Effects of obesity on the energetics of gradient walking

Abstract

Walking is a popular and convenient form of physical activity and can play an important role in the treatment and prevention of obesity. During level walking, obese adults are reported to have a greater net metabolic rate (W/kg) but a similar metabolic cost (J/kg/m) compared to non-obese adults. Individuals routinely walk up or down grades, but the metabolic response to gradient walking in obese individuals is not well understood. PURPOSE: To quantify metabolic rate and metabolic cost during level and gradient walking in obese and non-obese adults. A secondary purpose was to determine metabolic prediction equations' ability to estimate energy expenditure in obese adults. METHODS: Thirty-two obese (18 female, mass = 103.2 (15.8) kg, BMI = 35.0 (4.5) kg/m2, mean (SD)) and nineteen non-obese (10 female, mass = 64.9 (10.6) kg, BMI = 21.6 (2.0) kg/m2) volunteers participated in this study. We measured oxygen consumption while subjects stood and walked on a dual-belt force measuring treadmill at eleven speed/grade combinations ranging from 0.50 m/s to 1.75 m/s and -3° to 9°. We calculated gross and net (gross-stand) metabolic rate and metabolic cost for each condition. A two-factor repeated measures ANOVA determined how group (obese vs. non-obese) and speed/grade affected metabolic rate/cost. Bland-Altman plots and linear regression was used to determine the accuracy of prediction equations compared to measured oxygen consumption. RESULTS: Net metabolic rate increased with walking speed and grade, ranging from 1.91 (0.06) W/kg at 1.25 m/s, -3° to 5.91 (0.13) W/kg at 1.50 m/s, 3°, (mean (SE)). Obese individuals walked with a smaller gross metabolic rate (p<0.001), net metabolic rate (p=0.013), gross metabolic cost (p<0.001) and net metabolic cost (p=0.006) compared to non-obese adults. Body fat percentage, VO2 peak, and step width did not explain the variance in metabolic rate. Positive joint work was related to net metabolic rate during level walking, but not during uphill walking. ACSM and Pandolf prediction equations did not accurately predict metabolic rate/energy expenditure at all speed/grade combinations. Thus, we developed a new prediction equation for obese adults that is more accurate in predicting the energetics of walking. CONCLUSIONS: The smaller metabolic rate in obese adults suggests these individuals have better economy when walking on level or uphill/downhill grades. The mechanism by which economy is improved in obese adults has yet to be discovered.