A deep understanding of the movement mechanism of the human fingers is essential for achieving a human-like actuation mechanism for anthropomorphic robotic hands. After carefully analyzing a set of anatomical theories of human fingers, we propose a novel and more reasonable actuation mechanism of the human index finger within the sagittal plane. This actuation mechanism can accomplish independent movement of interphalangeal (IP) joints and metacarpal (MCP) joint. We also consider hyperextension in this new actuation mechanism, which improves the completeness when interpreting the human actuation. We further exploit this actuation mechanism in our robotic finger design, which have accomplished all 33 static and stable grasping postures in the GRASP taxonomy.