Berkeley's hula hoop study found the hoop's orbit and wobble are linked, not separate

UC Berkeley engineers say the two motions of a hula hoop, its orbit around the body and its up-and-down wobble, cannot be pulled apart into separate sets of body movements, contradicting about 20 years of earlier analysis. The study, published in Nonlinear Dynamics and announced by Berkeley on October 27, also found that the ankle and foot do more of the work than the researchers expected.

It is a small result about a 1950s toy, but it comes from swapping a standard data method for a less common one, and the team argues the same swap could improve models of how people walk with aids or how prosthetics and robots move.

Sensors built for earthquakes, strapped to a leg

Mechanical engineer Oliver O'Reilly led the work with postdoctoral researcher Chrystal Chern and Theresa Honein, a recent Berkeley Ph.D. now at the American University of Beirut. They fixed sensors that Chern normally uses to track model buildings during simulated earthquakes to the hoop, her right leg and her foot.

The sensors took 120 readings a second of rotation, acceleration, gravity and magnetic field. The hips were not tracked, because a sensor there would have hit the hoop. Chern, a former gymnast, was the hooper, and the Berkeley account describes data from her alone.

Why the old method gave a strange answer

Earlier studies had used principal component analysis, a linear technique that boils large data sets down to their strongest patterns. Those studies concluded that a back-and-forth motion drove the orbit and a separate vertical bounce drove the wobble.

When the Berkeley team ran the same kind of analysis on its own data, the numbers did not match. "We were really surprised," O'Reilly said, adding that the dominant results suggested the body and the hoop did not influence each other at all, which could not be right.

The team switched to functional network analysis, a nonlinear approach that Honein first came across through a Hamburg University of Technology researcher. This time the results made physical sense.

The thigh twist that ties it together

The new analysis shows the orbit is driven jointly by the thigh bone, the shin bone and the metatarsals of the midfoot. The thigh and shin twist like faucet knobs, the shin rises and falls as the legs bend, and the foot rocks from its inner to outer edge.

The wobble comes from the thigh bones rotating in their sockets. Because the thigh cannot turn that way without also twisting the shin, the movements behind the orbit and the wobble are bound together. That link is what earlier linear analyses missed.

With a single hooper and no hip data, the study does not show how much the pattern varies between people. The uses in rehabilitation, prosthetics and robotics are suggestions from the team rather than tested results.