Day 12 alternates 10 step jacks with 2 shoulder taps, five times through, for 50 step jacks and 10 taps per set, with 2 minutes rest between sets. The shoulder taps are the plank version: a high plank, one hand up to the opposite shoulder, back down, other hand. Two taps is one per hand.
The step jack is a jumping jack with the jump removed: step one foot out to the side as both arms rise overhead, step it back as the arms lower, then the other foot, each step counting as a rep. It keeps the arms swinging and the pulse up while giving the joints a break from impact, and fifty of them per set is plenty of time for the shoulders to notice the arm swing. The plank shoulder taps punctuate the list, and with only 2 at a time they are less about fatigue than about repetition: five times per set you lower into a plank, hold the hips square while a hand lifts, and come back up. That repetition is where the core learns.
Step wide enough on the step jacks that the inner thigh feels the movement, and reach the arms fully overhead on every rep. Keep the rhythm continuous - the step out and the arm swing should arrive together. In the shoulder taps, plant the feet a little wider than the hands, tighten the glutes and abs before the first hand lifts, and touch the shoulder without shifting the hips.
Low impact, high volume, and a plank every fifty reps - Day 12 keeps the pulse honest without leaving the ground.
Make it Harder: Turn the step jacks back into full jumping jacks for the last two blocks of every set.
Make it Easier: Tap the shoulders from a standing position instead - hands to opposite shoulders, 2 taps - and keep the step jacks as written.
