Why is weight painting 3D model stiff when rig?

A 3D model feels stiff when rigged despite weight painting usually because vertex weights aren’t smoothly blended between adjacent bones. If weights are too concentrated on one bone (e.g., an entire forearm controlled by a single bone) or lack gradual transitions at joints (like elbows), the model can’t bend naturally—for example, an elbow won’t flex if forearm and upper arm weights don’t overlap. To fix this: - Use soft brushes to blend weights at articulation points (elbows, knees). - Ensure each vertex is influenced by 2–3 nearby bones. - Check for vertices assigned to only one bone (locked weights)—these are a common cause of stiffness.