Most lifters have heard of 21 years. This intensity method is most commonly observed with barbell curls. They do 7 repeats only in the lower range of motion, 7 partials in the upper range and then 7 full repeats.
But this is not the only way to use 21 seconds. Instead of training different parts of the range of motion, you can also change the grip every 7 reps. That's how it looks. Note the handle changes:
Breast Support 21s
Run the unit on a machine with a low cable and a 30 degree inclination. Do 7 reps with three different grips: Pronate, Neutral and Supinated.
Hold your chin and make sure you position your bench so that the cable remains taut at all times. Your weight stack should not "clink" with each repetition. You want full tension throughout the set.
This exercise works the lattism, the rhomboids and the rear deltas as well as the traps to varying degrees due to the different grip. In addition, the chest-based variation helps effectively combat the upper back muscles, keeping your centerline static throughout the range of motion (less risk of cheating) repeating each grip variation with 60-90 second rest between sets. The loading should be easy.
Reverse 21s for hypertrophy
13 set-rep programs for new growth