Move your watch to your other hand ... 

Some of the sport watches . (Which seem to be able to do everything under the sun !) aren't all that light . Having that weight at the end of one arm is like adding weight to the bottom of a pendulum–it changes the speed it swings and the effort involved. Did you ever think It could be changing how your whole body moves? And not only that, if you look at it often ,you’re doing a movement with one arm that you never do with the other. So on your next run switch it up .Place the watch on your other wrist . If all’s been going well in your running, this will just help even out the tiny stresses on your body and improve your coordination one extra little bit. (Kind of like the brushing your teeth with your non dominate hand drill )  . On the flip side ,if you have been struggling with a chronic ache or pain anywhere in your body, you might be surprised ,it could just help.