Day 25 runs the jack family against itself. Four 15-second intervals - half jacks, jumping jacks, half jacks, jumping jacks - three to seven sets, one minute of rest. One exercise is the full version, one is its lower-effort twin, and alternating them is the whole session.
A half jack, as drawn, keeps the arms down at the sides while the feet jump wide and back together - the legs do the whole job and the shoulders sit it out. That makes it lower in effort than a full jumping jack without being slower, so your cadence stays high while the arms recover. Then the full jack brings the arms back overhead and the demand jumps again. Same footwork throughout, two different amounts of body in play, and a heart rate that never quite gets to settle.
On half jacks, keep the feet snapping wide and back with real intent - without the arms to keep you honest, this is the interval that quietly turns into a shuffle. On full jacks, take the arms all the way overhead and let hands and feet arrive together. Land softly on both, through the whole foot.
Arms out, arms down, and a session that proves how much of a jumping jack lives above the waist.
Make it Harder: Bring the hands to shoulder height on the half jacks so the arms are held up rather than resting at your sides.
Make it Easier: Step the feet wide and back rather than jumping them, on both intervals.
