. Effects of obesity and sex on the energetic cost and preferred speed of walking. J Appl Physiol 100: 390 -398, 2006. First published October 6, 2005 doi:10.1152/japplphysiol.00767.2005.-The metabolic energy cost of walking is determined, to a large degree, by body mass, but it is not clear how body composition and mass distribution influence this cost. We tested the hypothesis that walking would be most expensive for obese women compared with obese men and normalweight women and men. Furthermore, we hypothesized that for all groups, preferred walking speed would correspond to the speed that minimized the gross energy cost per distance. We measured body composition, maximal oxygen consumption, and preferred walking speed of 39 (19 class II obese, 20 normal weight) women and men. We also measured oxygen consumption and carbon dioxide production while the subjects walked on a level treadmill at six speeds (0.50 -1.75 m/s). Both obesity and sex affected the net metabolic rate (W/kg) of walking. Net metabolic rates of obese subjects were only ϳ10% greater (per kg) than for normal-weight subjects, and net metabolic rates for women were ϳ10% greater than for men. The increase in net metabolic rate at faster walking speeds was greatest in obese women compared with the other groups. Preferred walking speed was not different across groups (1.42 m/s) and was near the speed that minimized gross energy cost per distance. Surprisingly, mass distribution (thigh mass/body mass) was not related to net metabolic rate, but body composition (% fat) was (r 2 ϭ 0.43). Detailed biomechanical studies of walking are needed to investigate whether obese individuals adopt novel energy saving mechanisms during walking. locomotion; energy cost per distance; cost of transport; economy; body mass distribution WALKING IS A POPULAR AND CONVENIENT form of exercise that can play an important role in weight management (24, 26) . Effective weight management requires an accurate knowledge of how much metabolic energy is expended during exercise. Obese individuals expend much more metabolic energy during walking than normal-weight individuals (3, 16, 18, 31, 32) . However, the energy expended across walking speeds has only recently been established for obese female adults (6, 31, 32), and it is not well understood for obese men. When gross metabolic rate is expressed per kilogram of total body mass, the difference between individuals who are obese vs. normal weight is much reduced (1, 6), which suggests that total body weight is the primary determinant of the cost of walking. Measuring the net metabolic rate (gross Ϫ standing) can give a better measure of the cost of the walking movement itself. Some previous studies of energy expenditure during level walking suggest a 10 -15% greater net metabolic rate of walking (per kg of total body mass) for class II obese adults (6, 31, 32) . Other studies suggest that walking may be as much as 50% more expensive for adults with a BMI Ͼ35 kg/m 2 (3, 16, 18, 32). The greater net metabolic rate of obese adults may be partly due to heavier and larger legs that require an increase in step width and leg swing circumduction (lateral leg swing) (40). Both factors have been shown to substantially increase net metabolic rate in normalweight subjects (10, 39). One might predict that walking would be more expensive for obese women because women carry more of their body fat in the hips and thighs (gynoid adiposity) than men (android adiposity) (4). However, it is not known whether the energy expenditure during walking is different for obese women vs. obese men and whether any difference is due to the distribution of adipose tissue. It seems logical to expect that the net metabolic cost of walking is affected by the distribution of adipose tissue. Experiments on normal-weight individuals show that walking is more expensive when mass is placed on the thighs or lower legs compared with waist loads (41). This increase in net metabolic rate is partly due to the increase in mechanical work required to swing legs that have a greater mass and moment of inertia (38). Women may have relatively heavier legs (thigh mass/body mass) than men due to differences in the distribution of body fat (4), which might result in a greater net metabolic rate. Dual-energy X-ray absorptiometry (DEXA) provides a means of accurately determining leg segment masses and moments of inertia (13), but no leg mass or moment of inertia data have been reported for obese individuals. Even among normal-weight individuals, sex may affect the net metabolic rate measured during walking. Normal-weight women and men have similar gross energy expenditures during walking (41). However, normal-weight women have smaller standing metabolic rates (per kg body weight) than normalweight men (37) because of their smaller lean body mass (greater body fat percent) (7). The similar gross and smaller standing metabolic rates of normal-weight women would presumably result in a greater net metabolic rate than normalweight men during walking. In normal-weight adults, the gross energy consumed per unit distance vs. walking speed relationship is U-shaped (30, 35). The minimum energy cost required to walk a given distance occurs at ϳ1.4 m/s (ϳ5 km/h or 3 miles/h) (30, 34, 43) , which is also the preferred walking speed of normal-weight adults (37). Class II obese adults prefer to walk more slowly than Address for reprint requests and other correspondence: R. Browning,
We define two scenarios: One without mobility (static) and another with mobility (dynamic) at a walking speed of 1.4 m/s -based on an estimation of the preferred walking speed REF .
Effects of obesity and sex on the energetic cost and preferred speed of walking
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