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I think it depends on how adept you are at isolating and identifying your muscles kinesthetically.
If you know what you're doing and trust yourself to stay true, putting your hands behind your neck/head can provide support and the winged elbows will act as added resistance making the crunch harder. But, the temptation is there to retract your elbows to assist and that in and of itself can pull your head down/in and forward as a form of momentum to help. And as you fatigue, the temptation increases.
If you're good, and if you can truly isolate and flex your abs through your crunch, you can do a lot of different things with your arms & hands, with or without weights, to affect your crunch technique.
It's all about your skill.
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