Snake hips, good core strength, and a willingness to look slightly ridiculous all come in handy for those who want to master the art of hula hooping. Now, scientists have concluded that an hourglass figure could also be beneficial.
The study, involving gyrating robotic figures of different dimensions, suggests that a narrow waist and wide hips are optimal for holding a rotating hoop in a stable position. But those with less curvy physiques might be able to compensate by performing hip movements wider and at a faster pace, the analysis concludes.
“You have to throw the hoop fast enough as an initial condition. You also need sufficient body curvature. You also need a sufficiently tilted body,” said Olivia Pomerenk, a doctoral student at New York University and co-author of the analysis.
The paper is the culmination of about five years of research by applied mathematicians at New York University, who began investigating because “there is such a fundamental question about how the hoop actually stays in place.”
“It seems like your body is just pushing the hoop outward, you can’t immediately figure out what’s holding it back,” said Pomerenk, who joined the project more recently. “It’s a seemingly simple system, but actually involves some pretty complex math.”
To solve this mystery, the researchers 3D printed a series of plastic shapes: cylinders, cones, and hourglass-shaped figures (hyperboloids) with varying slopes and curvatures. “A cylinder has neither slope nor curvature. A cone has a slope but no curvature – it’s tilted but it’s a flat line,” Pomerenk said. An hourglass has both a slope and a curvature.
The shapes were covered in rubber, mounted on revolving poles and recorded using high-speed cameras while hoops spun around them.
Experiments suggest that three crucial conditions must be met for reliable hoop rotation to be maintained. First, the hoop must be thrown with sufficient vigor.
“An initial frustration encountered by novices is how to throw the hoop to initiate a twirl, and our experiments also involve false starts in which the hoop loses contact with the body if the throwing speed is low,” the paper said. .
The best throwing strategy is to start with the hoop and the center of the body moved to the same side, the document suggests.
The analysis, published in the Proceedings of the National Academy of Sciencesfound that below a critical waist-to-hip ratio and when the waist was too angular, rather than following a gentle curve, it was impossible to achieve stable rotation.
In real life, hula hooping should be possible for most people, Pomerenk said, because in general, “there are no humans without slope or curvature.” Those with less optimal body shape may need to spin more vigorously to keep the hoop spinning. Another option is to use a hoop with a larger circumference, the paper suggests. The weight of the hoop seems less important in determining its dynamics.
According to the authors, the findings could have useful applications in how to harvest energy from vibrating objects or manipulate objects using rotational forces instead of grasping them.