Mujika & Padilla 2000, Detraining review

Two-part Sports Medicine review of detraining — the loss of training-induced adaptations under an insufficient training stimulus. In short-term cessation (under 4 weeks), highly trained athletes lose VO₂max rapidly, by roughly 4–14%, driven mainly by a 5–12% fall in blood and plasma volume that lowers stroke volume and maximal cardiac output; recently trained people lose less (about 3.6–6%). These early losses are thus central and cardiovascular, whereas the longer-term losses (Part II) become increasingly peripheral and muscular — reduced capillary density, oxidative-enzyme activity and fibre cross-sectional area. Trained athletes retain a residual floor above the untrained. Strong.