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Why Do We Bend Our Arms While We Run?

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Why do people walk with their arms down by their sides, but when they run, their arms are typically swinging while bent at the elbow?

A recent study was conducted to see if arm positioning was affecting energy efficiency. Results showed that walking with bent arms was less energy efficient than walking with straight arms. Bent arms require less energy to swing back and forth. As a result, bent arms should be more efficient for people that run and walk. If this is true, why don’t people typically walk with bent arms?

The study looked at four men and four women on treadmills. They performed both activities with straight arms as well as bent arms. Scientists used infrared cameras and motion-capture software to monitor and record the movements of their volunteers. They were then able to construct a 3D digital model of their bodies.

Two weeks later, the same subjects repeated the treadmill activities, but this time wore breathing masks. The researchers wanted to collect metabolic data to represent the participants’ energy use. As would be expected, people that ran with straight arms felt awkward. And, scientifically, there didn’t seem to be any difference in their energy efficiency as a result.

When the participants walked with bent arms, it showed an energy expenditure of about 11%. This was most likely due to the fact that it requires more energy to keep their arms bent while moving at a slower speed than running.

Arm swinging costs more energy while running, but keeping arms by your side while running takes even more energy. When you swing your arms, it reduces the motion of the torso, which helps to reduce the energy level a bit.

Despite all of this knowledge, it just feels natural to walk with your arms by your side and arms bent when running. Still, it is certainly good information to have when people take to the treadmill for their workouts.