On my run today I noticed another runner carrying a water bottle in one hand and an ipod in the other. What struck me about this runner was the total lack of upper body movement as he run, no arm movement, no upper body rotation, nothing.
Once the runner had disappeared from sight I imitated how he ran to see what effect it would have on my running. I found it quite difficult and unnatural to completely immobilise my upper body and the effect on my lower body was dramatic. Immediately my stride length shortened significantly and my pace slowed to a shuffle, I had no push off, no knee lift, all feeling of fluidity disappeared. I felt like a poorly constructed robot.
Try it yourself and see the effect it has. If you run holding a water bottle notice if the arm you hold the water bottle in moves less than your other arm, if it does then it is affecting the way you run making you less efficient and setting up possible biomechanical asymmetries leading to injuries. Get a hip holder for your water bottle or a camel back.
Studies have shown the energy cost of running with a camel back is far less than running with a hand held water bottle.
For those of you who want to know why read on....
Keeping your upper body completely still you effectively turn off your core muscles and significantly reduce the load on your hip flexors and glutes.
Let me explain - as your leading leg travels forward (say right leg) and your left leg goes behind you your arms go the opposite direction - ie right arm goes backwards , left arm goes forward.
So now you have a diagonal stretch from your left hip to your right shoulder. This places a tension on your hip flexors and abdominals. This tension places a load on the muscles, tendons and fascia which when released acts like a rubber band pulling your left leg forward and right arm forward. As you drive the left leg and right arm forward your left arm and right leg travel backwards which loads the opposite diagonal ( right hip to left shoulder) and the cycle continues.
Your gluteal muscles are loaded by the rotation of your pelvis. As your right foot lands your pelvis rotates to the right placing a stretch or load on your right gluteals which when released helps you drive off the right leg.
So the rotation and arm movement of your upper body effectively loads the muscles in your lower body.
Notice that the faster you go the more arm swing you have - sprinters swing their arms a lot more than marathon runners. Sprinters use more force so the greater arm swing place a bigger stretch or tension on their muscles allowing them to generate more force.
Muscles work most effectively when they are placed under a tension first, if you dont do this then then amount of force the muscle can generate is greatly diminished . Take an extreme example - say you want to jump vertically as high as possible. TO load or place the muscles of the hip and leg under tension the first thing you do is squat down which is quickly followed by your vertical jump. Now imagine how high you could jump if you werent allowed to squat down at all - you'd be lucky to make it off the ground.
This is what happens if you dont move your arms when you run - you greatly reduce the force the glutes, hip flexors , core , hamstrings and quads can generate.