Berkeley Study Finds Hula Hooping Runs on Your Shin and Foot, Overturning 20 Years of Thinking
Sensors and nonlinear math show the hoop's orbit and wobble are coupled, not controlled separately, with the ankle and foot playing a bigger role than expected.
Keeping a hula hoop spinning looks like a hip trick. A new study from the University of California, Berkeley says it is really a coordinated effort running from the thigh through the shin to the bones of the foot, and that it upends roughly 20 years of conventional thinking about how the motion works.
The paper, 'The symphony of gyrations producing steady motions of a hula hoop: insights from a network analysis,' appeared in the journal Nonlinear Dynamics. Oliver O'Reilly, a Berkeley professor of mechanical engineering who has also studied why shoelaces come untied and how the droid BB-8 rolls, led the project. Chrystal Chern, a structural engineering postdoctoral researcher and former gymnast, did the hooping while sensors recorded. Theresa Honein, a recent Berkeley Ph.D. graduate now an assistant professor at the American University of Beirut, brought in the key analysis method.
Earlier studies used principal component analysis, a linear technique, to sort hula-hoop motion into patterns. They concluded that two separate sets of movements controlled the hoop: a back-and-forth motion driving its orbit around the body and a vertical bounce producing its wobble. The Berkeley team set out to build on that work, attaching a specialized sensor to the hoop, the right leg and the foot. It captured 120 readings per second of rotational velocity, acceleration, gravitational force and magnetic fields. The hips could not be tracked because the sensor would have struck the hoop.
When the team tried to reproduce the earlier results with its own data, the numbers did not agree. The linear analysis suggested that body motion and hoop motion did not affect each other at all, which the researchers judged impossible. They reanalyzed the data with functional network analysis, which relies on nonlinear math, and the results became consistent.
The new picture is different. The hoop's orbit is produced jointly by the femur, the tibia and the metatarsals. The femur and tibia twist together, the tibia rises and falls as the legs bend and straighten, and the metatarsals shift as weight rolls between the inner and outer edges of the foot. The wobble is linked to thigh rotation in the hip sockets. Because moving the femur also twists the tibia, rotation and wobble are coupled, and cannot be cleanly separated, contradicting the earlier view that they are controlled independently.
O'Reilly said the ankle and foot played a larger role than the researchers expected. 'We were really surprised,' he said of the first results.
The team stressed that the study is not mainly about toys. Its methods could lead to better models of how people walk with a walker, helping physical therapists focus on the movements that matter most during rehabilitation. Honein noted that similar motion analysis could also inform prosthetic design and robotics. O'Reilly called the work 'a wonderful collaboration that started out with curiosity' and grew into something much larger than the team anticipated.
