Why speed skaters swing their arms side-to-side instead of forward and back like runners

Speed skaters swing their arms backward and to the side rather than forward and back like sprinters.
The side-to-side movement of the arms in related to how speed skaters push off the ice.
Speed skaters push off in a diagonal motion meaning the arms must move from side-to-side to prevent the body from twisting.

Speed skating is a fast-paced sport where athletes whip around an oval track at speeds of up to 40 mph. If you're watching the Winter Olympics, you may notice that at the start of the race, skaters swing their arms backward and to the side, rather than forward and back like sprinters on a track. As the skaters pick up speed, they rest their arms on their back instead of pumping them.

The simple explanation for these arm movements is related to the direction of the leg and foot motion in skating, which is a little different than the technique used in running.

In running, an athlete propels herself forward by pushing off in the direction opposite the running direction. The right arm moves forward as the right foot pushes straight back. At the same time, the left arm moves straight back as the left foot moves forward.

"The forward and back motion of the right arm is in exactly opposite to that of right leg, and similarly for the left arm and leg," George Tuthill, a physicist at Plymouth State University, who conducted research in statistical mechanics for over 30 years, told Business Insider.

In speed skating, the only way to propel yourself forward is to push off the ice in a diagonal motion, rather than straight back.

"If a skater tried to run on an ice surface, by trying to push off the ice in a direction parallel to the skate blade, they couldn't do it because there is almost no friction between the ice and skate blade. They would just slip and not go anywhere," Franco Normani, who runs the site Real World Physics Problems, said via email.

The skater has to push off the ice with a force that is perpendicular to the skate blade, which is positioned horizontally (angling outward and forward) on the ice.

"Speed skating is a unique way of human locomotion, in the sense that forward velocity is achieved by sideward push-offs," researchers explained in the International Journal of Sport Biomechanics.

But after the first few strokes, "the skaters use little or no arm motion on the straights, usually their arms are behind their backs, but their upper bodies are nearly horizontal," said Tuthill.

The skater crouches forward (as if he was skating in a squat position) to reduce drag caused by air resistance on the front of his body, and to help him balance.

Resting the arms on back is mostly an energy-saving technique. Arms are also not as in important as they are in running because skaters are using the ice, rather than just their own bodies, to maintain momentum, according to Yahoo! Sports' Jay Bushee.

In longer races, speed skaters may place both arms on their back and swing only the outer arm on the curve, which helps with balance.

"When you're swinging both arms, you're using twice as much energy," former speed skating Olympian Dan Jansen told Newsday. "So when you want to save a little energy, for later in the race, you can put one arm on your back."

The swinging arms may come back out during the final moments of the race to give the skater an extra boost for the finish.