Snake hips, good core strength and a desire to look a little silly are all good for those who want to master the art of hula hooping. Now scientists have found that an hourglass shape may be beneficial, too.
The study, which involves rotating robotic figures of varying dimensions, suggests that a narrow waist and large hips are optimal for keeping the hoop rotating in a stable position. The analysis concluded that those with less curvy bodies may be able to compensate by making larger hip movements at a faster pace.
“You need to throw the hoop fast enough as your initial condition. You also need enough body curvature. You also need enough body curvature,” said Olivia Pomerinck, a doctoral student at New York University and co-author of the analysis.
The research is the culmination of about five years of research by applied mathematicians at New York University, who began the investigation because “there is a fundamental question about how the collar actually stays in place.”
“It’s as if your body is just pushing the hoop out, and you can’t immediately understand what’s stopping it,” said Pomernick, who recently joined the project. “It is a seemingly simple system, but in reality it involves very complex mathematical operations.”
To address this puzzle, the researchers 3D printed a series of plastic shapes: cylinders, cones, and hourglass-like shapes (hyperbolic shapes) with different slopes and curvatures. “A cylinder has neither inclination nor curvature. A cone has inclination but no curvature; it has an angle but is a flat line,” said Pomerinck. An hourglass has inclination and curvature.
The figures were encased in rubber, mounted on rotating poles, and recorded using high-speed cameras as the hoops rotated around them.
Experiments indicate that three critical conditions must be met to keep the collar rotating reliably. First, the collar must be released with sufficient force.
“The first frustration beginners face is how to release the hoop to initiate rotation,” the paper says, “and our experiments similarly involve false starts in which the hoop loses contact with the body if the release speed is low.”
The study indicates that the best release strategy involves starting by shifting the collar and center of the body to the same side.
Analysis published in Proceedings of the National Academy of SciencesI found that it was below the critical waist-to-hip ratio and when the waist was too angular, rather than following a smooth curve, it was impossible to achieve consistent rotation.
In real life, hula hooping should be possible for most people, Bomernick said, because generally “you don’t have humans that don’t have an incline or a curvature.” Those with less ideal body shapes may need to rotate more forcefully to keep the hoop spinning. The study suggests that another option is to use a collar with a larger circumference. The weight of the collar seems to be less important in determining its dynamics.
According to the authors, the findings may have useful applications in how to harvest energy from vibrating objects or manipulate objects using rotational forces rather than gripping them.