Which Bodies does MuJoCo.Humanoid have

Question

Mujoco.Humanoid’s documentation says it has 14 bodies (nbody=14) https://gymnasium.farama.org/environments/mujoco/humanoid/ but I can only count 13 in the xml file https://github.com/Farama-Foundation/Gymnasium/blob/main/gymnasium/envs/mujoco/assets/humanoid.xml

        # bodies
        torso = 0
        lwaist = 1
        pelvis = 2
        right_thigh = 3
        right_sin = 4
        right_foot = 5
        left_thigh = 6
        left_sin = 7
        left_foot = 8
        right_upper_arm = 9
        right_lower_arm = 10
        left_upper_arm = 11
        left_lower_arm = 12

am I missing something, which is the 14th body

Thanks!

[Kallinteris-Andreas]

Damn it, Was really hoping the answer, would not be worldbody

[rodrigodelazcano]

Yes, this is another redundant observation. Reading the MuJoCo docs I found an explanation (https://mujoco.readthedocs.io/en/latest/computation.html?highlight=moment arms#actuation-model)

qfrc_actuator is the actuator force in joint coordinates, joint_moment_arm * actuator_force (including joints without actuators, nv size). This means that the free joint of the head is also included in the qfrc_actuator vector and will always be 0. Then there is this other attribute data.actuator_force which returns the actuator output torques (size nu) and is equal to the Gymnasium action since Humanoid uses motor actuators.

My suggestion is to keep qfrc_actuator in the observation space since the joint moment arms is relevant for extracting information about the bodies inertia but removing the elements related to the un-actuated free joint.