This video shows a scene from the Apollo 16 mission to the Moon, April 1972. Astronaut John Young does a leaping salute to the flag. Note how long it takes him to return to the ground. By displaying the 30 fps video frame by frame, we’re able to calculate that he was “airborne” for a total of 1.4 seconds. That’s a long time for a vertical jump in a semi-flexible spacesuit with a bulky life-support backpack.
On Earth, that same jump under the same constraints would return you to the ground in only 0.23 seconds. That’s because Earth’s gravity is 6X stronger than the Moon’s. Turns out that has a huge affect on human locomotion. Walking on the Earth is very different from walking on the Moon.
The following video shows a scene from the Apollo 17 mission, Dec 1972. Astronaut Harrison “Jack” Schmitt is returning to the lander after deploying scientific instruments. His locomotion is clearly not normal “walking” but more of a “skipping/hopping” action:
There’s a good reason for this, and it’s based on the physics of walking. The process of walking can be described as a controlled fall. When you walk, you extend a leg forward which changes your balance. You are momentarily in an unstable posture. In physics terms, you’ve moved your center of gravity (COG) beyond the last point of support. Take a look at this graphic:
When you take a step forward you unbalance your body by moving your COG slightly forward and beyond the last point of support (the toe of your foot on the other leg). This causes the front of your body to start falling. It takes only a small amount of time (Δt) for your forward foot to reach the ground — typically 0.5 seconds for a normal gait. When you first learned to walk it was difficult to get the timing right, as I’m sure you recall. But as your skill developed it eventually became part of your muscle memory.
Now imagine trying this on the Moon where Δt has increased to 2.5 seconds [the time to fall a given distance is inversely proportional to √g]. You’d find yourself “waiting” for your foot to hit the ground, and that would render your muscle memory useless and inefficient. The time to walk from point A to point B would be longer than tolerable to accomplish mission objectives.
Apollo astronauts actually trained in a reduced gravity simulator that replicated lunar conditions. But as Neil Armstrong later commented “It was a lot easier walking on the Moon than in the simulator.” All the Apollo astronauts found their own natural gait on the Moon, much like children learning to walk. What they found was that this “skipping/hopping” style was the best way to get around.
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