Although remarkable progress has been achieved in video generation, synthesizing physically plausible human actions remains an unresolved challenge, especially when addressing fine-grained semantics and complex temporal dynamics. For instance, generating gymnastics routines such as “two turns on one leg with the free leg optionally below horizontal” poses substantial difficulties for current video generation methods, which often fail to produce satisfactory results. To address this, we propose FinePhys, a Fine-grained human action generation framework incorporating Physics for effective skeletal guidance. Specifically, FinePhys first performs online 2D pose estimation and then accomplishes dimension lifting through in-context learning. Recognizing that such data-driven 3D pose estimations may lack stability and interpretability, we incorporate a physics-based module that re-estimates motion dynamics using Euler-Lagrange equations, calculating joint accelerations bidirectionally across the temporal dimension. The physically predicted 3D poses are then fused with data-driven poses to provide multi-scale 2D heatmap-based guidance for the video generation process. Evaluated on three fine-grained action subsets from FineGym (FX-JUMP, FX-TURN, and FX-SALTO), FinePhys significantly outperforms competitive baselines. Comprehensive qualitative results further demonstrate FinePhys's ability to generate more natural and plausible fine-grained human actions.