The ten-exercise companion to 5BX, written in 1961 for women and built on the same idea: no equipment, no gym, and a hard twelve-minute ceiling. Four short warm-up moves, then six longer ones working down the body and finishing on the spot. The original ran to 48 levels, and the times never changed as you climbed. Only the repetitions did. So treat each window as the target and fit in as many good repetitions as you can. You need: floor space to lie down. Apple Health logs it as functional strength training.