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Keywords:

  • locomotion;
  • energy cost per distance;
  • cost of transport

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Research Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgement
  8. References

Objective: We tested the hypotheses that walking is more expensive for obese women, and they prefer slower walking speeds that minimize the gross energy cost per distance despite a greater relative aerobic effort [percent of maximal oxygen uptake (o2max)/kg].

Research Methods and Procedures: Twenty adult women, 10 obese (BMI = 34.1 ± 3.2 kg/m2) and 10 normal weight (BMI = 20.4 ± 2.1 kg/m2) volunteered. To determine the metabolic rate and energy cost per distance vs. speed relationships, we measured o2 and V˙CO2 while subjects walked on a treadmill at six speeds (0.50, 0.75, 1.0, 1.25, 1.5, and 1.75 m/s; 5-minute trials, with a 5-minute rest period between trials). We measured preferred walking speed on a 50-m section of level sidewalk and o2max using a modified Balke treadmill protocol.

Results: Walking was 11% more expensive for the obese subjects, but they preferred to walk at similar speeds as normal weight subjects (1.40 vs. 1.47 m/s, p = 0.07). Both groups preferred walking speeds at which their gross energy cost per distance was almost minimized. Obese subjects had a smaller o2max/kg, so they required a greater relative aerobic effort at the preferred speed (51% vs. 36%, p = 0.001).

Discussion: Obese women preferred a walking speed that minimized energy cost per distance, even though this strategy required a greater relative aerobic effort than walking more slowly. Our results suggest that walking slower for a set distance may be an appropriate exercise recommendation for a weight management prescription in obese adults.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Research Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgement
  8. References

Walking is a popular, convenient, and relatively safe form of exercise (1) that holds great promise for weight management (2, 3, 4). Weight management is most effective when individuals can accurately determine how much energy they expend during exercise, which, in the case of walking, is dependent on speed. However, it is not known how much energy obese adults expend while walking at different speeds.

Energy expenditure during walking influences the preferred walking speed of normal weight adults (5). The rate of metabolic energy consumption (e.g., watts) increases curvilinearly with walking speed (5, 6). As a result, the amount of energy consumed per unit distance has a U-shaped curve when plotted vs. walking speed (6, 7, 8). Hence, there is one speed for each individual that minimizes the amount of energy required to walk a given distance. For normal weight adults, the minimum energy cost per distance occurs at ∼1.4 m/s or ∼ 3 mph, which requires ∼36% of their aerobic capacity (6, 9) and corresponds to measured preferred walking speed (10). Older, normal weight adults prefer a slower walking speed than young adults, and this slower speed also corresponds to the minimum energy cost per distance (11). Interestingly, when normal weight adults walk backward, they also prefer the speed that minimizes energy cost per distance (12). These observations suggest that the body can sense the energy cost per distance and walk so as to minimize it.

Surprisingly few studies have examined the rate of metabolic energy consumption during level walking in obese adults. These studies report that, at a given speed, the metabolic rate for walking (per kilogram of total body mass) in obese adults ranges from 0% to 33% greater than in normal weight adults (13, 14, 15, 16, 17). This wide range of values may be because of differences in subject adiposity and walking speeds. The obese subjects in these studies had varying degrees of adiposity, with mean BMI values ranging from 32 to 52 kg/m2. Although the relationship between levels of adiposity and the metabolic cost of walking has not been clearly established, recent data suggest that walking is more expensive for adults with greater BMI values (15). In addition, some of the previously mentioned studies have compared normal weight and obese subjects at predetermined walking speeds (13, 14), whereas other studies measured the metabolic rate at each individual's preferred speed (16, 17, 18). Speed is an important consideration because of the curvilinear metabolic rate (watts per kilogram) vs. speed relationship. Faster walking speeds incur disproportionately greater metabolic rates.

Previous studies have indicated that obese adults prefer to walk more slowly, between 0.75 and 1.2 m/s (1.5 to 2 mph), depending on the degree of obesity (15, 16, 17, 19). The aerobic capacity per kilogram total body mass (o2max/kg)1 is much less for obese adults than for normal weight peers (17, 20, 21), and walking at their preferred speed requires a greater relative aerobic effort (%o2max/kg) (17). Obese adults may prefer to walk slower to reduce their acute metabolic rate (i.e., energy expenditure per unit time), thus making walking more comfortable.

