Sonic Boom is a shoulder endurance workout built on a piece of physics trivia: the crack of a whip is a genuine sonic boom, the tip briefly breaking the speed of sound. Arm circles, wide arm circles and chest expansions - 140 reps of continuous arm movement per set - are how you build shoulders capable of that kind of arm speed. No equipment, no floor work, nothing but rotation in volume.

The circuit runs without rest between exercises, and its threat is invisible on paper. Every movement here is unweighted and individually trivial - which is the trap. Arm circles keep the deltoids under uninterrupted tension; the wide variety sweeps the full arc of the shoulder joint, recruiting more of the surrounding musculature; and chest expansions pull the arms horizontally through their range, opening the chest and firing the upper back as a counterweight to all that circling. Alternating the three patterns means the shoulders never leave the working zone for the entire set, they just change direction - and somewhere around rep 80, arms with nothing in their hands begin to weigh a surprising amount. That accumulating burn is the point: sustained, unloaded volume builds the muscular endurance that fast arms are made of.

Keep circles controlled and round - actual circles, not shivering ovals - with arms at shoulder height and shoulders pressed down away from the ears. Let the wide circles travel through the fullest pain-free range you have. On chest expansions, swing to a full stretch across the chest and a full opening behind, keeping the arms at shoulder height throughout. Slowing down is allowed; lowering your arms is the only real failure.

The whip cracks because the tip outruns sound. Give these shoulders enough sets and something's going to crack besides your patience.

Make it Harder: Hold a water bottle in each hand for every exercise.

Make it Easier: Halve every block to 10 reps and shake the arms out between exercises while staying in motion.