I was just wondering why it is easier for me to do a crunch when I have my arms straight out at my sides (not on the floor, but in the air straight out towards my feet) then when I have my arms behind my head? Also I find that I shake alot less when I do it that way, is it normal to shake? I can also get my shoulders farther off the ground with my arms straight out as well.... why is this? Thanks.

When you do a crunch with your arms behind your head, you've changed your center of gravity.

Is it just as effective to do it either way???

Just like you discovered, crunches are harder with your hands behind your head. When crunches with your arms in front of you get easy, you can progress to the ones with your hands behind your head.

Because you change the centre of gravity and the length of the lever, crunches can be easier or more difficult. Generally, I have beginners start with their arms folded on the chest, then work up to different arm positions. Once crunches get easy to do, they lose their effectiveness, and one must try a different position.

I try to work on different arm and leg positions when doing crunches. It allows for variety while also varying the intensity of the crunch, since I don't add weights.

Thanks! I think Im going to try and do them whichever way is easier right now until that gets too easy, I dont really like the feeling of shaking while doing them with my arms behind my head! lol