2.5: Centre of Mass

The centre of mass is a fundamental concept in sports biomechanics, defined as the unique point at which a body or object is completely balanced in all directions. It is the point where the body's entire mass is concentrated, and it serves as the exact pivot around which gravity acts. In human movement, understanding the position of the centre of mass is crucial because it dictates a person's stability, balance, and flight path when they become a projectile.

Unlike a symmetrical solid object, such as a bowling ball, the human body is a complex system of segments that can change shape. Because of this, the centre of mass is not fixed; it shifts dynamically as an athlete moves. When a person stands in an upright, anatomical position, their centre of mass is located deep within the pelvic cavity, roughly at the level of the second sacral vertebra. However, if an athlete raises their arms above their head, the distribution of mass shifts upward, causing the centre of mass to rise toward the chest.

Remarkably, the centre of mass does not even have to remain inside the physical boundaries of the human body. When the body bends into a highly curved shape, the mathematical point of balance can shift completely outside the torso. Elite athletes exploit this phenomenon to gain a mechanical advantage. For example, high jumpers utilise the Fosbury Flop technique, arching their backs severely as they clear the bar. This extreme arching forces their centre of mass to pass underneath the bar while their physical body passes over it. Since the centre of mass's flight path is limited by the lift force generated at take-off, this technique allows athletes to clear a higher bar with less energy.

The position of the centre of mass also directly governs an athlete's stability, which is the ability to resist being unseated or knocked over. Stability is determined by the height of the centre of mass, the size of the base of support, and the location of the line of gravity. The line of gravity is an imaginary vertical line extending straight downward from the centre of mass to the ground. To remain stable, this line must fall within the base of support—the entire area of the ground bounded by the body parts in contact with the surface.

To maximise stability, an athlete will lower their centre of mass and widen their base of support. A rugby player preparing for a tackle or a wrestler defending a takedown will bend their knees and spread their feet wide. This low, wide stance ensures that even if an external force strikes them, their line of gravity is highly unlikely to be pushed outside their wide base of support. Conversely, when an athlete wants to initiate quick movement, they deliberately destabilise themselves. A sprinter in the blocks leans forward heavily, intentionally shifting their centre of mass right to the very edge of their base of support so that the slightest application of force will cause them to "fall" forward into rapid acceleration.