Detraining: Loss of training-induced physiological and performance adaptations. Part I Short term insufficient training stimulus

Detraining is the partial or complete loss of training-induced adaptations, in response to an insufficient training stimulus. Detraining characteristi cs may be different depending on the duration of training cessation or insu fficient training. Short term detraining (less than 4 weeks of insufficient training stimulus) is analysed in part I of this review, whereas part II w ill deal with long term detraining (more than 4 weeks of insufficient train ing stimulus). Short term cardiorespiratory detraining is characterised in highly trained athletes by a rapid decline in maximal oxygen uptake (V over dot(2max)) and blood volume. Exercise heart rate increases insufficiently to counterbalance the decreased stroke volume, and maximal cardiac output i s thus reduced. Ventilatory efficiency and endurance performance are also i mpaired. These changes are more moderate in recently trained individuals. F rom a metabolic viewpoint, short term inactivity implies an increased relia nce on carbohydrate metabolism during exercise, as shown by a higher exerci se respiratory exchange ratio, and lowered lipase activity, GLUT-4 content, glycogen level and lactate threshold. At the muscle level, capillary densi ty and oxidative enzyme activities are reduced. Training-induced changes in fibre cross-sectional area are reversed, but strength performance declines are limited. Hormonal changes include a reduced insulin sensitivity, a pos sible increase in testosterone and growth hormone levels in strength athlet es, and a reversal of short term training-induced adaptations in fluid-elec trolyte regulating hormones.