In reference to my last BACK & BICEP post ( P.S. sorry about the typo on the photo, hopefully you didn't notice…..'CABLE CABLE' oops!) "21" refers to the number of total reps you do in one set. However, this particular "21" is divided into three 7-rep segments that targets the entire bicep.
Here is how you do a set of 21s:
Grab a weighted barbell: doing a regular bicep curl-
1- 1st 7 Reps: go from the bottom of the movement up to the half way point (arms stop at 90 degrees and hands at elbow level).
2- 2nd 7 Reps: go from the halfway point up to the top of the bicep curl (hands near shoulder level).
3- 3rd 7 Reps: start at the bottom of the movement and complete a full range of movement all the way up (a full bicep curl with no stopping at halfway).
That is one set and how you do a 21s.