Relatively little is known about how excess body mass affects adolescents' capacity to perform sustained exercise. We hypothesized that most of the difficulty that severely overweight adolescents have with sustained exercise occurs because the metabolic costs of moving excess mass result in use of a high proportion of their total oxygen reserve. We compared results from a maximal cycle ergometry fitness test in 129 severely overweight adolescents who had BMIs of 41.5 +/- 9.7 kg/m2 and ages of 14.5 +/- 1.8 years (range: 12.1-17.8 years) and 34 nonoverweight adolescents who had BMIs of 20.1 +/- 2.9 kg/m2 and ages of 14.5 +/- 1.5 years (range: 12.0-18.1 years). Oxygen uptake (Vo2) was compared at 3 times: during a 4-minute period of unloaded cycling (ULVo2), at the lactate threshold estimated by gas exchange (LTVo2), and at maximal exertion (Vo2 max). Heart rate was obtained at rest and at Vo2 max. Participants also completed a 12-minute walk/run performance test to obtain distance traveled (D12) and heart rate. Absolute LTVo2 and Vo2 max and LTVo2 as a percentage of Vo2 max were not different in overweight and nonoverweight adolescents during the cycle test. However, absolute ULVo2 was significantly greater in overweight adolescents: ULVo2 accounted for 35 +/- 8% of Vo2 max (and 63 +/- 15% of LTVo2) in overweight adolescents but only 20 +/- 5% of Vo2 max (and 39 +/- 12% of LTVo2) in nonoverweight adolescents. Resting heart rate before initiating the cycle test was significantly greater in overweight than nonoverweight adolescents (94 +/- 14 vs 82 +/- 15 beats per minute). However, maximal heart rate during the cycle test was significantly lower in overweight adolescents (186 +/- 13 vs 196 +/- 11 beats per minute). During the walk/run test, mean D12 was significantly shorter for overweight than for nonoverweight adolescents (1983 +/- 323 vs 1159 +/- 194 m). D12 was negatively related to BMI SDS (r = -0.81) and to ULVo2 (r = -0.98). Overweight and nonoverweight adolescents had similar absolute Vo2 at the lactate threshold and at maximal exertion, suggesting that overweight adolescents are more limited by the increased cardiorespiratory effort required to move their larger body mass through space than by cardiorespiratory deconditioning. The higher percentage of oxygen consumed during submaximal exercise indicates that overweight adolescents are burdened by the metabolic cost of their excess mass. Their greater oxygen demand during an unloaded task predicted poorer performance during sustained exercise. Exercise prescriptions for overweight adolescents should account for the limited exercise tolerance imposed by excess body mass, focusing on activities that keep demands below lactate threshold so that exercise can be sustained.
