Seven, seven, seven, total bicep agony.
Keep your elbows pinned to your sides the whole 21 reps, the burn is the point.
21s split the barbell curl into two halves and a full rep, seven of each, so your biceps never get the rest they'd sneak at the top or bottom of normal curls. The partials keep constant tension through the exact ranges where most lifters relax, and by the final seven full reps the muscle is already cooked. It's a finisher, not a strength builder, which is exactly why it works.
Way lighter than your normal curl, roughly half is a good start. If you can't finish the last seven full reps with pinned elbows, the bar is too heavy.
Bottom half first, top half second, full reps last is the classic sequence, and it's brutal for a reason. The full reps land when you're most fatigued, forcing strict form.
Once a week as a finisher is plenty. It's a high-fatigue burnout scheme, so let strict curls do the main work and use 21s to empty the tank at the end.
