Physically speaking, hula hooping entails the steady, parallel oscillation (or periodic motion to and fro) of an unstable ring around a person's waist. Other examples of oscillating objects might include a swinging pendulum or a vibrating object. When you stand in the middle of a hoop, you become the center of the ring's rotation -- or the axis. As the axis, you also represent the source of the hoop's movement.
When you move your body to sling the toy around you, you exert a turning force called torque. This outward, parallel force is necessary to maintain the centripetal force, which keeps an object spinning around an axis. The exact force required depends on the size and weight of the hoop, as well as the size of your waist.
Inertia contributes a helping hand and enables the hoop to continue its angular momentum after your initial application of force, but not for long. As the hoop moves against your body and through the air, friction inevitably slows it down and causes it to fall. If you don't want gravity to win this force fight, you have to expend more effort to keep it going in regular pulses, staying just a little ahead of the spinning circle to aid in the ongoing momentum.
Of course, you don't need to understand the physics to actually do it. Hooping comes naturally to most of us, but from a biomechanical standpoint, it's a rather complex task. You won't find any hula-hooping robots out there and for good reason.
As with many physical activities that involve the coordinated use of multiple body segments, scientists are still working out exactly how hooping comes together in the brain. In 2004, a 15-page study in the journal Biological Cybernetics took a long, hard look at humans hooping and surmised that a great deal of the action comes down to concurrent oscillatory motion of the hips, knees and ankles [source: Balasubramaniam and Turvey].
A 2008 study in the journal Human Movement Science added that while all participants employed the same basic movement to maintain the hoop's rotation, the contribution from the hips, knees and ankles varied from person to person [source: Cluff et al.]. In other words, individual style and rhythm factor heavily into this activity.
Where did this practice come from? Circle up with us on the next page for a historical angle on the hula hoop.