What’s the physics behind it? Without analyzing the information, on the surface it would seem that the arm movement is a lot of useless expended energy, since main forces at work are the force of the foot against the ground and whatever drag the body encounters. You just don’t see sprinters with their arms trailing or tucked behind them, which would seem to be more aerodynamic. What is the nature of the boost that chugging with the arms gives?
The arms provide the torque that counters the torque your legs generate as you swing them. Without your arms providing this counter torque, your torso would twist back and forth too much instead. Try running while you swing your arms in the same direction as you swing your legs, and see.
If you ran like Fred Flintstone, it wouldn’t make any difference. In other words, if your hips stayed perpendicular to your direction of travel, and only your legs churned.
What happens when you run, is that your hips rotate. If your right leg is the current driving leg, then your right hip will rotate back and your left hip will be forward. I am not sure of the MAIN reason for this, but some obvious benefits would be increased stride length and transmission of energy from muscles above the hip.
The muscles in your back that allow you to twist your torso on your hips are not as strong as your arm muscles. If you move your right arm forward and your left arm backward, Newton’s Third Law tells us that your right hip will tend to go back and your left him forward. (I am sitting in an office chair pumping my arms, and the chair rotates under me. Test this out yourself.)
So, the pumping of your arms helps push your hip back. This is a positive force in moving you forward, and helps improve stride length.
It is not the most efficient way to add energy, since a lot of the effort of pumping your arms is probably lost internally, but some of it will positively effect your speed and that is what running is all about… going as fast as possible.
Also, it would actually take more energy to conciously hold your arms stiff and unmoving, as opposed to letting them swing, which is their natural motion while a person is moving forwards. Tensing muscles uses energy, which could be delegated elsewhere. It’s better to put the natural movement tendancy to work for you, rather than strain against it. Even when you walk, your arms move slightly. It’s more noticeable in some than others though. Tensed muscles use a lot of energy, especially for prolonged periods.
It would be very bad for a distance runner to tense their arms and shoulders while racing, and equally as bad if not worse for a sprinter to do so. In fact, we were told by our coach in Cross Country and Track to let our arms flop loosely for a bit to untense them during a race before resuming their piston movement. Our coach trained all the runners, both Cross Country and Track, and he used largely the same training methods for both kinds of runners. The sprinters just didn’t run for as long, so their arms/shoulders didn’t get a chance to tense. They did calisthentics(sp?) before the race to loosen up though.
This is why it is wise to build your upper body strength when you initially start training for running of any kind. The last part of a distance race is a “sprint” after all. (Though, it’s not nearly as fast as a “fresh” runner would go, it’s still the fastest pace that runner can produce at that point in time.) The strength you build in your upper body helps you stay balanced in more than one sense. It helps you perform better because you have better stamina, and it also puts the athelete in better overall health, as well as helping your form/posture.
A maxim I’ve often heard is that your legs are in synch with your arms. If you want to turn over your legs faster, swing your arms faster. Try running with your arms at your side and see how fast you run.