropomimetic fingers with dual-layer elastic fingertips. (c) TFCC, the primary stabilizer of the distal radioulnar joint, implemented with four radioulnar ligaments and a TPU-printed fibrocartilage disc.

Condition	Activated Muscles	Functional Scenario
Relaxed	None	Passive wrist compliance
Wrist muscles	ECRL, ECRB, ECU,	Hand position stabilization
	FCR, FCU	(typing, tapping)
Finger muscles	FDP, ED, EIP, EDM	Object grasping,
		inertial motion
Combined	Wrist + finger	Stabilized object grasp

TABLE 2 : Muscle activation conditions

FIG. 8 : Wrist stiffness ellipses under the four muscle activation conditions for the three skeletal configurations. Panels (a) and (b) share the same axis range (

\pm 300

N/m), whereas panel (c) uses a twofold wider range (

\pm 600

N/m) to accommodate the higher absolute stiffness of the geometric ellipsoidal skeleton; ellipse sizes are therefore not directly comparable between (c) and the other two panels. Absolute stiffness values are listed in Tables 3 – 5 .

Condition	$\theta_{\mathrm{min}}$	$\theta_{\mathrm{max}}$	Eccentricity $e$	$K_{\mathrm{max}}$ [N/m]	$K_{\mathrm{min}}$ [N/m]
Relaxed	182.4°	275.9°	0.816	77.4	14.2
Wrist muscles activated	189.1°	97.4°	0.903	277.5	26.9
Finger muscles activated	151.7°	254.0°	0.431	140.7	80.1
Combined activation	197.9°	302.2°	0.418	156.1	90.8

TABLE 3 : Stiffness ellipse parameters for normal skeleton

Condition	$\theta_{\mathrm{min}}$	$\theta_{\mathrm{max}}$	Eccentricity $e$	$K_{\mathrm{max}}$ [N/m]	$K_{\mathrm{min}}$ [N/m]
Relaxed	187.6°	80.7°	0.443	44.0	24.5
Wrist muscles activated	197.9°	100.1°	0.707	131.6	38.6
Finger muscles activated	183.4°	58.6°	0.186	183.6	149.5
Combined activation	187.4°	301.0°	0.213	123.5	97.2

TABLE 4 : Stiffness ellipse parameters for fused proximal row skeleton

Condition	$\theta_{\mathrm{min}}$	$\theta_{\mathrm{max}}$	Eccentricity $e$	$K_{\mathrm{max}}$ [N/m]	$K_{\mathrm{min}}$ [N/m]
Relaxed	180.5°	86.0°	0.691	168.5	52.1
Wrist muscles activated	187.8°	96.5°	0.911	518.3	46.1
Finger muscles activated	185.2°	92.3°	0.559	282.2	124.4
Combined activation	189.3°	94.0°	0.648	397.7	140.0

TABLE 5 : Stiffness ellipse parameters for geometric ellipsoidal skeleton

FIG. 9 : Carpal bone kinematics and wrist joint stiffness under the three muscle activation conditions (wrist muscles, finger muscles, and combined), measured during 330° loading.

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