The mechanics above the waist are often overlooked in runners.
However, they are quite important.
Collins, Adamczyk & Kuo (2009) — the seminal walking study — found:
Holding the arms still voluntarily: required 12% more metabolic energy than normal swinging.
But the vertical ground reaction moment (the rotational force the legs generate against the ground, which arm swing exists to counteract) increased by 63% without arm swing.
Walking with reversed (anti-normal) arm phasing caused metabolic rate to increase by 26%, even though it required minimal shoulder muscle effort — because the ground reaction moment penalty is the dominant cost, not the muscular cost of swinging itself.
Here is a great exercise to improve this upper body strength and mechanics.
It works the obliques unilaterally, which is essential for proper efficient running form.