Cardiorespiratory and metabolic characteristics of detraining in humans

Detraining can be defined as the partial or complete loss of training-induc ed adaptations, in response to an insufficient training stimulus. Detrainin g is characterized, among other changes, by marked alterations in the cardi orespiratory system and the metabolic patterns during exercise. In highly t rained athletes, insufficient training induces a rapid decline in (V) over dot O-2max, but it remains above control values. Exercise heart rate increa ses insufficiently to counterbalance the decreased stroke volume resulting from a rapid blood volume loss, and maximal cardiac output is thus reduced. Cardiac dimensions are also reduced, as well as ventilatory efficiency. Co nsequently, endurance performance is also markedly impaired. These changes are more moderate in recently trained subjects in the short-term, but recen tly acquired (V) over dot O-2max gains are completely lost after training s toppage periods longer than 4 wk. From a metabolic viewpoint. even short-te rm inactivity implies an increased reliance on carbohydrate metabolism duri ng exercise, as shown by a higher exercise respiratory exchange ratio. This may result from a reduced insulin sensitivity and GLUT-4 transporter prote in content, coupled with a lowered muscle lipoprotein lipase activity. Thes e metabolic changes may take place within 10 d of training cessation. Resti ng muscle glycogen concentration returns to baseline within a few weeks wit hout training, and trained athletes' lactate threshold is also lowered, but still remains above untrained values.