Your humanoid 3D model is stiff usually due to incomplete rigging, inaccurate weight painting, or too-tight bone rotation limits.
Incomplete rigging (e.g., missing spine or finger bones) cuts off natural movement. Poor weight painting makes the mesh deform unevenly instead of flowing with bones. Tight rotation limits—like elbows that can’t fully bend—force rigid poses.
To fix it, check your rig’s bone hierarchy (add small spine segments for flexibility) and use soft brushes for weight painting, blending larger areas (torso) before details (wrists).