1988 Seoul, South Korea. Olympic diver Greg Louganis attempts a reverse two-and-a-half pike and hears a “big hollow thud…” his head hitting the springboard. [1] A few stitches and he returns to ultimately claim the Gold Medal. Not all of us are that lucky. Can physics help us understand the risks?
Diving board injuries are not always explicitly noted in our national statistics, but the estimates are about 12,000 injuries annually. The incidence of an injury resulting in paraplegia or quadriplegia is roughly 0.04/per 100,000 people. For comparison, the chance of being hit by lightning is 0.05/100,000. And that injury rate drops 10-fold to 0.004/100,000 when the pool is deeper than 4 feet. [2]
A group of researchers were interested in the physics of diving; after all, various seabirds and dolphins all dive head first into the water without apparent injury. Could an analysis of the impacts of diving provide some limits to what humans might attempt safely?
The research utilized a diving model, measuring the physical forces of entry into the water from heights using a variety of strain gauges and high-speed photography. More specifically, they looked at the forces based upon 3 “shapes.”
- Curved – like the shape of a dolphin’s head, a human head-first dive model
- Pointy – like the form of the gannet head, a seabird, modeling a human hands-first dive
- Flat – like the shape of the feet of a water-walking basilisk lizard, our human feet-first dive
Ideally, like in the Olympics, we enter the water smoothly, without a splash, allowing the forces of entry to dissipate slowly – our speed slows smoothly. The hand-first dive, the pointy dive of seabirds, is the exemplar. That slow dissipation of energy is the key. For example, in motor vehicle accidents, the sudden, rapid deceleration of impact causes our tissues to violently move back and forth, creating tears and other injuries. At the opposite end of the “smoothness” scale is the foot-first entry, where things slow down immediately, like a car accident, increasing the changing forces on our body. [3]
Hand-first diving protects the head and allows the slowest diffusion of impact energies. Feet-first diving creates the fastest distribution of impact energies primarily to the lower torso and, under the worst of circumstances, results in knee injury or fracture of the lower leg. Head first diving lies somewhere in between concerning the dissipation of energy, but that energy is being transferred to our most vulnerable areas, our head and spine. As a result, head-first diving is the source of the most significant diving injuries and fatalities.
Based on their modeling, the researchers suggest that head-first dives over a distance of 8 meters or more can be hazardous to your health. That critical distance increases to 11 meters for hands first and 15 meters for feet first. Personally, any diving by me is limited to about a meter, except for the iconic “cannonball,” in which I might increase my diving height by an additional foot.
Our animal friends have made adaptations to increase the safety of their diving. The gannets are more conically shaped and enter the water with a straight neck to dissipate forces down the spine. The dolphins have shortened and fused cervical vertebrae creating a stiffer neck, which can more safely dissipate the energy of impact.
[1] Here is the YouTube video of the accident
[2] Review of Spinal Cord Injury Statistics Related To Diving And Diving Board Use American Institutes for Research
[3] The medical rule of thumb is that it isn’t the fall that kills you; it is the stopping.
Source: Slamming dynamics of diving and its implications for diving-related injuries Science Advances DOI: 10.1126/sciadv.abo5888
