When talking about running form I think we sometimes get very caught up with what the lower part of our body is doing. Of course the feet and knees, and legs in general are very key to the act of running. But, neglecting the top half of your body means negating the other half of the formula.
Certainly posture get some mention, and core exercises for training abdominal strength are very popular with runners. It’s hard to maintain a good standing posture while on the back end of a long race. But how often do people really think about what their arms are doing?
When I do some people watching while jogging at the park or during a race, I come across a multitude of different arm swings. Some people do a very awkward, almost side to side swing, while others do a very abbreviated and seemingly strained short little half swing. Since it seems that nobody is really born running with good form any more, and if they are they seem to forget in adulthood, it should be no surprise that arm swing is equally as bad as what’s happening with the feet.
But, is arm swing really that important? Isn’t the real work going on down below your waist? Well, in fact the arm swing is actually quite important indeed.
You see, your body isn’t just a collection of independent muscles and bones, that seemingly agree to cooperate on occasion to produce force and motion. We label and refer to them by individual names, but for your body’s purposes it is all one unit. Beyond your musculature, there is a body-wide suit of connective tissue referred to as fascia.
Once thought of as just a sort of connective mesh, the fascia is now better understood to be a powerful force generating and amplifying apparatus. These fascia link between muscle and connective tissue like a series of very specific straight and spiral lines that run from foot to forehead and back again.
“One line of fascia tissue crisscrosses your abdomen and drops over your hip and down your shin to your foot, where it loops under your arch like a stirrup. Meaning? The angle of your pelvis affects the strength of your arches. Similarly, your thumbs are ultimately connected to your pecs by fascia. Which is why people who frequently use a PDA in a slumped posture are setting themselves up for problems.” 
And so it is also with the fascia and muscles that run between your legs and arms, chiefly used while running. It has been demonstrated in a lab that eliminating arm swing actually increases the energetic cost of running.  Conversely, that means that a proper arm swing, operating fluidly in conjunction with your running stride, serves to reduce the energy needed to propel yourself forward.
One would naturally assume that the reduction in energy cost would be a result of maintaining lateral balance, as the arms naturally move in the opposite manner to the leg of the same side. It’s more difficult to move forward when you’re off balance, or swaying back and forth.
But, this same study demonstrated that the arm swing itself did not assist with lateral balance. I suspect that it is actually the connected fascia, and the force amplification that results from the elasticity of this tissue, which is the cause of the energetic savings. I think this might function in a similar way that the achilles tendon stores and releases mechanical energy during the pumping action of the calf muscles in a forefoot landing. Tendons are after all, essentially rubber bands.
Far from being just about energy, the arm swing is critical to speed production and maintaing cadence. Heck, even just swinging your arms seems to have an excitatory effect on the lower limbs.  But, as legendary track and field coach Tom Tellez elucidates, the arm action has a huge effect on a sprinter’s ability to maintain speed and tempo during a race.
The biomechanical effect that he hints at must be the interaction of the fascia between the muscles and the coordinated movement of the arms and legs at the precise moments of impact. Linking up these movements seems to produce far more power than if done out of synch. It’s not just about balance, it’s about tension and transferring the power generated by your body at the exact moment that it will do the most good.
So, what should your arm swing look like? Well, Coach Tellez provides a good demonstration in that video, but of course that is intended for sprinting. Just like with your kick, the amplitude of your arm swing will scale with your speed and stride length. At the least, try to focus on clean back and forth arm movement, while limiting the awkward sideways or angled movement I mentioned above. Your arms should almost mirror the movement of your legs, and since your legs aren’t moving sideways or twisting behind you, neither should your arms.
The most important thing is to just be mindful that your arms are doing more than just flailing around uselessly while your legs are supposedly doing all the work. Your body is a unit.