21's are not sets of 21 reps, well twenty one whole ones anyway. They are best used on the biceps or quads, but they could be used for any body part if you feel the need to get a unbearable pump going.
There is one main rule with 21's. Use them only once per body part and only once or twice a month, more is not better.
I will use the barbell curl as an example but the other main exercise they are used for is squats.
Pick a weight you would normally do 7-10 reps with, if you are trying these for the first time 10 might be better.
Start with the bar at the top of the lift. e.g. at shoulder height. Now instead of lowering the bar all the way, lower it about one third of the way and raise it back up. Lower it again a third and back up. You do 7 reps of the top third of the movement.
Without stopping lower the bar all the way, only this time raise it a third and lower it. Again 7 reps, by now you should be pumped but it's not over yet. Now you do 7 full reps keeping the form as good as possible, you probably will swing but try to keep that minimal.
There you have it a set of 21's and you barbell curls are done with in one set. You can see how hard this may be for the squat so don't let you ego take control with the weight.
| What Are 21's?
One of Arnold's favorites. Using the curl as an example, you would do 7 full reps followed by 7 half reps from the starting position to half way up followed by 7 half reps from half way up to the top of the movement. All of this counts as one set.
First 7 Reps
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21's are fun and an efficient way to new muscle growth, just remember to not overdo them.
Enjoy your training and life,