No study has comprehensively determined how the metabolic rate or energy cost per distance for walking varies across a range of speeds for obese adults. As a result, it is not known whether preferred walking speed corresponds to the minimum energy cost per distance. The purpose of this study was to compare the metabolic rates and energy cost per distance of walking vs. speed relationships and the preferred walking speed for obese vs. normal weight women.

We hypothesized the following for obese vs. normal weight women: 1) walking would be more expensive (per kilogram total body mass); 2) preferred walking speed would be slower; and 3) preferred walking speed would correspond to the speed that minimizes energy cost per distance.

Research Methods and Procedures

  1. Top of page
  2. Abstract
  3. Introduction
  4. Research Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgement
  8. References

Subjects

Two groups of young female adults volunteered for this study: obese (n = 10) and normal weight (n = 10). The physical characteristics of the subjects are shown in Table 1. All subjects were in good health and were not taking medications known to influence metabolism. Subjects were sedentary to moderately physically active (<90 min/wk), and their mass was stable (<2.5 kg change) over the previous 3 months. Subjects gave written informed consent that followed the guidelines of the University of Colorado Human Research Committee.

Table 1. . Physical characteristics of obese and normal weight subjects
Physical characteristicObeseNormal weight
  • Values are means (±SD).

  • *

    p < 0.05, obese vs. normal.

Age (years)25.5 (6.9)26.6 (5.5)
Height (m)1.67 (0.06)1.68 (0.06)
Body mass (kg)95.4 (14.9)*58.7 (9.5)
BMI (kg/m2)34.1 (3.2)*20.4 (2.1)
Percent body fat (%)46.0 (2.6)*27.8 (6.2)
Lean body mass (kg)51.5 (8.0)*42.1 (5.9)
Leg mass (kg)34.7 (6.6)*20.6 (3.4)

Protocol

Each subject completed three test sessions. In the first session, which followed a 12-hour fast, subjects had a physical examination, blood was drawn and analyzed, and body composition was measured. In the second session, which followed a 4-hour fast and took place within the next week, we familiarized each subject with the treadmill (Track Master 425; Full Vision, Inc., Newton, KS) and performed a maximal oxygen uptake (o2max) test. In the third session, which followed a 4-hour fast and took place the subsequent week, we measured each subject's preferred walking speed, and each subject completed six level walking trials. The trials began after 5 minutes of quiet standing on the treadmill and speeds were 0.5, 0.75, 1.0, 1.25, 1.5, and 1.75 m/s. The subjects rested for 5 minutes between each walking trial. Trial order progressed from the slowest to the fastest speed. Subjects were allowed to select a comfortable stride frequency during each trial.

Assessments

Physical Health and Activity

Each subject's health and physical activity level was assessed by examination and interview. Each subject completed a medical history form and was interviewed and examined by a physician. Resting heart rate and blood pressure were recorded. Blood was drawn to test for normal metabolic function. Resting levels of glucose, thyroid-stimulating hormone, blood cell counts, and profiles were determined and confirmed to be within normal ranges. Physical activity level was assessed through an activity questionnaire, and subjects who engaged in >90 minutes of moderate or vigorous activity per week were excluded from this study.

Body Composition

We measured each subject's body composition using a whole body DXA scanner (DPX-IQ; Lunar Corp., Madison, WI). The DXA scan measured fat mass, lean tissue mass, and bone mineral content of the total body and of the trunk, arm, and leg regions.

Maximal Oxygen Uptake

We determined each subject's o2max using a modified Balke treadmill protocol (22). The subjects were familiarized with the Borg Rating of Perceived Exertion scale (6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 23) and with treadmill walking. This familiarization was followed by a warm-up period where we gradually increased treadmill speed. We monitored heart rate and perceived exertion during the warm-up and set the testing speed to elicit a heart rate between 70% and 80% of age-predicted maximum and a Rating of Perceived Exertion of 11 to 13. During the test, a physician monitored heart function using a 12-lead electrocardiogram. Treadmill speed was held constant as grade was increased by 2% every 2 minutes. The subjects were encouraged to continue to exhaustion. We determined oxygen consumption by open circuit respirometry (CardiO2/CP Gas Exchange System; MedGraphics, St. Paul, MN), with expired gas data averaged over 30-second intervals.

