In the first one (1) a trio of specialists built a mechanical model to get an idea of the dynamics of arm-swinging and then recruited ten volunteers, who were asked to walk using four different types of arm swing: (a) normal, (b) bound, in which subjects’ arms were physically restrained from moving, (c) held, in which subjects held their own arms still and (d) anti-normal, where subjects actively swung their arms out of phase relative to normal. The energy expenditures were lowest in the normal condition and increased 7 per cent for bound, 12 per cent for held and 26 per cent for anti-normal.
In addition, the vertical ground reaction force was lowest during normal arm swinging, approximately 60 per cent higher during the bound and held modes and nearly 3-fold greater when arms were swung out of phase relative to normal. The vertical ground reaction force is the force exerted by the ground on a body in contact with it.
For example, a person standing motionless on the ground exerts a contact force on it (equal to the person’s weight) and at the same time an equal and opposite ground reaction force is exerted by the ground on the person. Hence arm swinging is the least energy spending mode of locomotion.
The second research (2) concluded that the arms act as mass dampers during human walking and running, although the evidence is clearest for running. A mass damper is a device mounted in structures to reduce the amplitude of mechanical vibrations. The natural arm swing also reduces head yaw (= to swerve off course momentarily) and shoulder movement.
There are a number of other possible benefits to arm swinging. They include reduced vertical displacement of the centre of mass, prevention of uncontrolled arm motions, increased walking stability. It has even been proposed that arm swinging may be an evolutionary relic from quadrupedalism that serves little or no purpose.