Inside the MuJoCo tendon hand model: bodies, joints, sites and sign conventions

contact@mulhamfetna.com (Mulham Fetna) — Wed, 16 Sep 2026 00:00:00 +0000

A fixed palm, five three-segment digits, five free-spinning servo horns, ten strings through seventy points, five motors. Every number here was read from the compiled model with MuJoCo 3.13.0.

Open (+50 N)

Neutral (0 N)

Fist (−50 N)

Index tendon only

Middle tendon only

Ring tendon only

Pinky tendon only

Thumb tendon only Previous Next

At a glance
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Quantity	Value	Notes
Bodies	22	world, palm, 15 phalanges, 5 servo horns
Joints / DoF	20 / 20	all hinges; no free joint — the palm is welded to the world
Knuckle joints	15	passive, 90° limits
Servo horn joints	5	unlimited and unactuated
Geoms	87	43 visual + 43 collision meshes + floor
Meshes	14	STL
Sites	70	tendon via-points and anchors
Tendons	10	5 flexor (actuated) + 5 extensor (passive)
Actuators	5	`<motor>` on flexors, ±50 N
Timestep / integrator	0.002 s / Euler	500 steps per simulated second
Total mass	0.398 kg	palm block 341.8 g

The kinematic tree
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flowchart TB
    W["world"] --> P["part_1 · palm + servo block
341.8 g · 20 sites"]
    P --> I1["part_2_4 · index proximal
4.6 g · index_mcp"] --> I2["part_3_4
1.8 g · index_pip"] --> I3["part_4_4
4.6 g · index_dip"]
    P --> M1["part_2_3 · middle
middle_mcp"] --> M2["part_3_3
middle_pip"] --> M3["part_4_3
middle_dip"]
    P --> R1["part_2_2 · ring
ring_mcp"] --> R2["part_3_2
ring_pip"] --> R3["part_4_2
ring_dip"]
    P --> K1["part_2 · pinky
pinky_mcp"] --> K2["part_3
pinky_pip"] --> K3["part_4
pinky_dip"]
    P --> T1["part_5 · thumb
5.4 g · thumb_cmc"] --> T2["part_6
2.4 g · thumb_mp"] --> T3["part_7
2.5 g · thumb_ip"]
    P --> H["servo_horn … servo_horn_5
0.3 g each"]

Body names are CAD part names, not finger names. Repeated instances get _2, _3, … appended, so the pinky chain is part_2 → part_3 → part_4 and the index chain is part_2_4 → part_3_4 → part_4_4. Address joints, sites, tendons and actuators in code — never bodies.

Joints and their signs
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Joint	Range (rad)	Bend direction
`index_mcp`, `index_pip`, `index_dip`	[−1.571, 0]	negative
`middle_mcp`	[−1.571, 0]	negative
`middle_pip`, `middle_dip`	[0, +1.571]	positive
`ring_mcp`, `ring_dip`	[−1.571, 0]	negative
`ring_pip`	[0, +1.571]	positive
`pinky_mcp`, `pinky_dip`	[−1.571, 0]	negative
`pinky_pip`	[0, +1.571]	positive
`thumb_cmc`, `thumb_mp`, `thumb_ip`	[−1.571, 0]	negative
`servo_*` × 5	unlimited	—

Bend direction is inconsistent, because each Onshape mate’s axis was exported as-is. Anything that reads joint angles must normalize per joint:

Reference on Mulham Fetna

Inside the MuJoCo tendon hand model: bodies, joints, sites and sign conventions

At a glance #

The kinematic tree #

Joints and their signs #

At a glance
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The kinematic tree
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Joints and their signs
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