You're right that I quoted a number too cleanly. The 7 percent figure is a rough midpoint, not a law. Across detraining studies the range runs from almost nothing to well over 10 percent, and the big moderators are how trained you were to start, how long the break runs, and whether any activity happened during it. The central-versus-peripheral split is the part I find genuinely useful. Plasma volume and stroke volume fall within the first week or two, which shows up as a higher resting heart rate and a lower stroke volume, and that reverses fast once you're training again. So part of what feels like lost fitness is really just less blood in circulation, not muscle going anywhere. Strength hangs on longer than aerobic capacity, which is why a lifter on a three week break often comes back nearly intact while a runner feels the difference. One honest caveat: the trained-versus-untrained comparison is real, but the clean numbers behind it come mostly from small studies with different protocols, so I'd treat the direction as solid and the exact percentages as soft. Resting heart rate is a reasonable cheap proxy for the central piece, though it'll also move with sleep, illness and heat, so trend it rather than reading one morning.