Preferred Walking Speed

We determined the preferred walking speed of each subject by measuring the time required to walk 50 m on a level (<1° inclination as measured by surveyor's line level) sidewalk. Subjects were instructed to walk back and forth six times along a 70-m section of sidewalk at a “comfortable walking pace.” Subjects were timed over the middle 50 m during each trial. The preferred walking speed was calculated as the mean of the last five trials. All preferred speed measurements were made during clement weather and at a time when the sidewalk was free of other pedestrian traffic.

Energetic Measurements

We measured the rates of oxygen consumption (o2) and carbon dioxide production (CO2) using an open circuit respirometry system (CardiO2/CP Gas Exchange System; MedGraphics). Before beginning the experimental trials, we calibrated the system and measured standing metabolic rate. For all trials, we allowed 3 minutes for the subjects to reach steady state (no significant increase in o2 during final 2 minutes and respiratory exchange ratio <1.0) and calculated the average o2 (milliliters of O2 per second) and CO2 (milliliters of CO2 per second) for the final 2 minutes of each trial. We calculated gross metabolic rate (watts per kilogram) using a standard equation (24). We subtracted the standing value from the walking values to derive net metabolic rate. We used net metabolic rate to quantify the energy consumption required for the walking task. Finally, we divided gross metabolic rate (watts per kilogram total body mass) by walking speed (meters per second) to resolve gross metabolic energy cost per distance (Joules per kilogram per meter). We used gross metabolic values for cost per distance because studies have shown that preferred walking speed corresponds to the minimum in gross energy cost per distance in normal weight adults (5, 6).

Statistical Analysis

We used independent Student's t tests to determine group differences in physical characteristics (i.e., height, body mass, lean body mass, percent body fat, o2max, standing o2, and preferred walking speed). We used two-factor repeated measures ANOVA to determine how walking speed affects metabolic rate, energy cost per distance, and relative aerobic effort. Finally, we performed warranted posthoc statistical analyses using Tukey's HSD procedure. Significance was defined as p ≤ 0.05.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Research Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgement
  8. References

Walking was more expensive for the obese subjects (Figure 1; Table 2), but they preferred to walk at similar speeds as normal weight subjects. Both groups preferred to walk at speeds where their gross energy cost per distance was nearly minimized (Figure 2). Obese subjects also had a smaller o2max/kg so they required a greater relative aerobic effort at the preferred speed (Figure 3).

image

Figure 1. Net metabolic rate vs. walking speed for obese (filled squares and thick line) and normal weight (open squares and thin line) groups. Metabolic rate increased with walking speed and was significantly greater for the obese at 0.75, 1.25, 1.5, and 1.75 m/s (p < 0.05). Lines are second-order least squares regressions. Values are means ± SE. Some error bars are obscured by the size of the square symbols. *Significant differences between groups at that speed. Obese W/kg = 2.357(v)2 − 2.126(v) + 1.829, r2 = 0.94. Normal weight W/kg = 1.842(v)2 − 1.413(v) + 1.46, r2 = 0.95; v = walking speed (m/s).

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Table 2. . Net metabolic rate (watts per kilogram) vs. walking speed for obese and normal weight subjects
 Speed (m/s)
 0.50.751.01.251.51.75
  • Values are means (±SE).

  • Metabolic rate increased as walking speed increased in both groups (p < 0.001). The metabolic rate was significantly higher for the obese at all speeds except 0.5 and 1.0 m/s.

  • *

    p < 0.05, obese vs. normal.

