In the 19th century, figure skating was a very literal term: Athletes were expected to carve elaborate figures into the ice while maintaining their physical composure. Before long, innovative skaters began adding jumps to their routines, and the difficult aerial maneuvers quickly became the focal points of the programs.

Norwegian speed skater Axel Paulsen invented the axel in 1882. Of the six types of jumps now performed in the sport, the axel is the only one in which a skater takes off in a forward motion. In a single axel, the skater builds momentum leading to the jump, takes off from one skate's forward outside edge, turns 1.5 times in the air, and lands backward on the opposite skate's outside edge. The landing foot absorbs impact forces of eight to 10 times the skater's body weight. A double axel demands 2.5 rotations; a triple, 3.5. (Landing a triple is worth 8.5 points in a skater’s base score, while a double is only 3.3 points.)

An axel is a required element in today's skating routines—you can't win a medal without it. The top male skaters land triple axels, while top female skaters do doubles. Just a handful of women have completed a triple axel in Olympic competition—Japanese skaters Midori Ito in 1992 and Mao Asada in 2010 and 2014, and now American skater Mirai Nagasu at the 2018 Winter Games.

To the untrained eye, it looks like the human version of spinning a coin on its side. But the physics, athleticism, and preparation involved make it one of the most difficult efforts in the Games.

To perfect a double or triple axel, athletes and their coaches need to pay attention to biomechanics. Inertia, or the degree to which their body mass is spread out in space, will affect how fast they can spin in the air. By contracting their body as much as possible, skaters increase their chances for faster rotational movement. They also need to pay attention to mass—specifically, that their competition attire doesn’t weigh them down any more than necessary. Nagasu’s team reduced the number of rhinestones and even factored in the weight of the glue used to make sure the skater wasn’t going to be slowed down by even a few extra ounces of weight.

In landing the triple axel, Nagasu has also defied traditional thinking about an inherent disadvantage for women: Because they tend to have wider hips and chests than men, contracting their bodies for rotation is more difficult.

Perceived limitations in sports—like the four-minute mile, long thought to be impossible—are often broken by determined athletes who overcome pessimism with practice. While many skaters performed single axels in the early 20th century, American gold medalist Dick Button pushed the envelope by landing the first double axel at the Winter Olympics in 1948. Canadian Vern Taylor landed the first triple axel at the 1978 World Figure Skating Championships. In the 21st century, the world's elite skaters are landing quadruple backward-takeoff jumps.