I wonder whether I should place my arms across my chest or behind my head when doing abdominal crunches. I am interested in both the effectiveness of the exercise as well as injury prevention (eg lower back or neck injuries).
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Your core likes it when all your limbs are close and stable to your body. When you put your arms to the side or behind your head, your core has to work extra hard in order to balance things out. So to answer your question, across your chest will make the exercise easier while behind the head makes it harder and works a bit more stabilizers. Keep in mind, putting them across your chest makes it easier to round your upper back, which is not what you want out of a crunch. Similarly, behind the head can also make you pull with your head, which is also not good. When you do put them behind your head, don't actually touch your head, maybe just with the fingertips, the exercise only benefits from your arms being away from your body, all actually touching your head does is make the exercise lazier and easier to get injured.
As long as you do not pull your head - it does not matter. You can also keep hands side by side, or straight. Try different positions, and choose best for you. From time to time change it - maybe something goes better over time?
You can stress muscles isometrically, or move chest and hips closer. That is other topic, but can influence hands position.
I think placing the hands behind the head is more difficult for doing crunches than placing hands on the chest. So it's upto you. Placing hands behind the head can cause injuries if you do it improperly that is if you pull your head while doing crunches so be careful and use proper form.
Placing my hands behind my head to do sit ups caused neck strain and l could not turn my head properly for a few weeks after this so it is not the correct way to do a sit up.
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1I don't know exactly what happened to you, but I watch people at the gym doing crunches, and I frequently see people flex their neck forward each time they crunch. In other words, they use the force of their arms to push their chin down to their chest. After their abs are fatigued and they should stop, they keep going, but all they are really doing at that point is flexing their neck back and forth. None of the trainers at the gym correct this. They don't care. This might have been what you were doing. – Chris Mar 31 '19 at 5:00