Obese1.28 (0.04)1.70 (0.05)*2.08 (0.07)2.72 (0.10)*3.97 (0.18)*5.35 (0.14)*
Normal weight1.14 (0.06)1.54 (0.05)1.97 (0.05)2.47 (0.05)3.41 (0.07)4.69 (0.17)
image

Figure 2. Gross energy cost per distance for obese (filled squares and thick line) and normal weight (open squares and thin line) groups. Lines are second-order least squares regressions. Minimum energy cost per distance speed was slower for obese (1.23 m/s) compared with normal weight women (1.33 m/s; p = 0.02). Minimum energy cost per distance was 3.00 vs. 3.01 J/kg per meter for obese and normal weight women, respectively. The solid vertical lines indicates preferred walking speed: obese (thick line) = 1.40 m/s; normal weight (thin line) = 1.47 m/s. Energy cost per distance at preferred speeds was 3.07 vs. 3.06 J/kg per meter for obese and normal weight women, respectively. Values are means ± SE. Obese J/kg/m = 2.859(v)2 −7.137(v) + 7.455, r2 = 0.80. Normal weight J/kg/m = 2.729(v)2 −7.256(v) + 7.833, r2 = 0.83; v = walking speed (m/s).

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image

Figure 3. Relative aerobic effort (%o2max/kg) for obese (filled squares and thick line) and normal weight (open squares and thin line) groups. The relative aerobic effort was significantly greater for the obese at all speeds (p < 0.05). The solid vertical lines indicate preferred walking speed: obese (thick line) = 1.40 m/s; normal weight (thin line) = 1.47 m/s. Walking at the preferred speed required 51% vs. 36% of o2max/kg for obese and normal weight groups, respectively. To achieve the same relative effort would require the obese to walk at ∼1.0 m/s. Values are means ± SE. *Significant differences between groups at that speed.

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Physical and Metabolic Characteristics

By design, the obese and normal weight groups were dramatically different in terms of physical characteristics. These differences included a significantly greater body mass, BMI, percent body fat, lean mass, and leg mass for the obese vs. the normal weight groups (Table 1). Standing metabolic rates were also different between the two groups (Table 3). Pre-exercise standing oxygen consumption rates normalized to total body mass were 21% less in the obese group compared with the normal group (3.13 vs. 3.94 mL O2/kg per minute; p < 0.001), likely a result of the proportionally less metabolically active tissue in the obese subjects (Table 3). No differences were found between obese and normal weight groups in absolute o2max (2.44 vs. 2.27 liters/min; p = 0.37), but the obese group had a 33% lower total body mass-specific o2max (25.8 vs. 38.7 mL/kg per minute; p < 0.001).

Table 3. . Standing and maximal metabolic rates for obese and normal weight subjects
Metabolic rateObeseNormal weight
  • Values are means (±SD).

  • Kilogram is total body mass unless specified.

  • *

    p < 0.05, obese vs. normal.

Standing O2 Consumption (mL O2/kg/min)3.13 (0.34)*3.94 (0.51)
Standing O2 Consumption (mL O2/kg lean/min)5.80 (0.63)5.49 (0.71)
Standing Metabolic Rate (W/kg)1.02 (0.12)*1.32 (0.17)
o2max (liters/min)2.44 (0.38)2.27 (0.47)
o2max (mL O2/kg/min)25.8 (3.1)*38.7 (4.4)

Metabolic Rate during Walking

The mean net metabolic rate was 11% higher for the obese subjects compared with normal weight subjects averaged across all walking speeds (2.81 vs. 2.54 W/kg; p = 0.01). The net metabolic rates (watts per kilogram) for walking were significantly greater for the obese group at 0.75, 1.25, 1.5, and 1.75 m/s (Figure 1; Table 2). Four of the obese subjects had slight, but significant, increases in oxygen consumption (<7%) during the final 2 minutes at the 1.75 m/s walking speed. Using a revised regression equation without those four subjects resulted in only slight changes (<1.5% at preferred walking speed) in the metabolic rate vs. speed relationship for the obese and no changes in the statistical significance of the data. As a result, these subjects were not excluded from further analysis. The metabolic rate vs. speed relationship for each group was curvilinear (second-order least squares regression, r2 = 0.94).

Preferred Walking Speed

Preferred walking speeds were similar between groups. The obese walked slower (1.40 m/s) than the normal weight subjects (1.47 m/s), but the difference was not statistically significant (p = 0.07).

