by Neil Anderson

"21's" are performed by splitting each set into 3 different combinations of that same movement for 7 reps each.  Using back squats as an example, you would complete:

7 reps from the bottom of the squat to half-way...
7 reps from the half to full up...then finally,
7 full reps from full flexion to full extension.   

 Totaled up this would be "21" reps without stopping.  This contributes to improved muscular development and better understanding of movement patterns for each movement.  It may be applied to almost any exercise/movement.  

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"21's" performed with biceps curls.