On Earth, humans break from a walk into a run—where both feet are lifted off the ground—when their speed reaches about 2.1 meters per second. Calculations predicted that the transition speed would be lower on the moon due to the reduced gravity, at about 0.86 meters per second, but lab simulations have suggested higher values than that. Short of flying a crew to the moon, the only way to test things out in lunar gravity is by hopping onto NASA’s adapted DC-9 aircraft that can fly a roller-coaster path—so the researchers did just that. During each cycle on the so-called vomit comet, volunteers had 20 to 30 seconds to test out varying walk and run speeds on a treadmill (similar to what’s pictured) when the gravity on them fell to one-sixth of that on Earth. The results confirm that the walk-to-run transition speed is indeed higher than predicted by theory, at an average of 1.42 meters per second. This is possibly due to the additional force that swinging arms and legs exert on the body, as the effect of such force is more pronounced in lunar gravity, the team reports this week in The Journal of Experimental Biology. The researchers hope such studies could lead to more agile space suits.