What is the minimum physical requirement to validate the pillar of Bowing (Ruku')?

The Pillars of Prayer Part 2: Bowing and Prostration Mechanics

• Bending the head slightly forward.
• Bending the back to the extent that both hands can reach and grasp the knees.
• Keeping the back perfectly parallel to the ground.
• Bending until the chest touches the thighs.