Doing it like that (called 'concentration curls') works a different part of the bicep, called the brachialis (it's the same muscle that gets worked in preacher curls), where regular standing or seated curls work the biceps brachii (the main, larger muscle on top). A good arms workout usually includes work for all 3 muscles of the bicep (the other is the brachioradialis, which stretches across the bicep and forearm, and which gets worked in any reverse type curl, such as hammer curls).
You're definitely right though that it isolates it more (hence, the name 'concentration curls') and recruits the back muscles far less (of course if you're also working yoru back on your biceps day, then that wouldn't be a bad thing). It would probably be pretty difficult to injure your back doing curls unless you were going way too heavy for your own strength, but for anyone who has prior back issues that may be a better option.
However I believe that lifting heavier will always yeild faster results, and since regular curls do recruit your back more, you can simply lift heavier - even though the back is helping, you're still getting more resistance on your biceps, which will make them grow more. If your gym has an EZ curl bar (looks like a shorter regular barbell but it's kind of squigly in the middle to make it easier to hold on to) you can curl with that for an even tougher curl (here's a pic of that).
Thanks for pointing that out Tamara! 
Edited by retrograde (07/27/05 09:15 PM)
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