Gross Energy Cost per Distance

The gross energetic cost per distance (Joules per kilogram per meter) of walking was not different between the groups at any speed (p = 0.30; Figure 2). The calculated speed (from second-order least squares regression) that corresponded to the minimum energy cost per distance was 8% slower for the obese (1.23 vs. 1.33 m/s; p = 0.02). The energy cost per distance vs. speed relationship was not different between the groups, even at the faster speeds, because of the effect of the lower standing metabolic rates of the obese subjects. By interpolating this relationship to their preferred walking speeds, we found that the gross energy cost per distance was almost identical for the two groups (3.07 vs. 3.06 J/kg per meter for obese and normal weight, respectively; p = 0.95). The difference in the energy cost per distance at the preferred speed and at the minimum energy cost per distance speed was small, but statistically significant, in both groups (3.07 vs. 3.00 J/kg per meter for obese, p = 0.003; 3.06 vs. 3.01 J/kg per meter for normal weight, p = 0.02).

Relative Aerobic Effort at Preferred Speed

The relative aerobic effort required to walk at the preferred speed was greater (p < 0.01) in the obese than normal weight women. Walking at the preferred speed required 51% of o2max/kg for the obese subjects and 36% of o2max/kg for the normal weight subjects. The obese subjects would need to walk at just 1.0 m/s to have the same relative aerobic effort as normal weight subjects walking at their preferred speed (Figure 3).

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Research Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgement
  8. References

Metabolic Rate

We accepted our hypothesis that obese women have a greater net metabolic rate (per kilogram of total body mass) for walking. The 11% difference in net metabolic rate is less than early published reports but similar to recent literature. An early study by Freyschuss and Melcher (14) found that oxygen consumption was 33% greater in Class III (BMI > 40 kg/m2) (25) obese men and women compared with normal weight controls walking at 1.0 m/s across a range of inclines. The greater difference in metabolic rate in that study may be because, in part, of the greater adiposity (percent fat) of their subjects. The metabolic rate vs. speed relationship for Class III obese has not been established, but recent data indicate that metabolic rate increases with BMI (15). In addition, the subjects in the study of Freyschuss et al. walked uphill, which may increase the metabolic rate disproportionately for obese subjects. In another early study, Bloom and Marshall (13) reported that the gross metabolic rate of walking at speeds ranging from 0.7 to 1.4 m/s was ∼20% greater in obese men and women compared with normal weight adults, although that study was difficult to interpret accurately because mean data were not presented. In contrast, a recent study by Melanson et al. (15) reported regression equations that predicted only a 10% greater net metabolic rate of walking for obese women (BMI = 35 kg/m2) compared with normal weight women (BMI = 25 kg/m2) when walking at 1.4 m/s.

Biomechanically, there are several factors that would suggest walking should be far more expensive for obese women than we found it to be. These factors include greater body mass and heavier legs. First, the mass of obese subjects was 36 kg heavier than the normal weight subjects. According to an equation developed by Pandolf et al. (26), normal weight subjects walking at 1.4 m/s while carrying a 36 kg backpack would require an increase in net energy expenditure of 15% (per total kilograms). Second, the obese subjects in this study had legs that were 14.1 kg heavier than the normal weight subjects. Adding mass to the legs of normal weight adults greatly increases the energy cost of walking (10, 27, 28). According to Inman et al. (10), adding 1 kg to the legs of normal weight adults increases oxygen consumption by 3.5%. Thus, the 14 kg of extra leg mass should theoretically result in a 50% increase in metabolic rate in the obese subjects in our study.

Other biomechanical factors that increase the metabolic rate of walking include decreased stability, wider stance, and wider lateral leg swing. Obesity has been shown to reduce postural stability (29). Greater instability requires compensatory muscle actions that may increase the energy cost of walking (30). A recent study has shown that lateral stabilization during walking comprises ∼6% of the metabolic rate in non-obese adults (31). Furthermore, obese adults walk with a wider step width and lateral leg swing (19). Normal weight adults consume 25% more energy when they double their step width (32). Walking with an enforced wide leg swing increased the energy cost of walking in normal weight subjects by up to 30% (33).

