Study suggests 'hourglass' body shape ideal for sustained hula-hooping

An “hourglass” body shape might be the answer to sustained hula-hooping, a workout involving the twirling of a hoop around the body to keep it spinning, a study has suggested.

Researchers from the New York University in the United States have performed robotic experiments and shown that “keeping a hoop at a level requires a sloped surface with ‘hips’ and a curvy ‘waist'”.

Along with explaining a “familiar but poorly understood activity”, the findings, published in the “Proceedings of the National Academy of Sciences” journal, could help inform robotic applications, such as controlling and manipulating objects without gripping.

The authors have explained that mechanical systems, in which components make physical contact with each other through sliding or rolling, are common in engineering and everyday life.

Analysing such systems can, however, be complicated, especially when one of the components moves against another having a “complex surface shape” — which arises in familiar but poorly-understood contexts, such as hula-hooping.

Describing the activity as “a unique form of mechanical levitation against gravity”, the authors have set out to identify conditions that allow to stably suspend an object rolling around a gyrating body.

The team has combined results from robotic experiments and models, in which hoops were made to twirl around surfaces possessing varied shapes.

Simple body shapes, such as a cylinder and cone — over which motions of the hula hoop vary in a relatively straight forward manner — were first analysed to find out if levitation could be achieved.

However, these “failed attempts” motivated the authors to look at more “complex geometries”.

An hourglass-shaped body — in which “the hoop is released low on the body and thereafter, rises up to a height just below the ‘waist’ or narrowest point” — was found to successfully suspend the hoop.

While a vertical balance was found to be achieved only for bodies with “hips” or a “critical slope of the surface”, stability required an “hourglass shape with a ‘waist'”, with a curvature exceeding a certain threshold.

“By revealing and explaining the mechanics of hula-hoop levitation, these results motivate strategies for motion control via geometry-dependent contact forces and for accurately predicting the resulting equilibria and their stability,” the authors have written.