Angular momentum is the quantity of rotation a body or system possesses. For a rigid body, it equals the moment of inertia multiplied by the angular velocity. Moment of inertia describes how mass is distributed around the axis of rotation: mass held farther from the axis gives a larger moment of inertia.
Angular momentum changes only when a net external torque acts on the system. This leads to one of the most useful principles in sports biomechanics: once an athlete is airborne, and air resistance is negligible, total angular momentum about the center of mass stays constant.
Practical Consequences
- Angular momentum for a jump or dive is generated mainly during ground or board contact, through the torque produced by the ground reaction force about the center of mass
- In the air, tucking reduces moment of inertia and increases rotation speed; opening out slows it
- An athlete cannot create or remove total angular momentum in flight, but can redistribute it between body segments
- Asymmetric arm or hip movements in the air can tilt the body and contribute to twisting
These ideas apply to diving, gymnastics, figure skating, trampoline, freestyle skiing, long jump, and high jump, as well as to swings and throws in which angular momentum is transferred between segments.
Observing It in Video
Video can show the visible effects of angular momentum. Frame-by-frame review can measure how many degrees a body or segment rotates between frames, giving an estimate of angular velocity during tucked and open positions. Comparing takeoffs side by side can help connect the position at takeoff with the rotation that follows in flight.
Limits and Pitfalls
- Calculating angular momentum requires estimates of segment masses and moments of inertia, which video does not provide directly
- Rotation out of the camera plane is difficult to quantify from a single 2D view
- Motion blur and low frame rates can make fast rotations hard to measure
- Faster visible rotation in the air reflects body shape as well as the angular momentum created at takeoff
Angular momentum explains why takeoff quality often determines what is possible once an athlete leaves the ground.