PhyMotion: Structured 3D Motion Reward for Physics-Grounded Human Video Generation

Generating realistic human motion is a central yet unsolved challenge in video generation. Reinforcement learning (RL)-based post-training has emerged as a promising direction, yet its success critically depends on the quality of the reward signal. Existing video rewards primarily rely on 2D perceptual signals, without explicitly modeling the 3D body state, contact, and dynamics underlying articulated human motion, and often assign high scores to videos with floating bodies or physically implausible movements. To address this, we propose PhyMotion, a structured, fine-grained motion reward that grounds recovered 3D human trajectories in a physics simulator and evaluates motion quality along multiple dimensions of physical feasibility. Concretely, we recover SMPL body meshes from generated videos, retarget them onto a humanoid in the MuJoCo physics simulator, and evaluate the resulting motion along three axes: kinematic plausibility, contact and balance consistency, and dynamic feasibility. Each component provides a continuous and interpretable signal tied to a specific aspect of motion quality, allowing the reward to capture which aspects of motion are physically correct or violated. Experiments show that PhyMotion achieves stronger correlation with human judgments than existing reward formulations. When used for RL-based post-training, it consistently improves motion realism across both autoregressive and bidirectional video generators under both automatic metrics and blind human evaluation. Ablations further show that the three axes provide complementary supervision signals, while the reward preserves overall video generation quality with only modest training overhead.