I’ve posted about this before, and someone entered the question on a search engine and came to my website today:
The answer, is NO! (That is, if you’re thinking of walking or running with your arms out like a zombie, holding into the top of the treadmill – don’t do it! You wouldn’t go for a walk in the park like that, so don’t do it on the treadmill!)
It makes your body go at an unnatural angle and prevents your bodies natural movement (ie, arms swinging and torso and core moving).
If however, you’re a bit scared of the treadmill, or you feel like it would be better to hold onto the side parts of a treadmill as you build up or reduce speed, then do that, but the trick is to not put the speed up too high that you might feel so uncomfortable that you might fall off. Run or walk at a natural and comfortable pace and you shouldn’t need to hold on!