Respiratory Limitations to Exercise Tolerance and Performance in Obese Adolescent Patients

Obese adolescents (OB) have an increased O2 cost of exercise, attributable in part to an increased O2 cost of breathing. The increased O2 cost of breathing could entail a “competition” between respiratory and locomotor muscles for the finite available O2, leading to fatigue and premature exhaustion. This would contribute to the inactivity which represents one of the main causes of obesity, impeding the increased level of physical activity which is one of the cornerstones of the treatment of the disease. In order to interrupt this vicious circle, we followed two approaches, attempting to relieve the respiratory limitation in obese adolescents performing cycling/walking exercises. In the first approach, respiratory muscles were acutely unloaded via normoxic helium-O2 (HeO2). Helium [He] has a lower density compared to nitrogen, and thereby HeO2 breathing requires less respiratory muscle work than air breathing. This unloading lowered the O2 cost of exercise and perceived exertion during moderate- and heavy-intensity cycling of relatively short duration (12 min). Following these findings, respiratory muscles were acutely unloaded by switching the inspired gas from ambient air (AIR) to normoxic HeO2 (AIR+HeO2) during constant work rate (CWR) cycling to exhaustion, in order to specifically evaluate the effects of the proposed intervention on exercise tolerance. The intervention extended exercise duration/improved exercise tolerance during both moderate (below the gas exchange threshold,GET) cycling exercises. In the second approach, a standardized program of respiratory muscle endurance training (RMET) was superimposed on a standard multidisciplinary body mass reduction program; RMET decreased perceived exertion and O2 cost of exercise during heavy-, but not during moderate-intensity cycling exercise, and improved peak cycling capacity. We then evaluated the hypothesis that the benefits of RMET on the O2 cost of exercise and exercise tolerance would be more pronounced during walking on a treadmill compared to the effects described during cycling, and/or could be observed also at low intensities. Indeed, in OB a 3-wk RMET program markedly lowered the O2 cost of moderate- and heavy-intensity walking and improved exercise tolerance. By contrasting the vicious circle of obesity → early fatigue → reduced exercise tolerance → reduced physical activity → obesity, the intervention more specifically directed to the respiratory function, such as RMET programs, could represent a useful adjunct in the control of obesity. Longer periods of RMET should be investigated.