If these physiological changes can occur very rapidlywhen training load is decreased, they also are prone to a rapidloss when the training stimulus is insufficient (44). In fact,Coyle et al. (11), as well as Houmard et al. (27), found thata few days were enough to decrease significantly the bloodvolume, which negatively affected the cardiovascularfunction by decreasing both the stroke volume (11) andthe cardiac output (13). Enzymatic activity, includingoxidative enzymes and glycogen synthase, decreases alsovery rapidly when the training stimulus is insufficient(12,27,37); this may contribute to the rapid decrease in ̇VO2max and glycogen stores reported in the detrainingliterature (10,11,13,27). Altogether, these results show that taper-induced positive physiological changes are only transitory and may return very rapidly to pretaper values or, eventually, to initial values in the case of a long-term insufficient training stimulus (45). Whence the difficulty to find the taper strategy that will allow the athlete to recover and overcompensate adequately from prior heavy training loads while avoiding detraining.