Determinants of increased energy cost of submaximal exercise in obese subjects

The energy cost of submaximal cycling exercises is studied in 23 obese (OS) and 13 lean control (LS) subjects at 1) a constant pedaling frequency (60 rpm) and at various work loads [external work loads (Wmec) up to 100 W] for one group of OS and LS, and at 2) constant Wmec (brake free and 60 or 70 W) and various frequencies (38-70 rpm) for a second group of OS and LS. The total energy expenditure (WO2) is calculated from O2 consumption (VO2) measured in both conditions and is compared with anthropometric data. The results show that at rest or at the same Wmec, WO2 is always greater in OS than in LS. At rest the quotients of WO2 over body surface area are not significantly different. At work the difference in WO2 cannot be explained by the muscular mechanical efficiency, which is not statistically different in OS (26 +/- 7.8%) and LS (25 +/- 4.6%). The calculated increase in the work of breathing of OS can account only for 5-15% of the energy overexpenditure. The energy cost of leg movement is estimated in brake-free cycling trials; it is significantly greater in OS than in LS (118 J compared with 68 J/pedal stroke), but when divided by leg volume the figures are not different (9.2 compared with 8.5 J X dm-3 X pedal stroke-1). Leg moving may account for approximately 60-70% of the energy cost of moderate exercise in cycling OS. The remaining difference in WO2 between OS and LS (20-30%) may be explained by an increase in muscular postural activity related to the lack of physical training of OS.