Humans naturally move their arms while running, it is a natural act! You have to make a conscious effort to walk without using your arms and an even bigger effort if you running. This leads us to believe there must be some kind of benefit by using this technique, otherwise, evolution should have left it out of our nature, since it is an energy consuming movement.
We move our arms contrary to our legs, so when your left foot goes. Your arms work as a counter-balancing force to the angular momentum generated by the legs(1,2). The angular momentum is the amount of rotation of a body and it depends on the velocity as well as the mass of the object. This compensation is important because it keeps the runner in a straight line, and it keeps the visual aim stable.
The left arm is counter-acting the angular momentum caused by the right leg so that the body of the runner is kept straight and the visual aim remains stable.
The movement of the arms can also help lifting the runner from the floor and bring an extra force forwards(3) to help the legs.
Keeping your visual aim stable and moving your body forward and up!!!! these simple tips will make your running more efficient.
Active vs. Passive Theories
Since 1939 it has been suggested that the arms are not passively moved as simple pendulums(4) but that they are actively moved by shoulder muscles, especially while moving the arm backwards by the posterior deltoid (the anterior deltoid also does some work while moving forwards, but it does not appear to have a great contribution)
It has actually been demonstrated through electromyography (a study where they stick you with needles in the muscle to measure contractility) that the deltoid muscle fires activity during walking even when the arm is immobilised (5). This means that the shoulder muscle is actively moving during walking.
Despite the evidence that the deltoid muscle acts during running there is another theory. The passive theory believes that the arm movement is a passive response to the movement caused by the forces exerted on the pelvis by the legs and transmitted by the spinal column to the shoulders and arms.
According to this theory the shoulders and arms would move at the same frequency, although slightly out of phase, as the pelvis and legs and it would adjust on its own. If your legs are moving faster your arms would adjust automatically. This theory acknowledges that the shoulder contracts on its own but it believes those contractions are necessary to stabilise the shoulder and do not contribute much to the movement of the arms.
Why should I care?
You probably should if you are trying to become a better runner, it is crucial to understand what makes us better and what slows us down. Running with your arms fixed (with a stroller) will diminish your performance, not only does it affect your balance but it also affects your running economy by increasing your metabolic needs. If you tend to run with an object in your hand, like a music player or a water bottle, you are increasing your angular momentum on one side and you will have to compensate, which means more wasted energy.
- Hamner, S. R., Seth, A. and Delp, S. L. (2010). Muscle contributions to propulsion and support during running. J. Biomech. 43, 2709-2716.
- Hinrichs, R. N. (1987). Upper extremity function in running. II. Angular momentum considerations. Int. J. Sport Biomech. 3, 242-263.
- Hopper, B. J. (1964). The mechanics of arm action in running. Track Technique 17, 520-522.
- Elftman, H. (1939). The function of the arms in walking. Hum. Biol. 11, 529-535.
- Fernandez Ballesteros, M. L., Buchtal, F. and Rosenfalck, R. (1965). The pattern of muscular activity during the arm swing of natural walking. Acta Physiol. Scand. 63, 296-310.