twist and shout

# To understand the power of Simone Biles, you need physics

A physicist explains how Simone Biles is able to pull off a signature move.

__Arms held high above her head__, Simone Biles breathes out. And then she runs.

As she approaches the far corner of the sprung floor mat, she appears to take off into flight, hurling her body through three twists and two flips in midair, before coming down to land with barely a hair out of place.

Watching it, it is as if she is able to manipulate the principles of physics — and in a way, she is.

The stunt garnered ecstatic cheers from the crowd on Sunday during the U.S. Gymnastics Championships in Fort Worth, Texas, and set a new bar for the upcoming Tokyo Olympic Games. There, Biles will once more have an opportunity to show us what a true athlete can achieve.

Mere mortals like us might feel pleased with ourselves when we manage a cartwheel, but Biles makes even the most harrowing gymnastics stunts look easy. The 24-year-old American gymnast has several moves named after her, some so difficult other gymnasts won’t even attempt them. on Sunday, Biles pulled a new eponymous and unprecedented power move, shooting the top athlete further into the stratosphere.

See it for yourself — the jaw-dropping floor routine starts at 3 minutes into the footage:

## What is “The Biles?”

The Biles actually refers to a few different gymnastics feats, some of which have never been completed by any other gymnast. But the latest iteration is a variation on what is also known as a triple-double, a complex movement that requires the athlete to complete three twists and two flips midair. Biles is the first woman to ever manage such a move on the mat. By the Championships’ end, Biles added her record seventh U.S. title to her myriad laurels.

To say she is the greatest female gymnast of all time is not an understatement — indeed, she may just be the greatest, having shown her ability to pull off moves previously considered the preserve of male athletes, like the notoriously difficult Yurchenko double pike. (Biles herself occasionally wears leotards embellished with a goat.)

These heroic feats leave audiences speechless, but to understand them, we must turn to the thing Biles’ athleticism seems to bend: physics.

“Once she’s in the air, she is a projectile.”

David Young, a physicist at Louisiana State University who has investigated the physics of this sport before, explains that part of what sets Biles apart is her ability to act as the conductor to a symphony of factors that play into each movement.

“What sets her apart is her strength, balance, coordination, and muscle control that produce faster rotation rates once she’s in the air,” he tells *Inverse *after examining the footage of her latest feat.

Young makes a few back-of-the-envelope calculations:

- Biles launches into the triple-double at about 14.7 miles per hour from a 71-degree angle
- She reaches a peak of 9.5 feet and travels 6.5 feet from where she launched
- The impact force from the landing is 382 pounds of force, which is more than three times the gymnast’s weight

“To get high enough means that you have to raise your gravitational potential energy.”

This interview has been lightly edited for style and clarity.

**INVERSE****:**** Biles reaches a peak twice her own height during this move — how does she manage to produce the energy needed to launch that high?**

** David Young: **The sprint is to gain translational kinetic energy. In order for her to do the rotations when she's in the air, she has to be able to get high enough. And to get high enough means that you have to raise your gravitational potential energy. The first thing she does really well is to maximize her hangtime by choosing a launch angle that conserves the majority of her kinetic energy gained during her sprint. It’s that launch speed and angle that ultimately is going to determine how high she goes. And how high she goes then determines how long she’s in the air. So that’s going to set the time that she has to perform whatever kind of twisting motions and flips that she needs to get accomplished.

**How does Biles spin herself three times?**

Once she’s in the air, she is a projectile, and the only force that acts on her is her weight due to gravity. At this point, her angular momentum must be conserved. Her angular momentum depends on how fast she is rotating around each axis and how her body’s mass is distributed about each axis. Her twisting motion occurs about the axis that passes through the length of her body. She can control her rotation rate about this axis by repositioning her arms during the motion, thereby changing her moment of inertia and thus her twist rotation rate. Once she is in the air, she pulls her arms in and across the center of her body, decreasing her moment of inertia and increasing her rotation rate about her twist axis. She keeps her arms folded in close to her chest to maintain this higher rotation rate. Her knees remain bent, too. This maintains a high rotation rate about her flip axis.

**Would it be possible for her to complete even more twists or flips?**

Assuming her rotation rates around each axis remain constant, to get three full flips in would require an extra 0.65 seconds, which requires a launch speed of 22.6 miles per hour, all other things being equal. This is not possible, even if we assume her max launch speed is 18 miles per hour, which is apparently her top sprinting speed.

However, if she *could* do three full flips, she would also be able to get in one-and-a-half more twists at her current rotation rate! What would this even be called?! What might be more likely would be to try to gain an extra half twist so that she would take off facing left and land facing right, still only completing two full flips.

**Why does she use an extra pad for landing this movement?**

The impact force is a little more complicated. I can use the Impulse-Momentum Theorem to calculate the average force during impact. I assumed her change in momentum is just equal to her mass (52kg) times her launch speed (6.6m/s). I need the time it takes her to come to rest once she contacts the pad. From slowing the video down I got about 200 ms (milliseconds), which is reasonable. The impact time is made longer due to the extra foam pad they stick in the corner where she lands. That’s why they use the extra pad. This helps lengthen the time over which the impact force is dissipated. This gives an average impact force of about 1700 N(Newtons) or 382 lbs-force. This is over three times her weight. This is the average, so she will undoubtedly experience even larger impact forces during her landing.

**How is she capable of completing this move?**

One might say that Simone runs faster than other gymnasts, and so when she launches, she stays in the air longer and thus can do more moves in the longer hangtime. However, you don’t gain much from the launch speed. Even considering speeds well beyond her maximum, you don’t get a lot of extra hangtime. My guess is that her hangtime is not much different than other world-class gymnasts.

What sets her apart is her strength, balance, coordination, and muscle control that produce faster rotation rates once she’s in the air. She then needs the skill, timing, and training to know when to unwind all that rotational energy and stick the landing.