Mechanical efficiency at different exercise intensities among adolescent boys with different body fat levels

This study investigated the mechanical efficiency (ME) and associated factors in obese, overweight, and normal-weight adolescent boys during incremental cycle exercise test to exhaustion. Forty-five sedentary adolescent boys (13-14 years old) were separated in three groups according to the percentage of fat mass as follows: 15 normal-weight (NW) (body fat: 16.0 + - 1.9%), 15 overweight (OW) (body fat: 24.0 + - 1.6%), and 15 obese (OB) (body fat: 31.0 + - 3.0%). All groups completed an incremental cycle exercise to exhaustion in which energy consumption (E, W), ME (%), lipid oxidation rate (LO, %), plasma epinephrine and norepinephrine concentrations were determined consecutively at rest and at three intensity levels corresponding to 50 and 75% of each participant's maximal heart rate (50%HRmax and 75%HRmax) and peak oxygen consumption (VO2peak). During the incremental cycle exercise test, plasma epinephrine, and norepinephrine responses as well as ME determined at 50%HRmax, 75%HRmax, and at VO2peak stages were significantly lower in OB compared to NW and OW individuals (ps < 0.01). Multiple linear regressions showed that body weight (B = -0.64, p < 0.001), energy consumption (B = -0.24, p < 0.05) and lipid oxidation (B = 0.69, p < 0.01) were significant predictors of ME at 50%HRmax. However, at 75%HRmax and O2peak, significant predictors of ME were epinephrine (B = 0.34, B = 0.49, respectively, ps = 0.01), norepinephrine (B = 0.26, B = 0.60, respectively, ps < 0.05) and power output (B = 0.62, B = 0.71, respectively, ps < 0.01). These findings suggest that excess in body weight exerts a negative effect on ME at a low intensity by increasing energy consumption for obese and overweight adolescent boys, while at higher intensities (75%HRmax and VO2peak) the lower ME could be better explained by the lower power output and catecholamine responses that were attenuated among obese and overweight adolescent boys.The publication of this article was funded by the Qatar National Library.Scopu