Centre of Gravity of the Human Body Visualised with AI
When introductory physics curriculum introduces static equilibrium and torque, textbook examples often default to inanimate objects: rigid beams, uniform ladders leaning against frictionless walls, or seesaws. While mathematically sound, these examples lack an intuitive, physical connection for students.
By shifting the lens to the human body, educators can transform an abstract lesson on Center of Gravity (CoG) into an interactive, phenomenological study of personal balance and everyday biology.
The Core Physics: Center of Gravity and Torques
From a classical standpoint, the human body is a multi-segment system composed of rigid links (bones) moved by actuators (muscles). The net force of gravity acts on every individual molecule of mass distributed across these segments. However, we can simplify this distributed system into a single conceptual point: the Center of Gravity (CoG).
The position of the total body CoG represents the average location of the mass of all body segments, weighted according to their individual shares of total body weight.
In a standard standing anatomical position, a human's overall CoG lies approximately just behind the navel, or at about 55% to 57% of a person's total standing height.
The Conditions for Balance
To remain in static equilibrium without toppling, a body must satisfy two fundamental conditions:
- Translational Equilibrium: The sum of all vertical and horizontal forces acting on the body must equal zero. For a standing person, the upward normal force exerted by the ground must perfectly counter the downward gravitational pull of the body's total weight.
- Rotational Equilibrium: The sum of all moments or torques about any axis must equal zero.
The pivot point of greatest vulnerability to falling is situated at the outer boundaries of the feet. If a person's vertical line of action of weight—the projection straight downward from the CoG toward the floor—falls outside the margins of their feet, a net gravitational torque is generated. Lacking an intrinsic mechanism to push backward against empty air, the body rotates about the edge of the foot, resulting in a fall unless a corrective step is taken.
Base of Support (BoS) and Stability
The surface of contact made by the body with the floor is called the Base of Support (BoS). When standing on two legs, the BoS is the geometric boundary surrounding both feet, including the space between them. When standing on one leg, the base of support becomes smaller and balancing can only be achieved if the CoG is above the smaller BoS.
A person's stability can be quantified by how closely their projected line of action of weight approaches the boundaries of this BoS. If the line shifts near an edge, the stability margin decreases, necessitating minute, active counter-torques from the calves and ankle joints (known in biomechanics as the Ankle Strategy).
Kinesthetic Classroom Demo: The Wall Test
Have students stand with their heels and back completely flat against a wall, then ask them to attempt to bend forward at the waist to touch their toes without bending their knees. They will instantly begin to fall forward.
The Physics Explanation: Under normal conditions, bending forward forces the hips to shift backward behind the feet to keep the combined system's center of gravity safely centered above the feet. The wall mechanically prevents the hips from moving backward. As the upper body moves forward, the center of gravity shifts past the toes, violating rotational equilibrium.
Leveraging Real-Time Visualization in Physics Pedagogy
In this app, we utilize human pose estimations with the help of MediaPipe to visualizing a dynamic indicator for the CoG directly over a live video stream..
Key Concepts Visualized for Students:
- Dynamic Segmentation: Students see that when they raise their arms overhead, the total body CoG shifts upward, making them structurally more top-heavy.
- Mass Profiles: By toggling between standard anatomical models (Male, Female, or Unisex distributions), students analyze how varying body geometries alter mass concentration and stability thresholds.
- Postural Sway: Tracking the minute shifts of the CoG over time allows students to observe and analyze postural sway, effectively demonstrating how the body actively preserves balance under quiet standing conditions.