Summing these biomechanical factors indicates that the energy cost of walking should be >100% greater for the obese; however, our observed difference was only 11%. Although the factors may not truly be additive, we would expect the increase in metabolic rate to be much greater than 11%. This discrepancy is likely because of the difference between experiments that acutely simulate the effects of obesity on locomotion and the long-term adaptive mechanisms acquired as adiposity increases. A mechanism by which obese may mitigate the cost of walking could be a more effective use of the body as an inverted pendulum. Heglund et al. (34) have reported that African women who carry loads on their heads have a smaller than expected increase in metabolic rate because of an improved recovery of mechanical energy. Another mechanism would be to walk with straighter legs and a more erect posture, which would reduce the muscle force required to support the body (35). Whatever the adaptive mechanisms are, they are quite effective at ameliorating the metabolic cost of walking but are not well understood.

Preferred Walking Speed

We rejected our hypothesis that preferred walking speed would be slower for obese compared with normal weight women. This result was surprising, considering data from previous studies. Mattsson et al. (17) measured a preferred speed of 1.18 m/s for moderately obese women, and Melanson et al. (15) reported a normal walking speed of 1.19 m/s for adults with a BMI >29 kg/m2. Spyropoulos et al. (19) measured a preferred speed of 1.09 m/s for Class III obese men, whereas Ohrstrom et al. (16) reported that Class III obese women preferred a speed of just 0.75 m/s. The faster preferred speed that we found might be because of methodological differences. We measured preferred walking speed outdoors, whereas other studies used a treadmill (16) or indoor walkway to measure speed (15, 17, 19). Although we could find no study that has directly addressed the issue of overground vs. treadmill preferred speed measurement, a review of the literature suggests that both methods result in similar speeds (11, 36, 37, 38, 39). We chose to measure preferred speed outdoors because we felt it was most likely to reflect the speed an individual would select during the common walking tasks of everyday life as well as during exercise.

Both the obese and normal weight groups preferred to walk at a speed that was near the minimum energy cost per distance, suggesting this strategy for selecting speed is used in both groups. Although both groups preferred a walking speed that was slightly faster than the minimum energy cost per distance speed, the change in energy cost per distance between the two speeds was very small. The difference between preferred and minimum energy cost per distance speeds was similar in both groups (0.16 and 0.14 m/s, respectively).

It is not known whether obese adults would maintain the same preferred walking speed during a longer trial (e.g., 30 minutes). We measured speed over 50 m, and three of the above-mentioned studies measured speed over 15 m (15, 19), i.e., all relatively short distances. Mattsson et al. (17) measured speed over a 70-m loop during a 4-minute trial, which is still a relatively short duration. Given the limited functional aerobic capacities of obese adults, they may prefer to walk slower if the duration or distance of the trial is extended (i.e., the sustained preferred speed). Future studies should measure the sustained preferred walking speed of obese adults over a typical recommended exercise session duration (e.g., 30 minutes), because this would improve the determination of energy expenditure in response to an exercise prescription.

Gross Energy Cost per Distance

The gross energy cost per distance vs. speed relationship was similar for our obese and normal weight subjects. This similarity was primarily because of lower standing energy expenditure (per kilogram) for obese subjects that offset their greater net energy cost of walking. The U-shaped energy cost per distance vs. speed relationship for normal weight subjects was comparable to previous literature (6, 11). Martin et al. (11) reported a minimum energy cost per distance of ∼3.2 J/kg per meter at a speed of 1.34 m/s for young sedentary adults, very similar to the 3.01 J/kg per meter at 1.33 m/s for our normal weight group.

Although the relationships were not significantly different, the energy cost per distance curve for the obese was shifted slightly toward slower speeds (Figure 2). This shift is a result of the steeper slope of the net metabolic rate vs. speed for the obese. We speculate that this shift may become more pronounced as adiposity increases, which would reduce the speed where the energy cost per distance is minimum, and might result in a slower preferred walking speed in Class III obese adults. Our results show that the minimum energy cost per distance is not affected by moderate obesity, but it is not known whether this applies to greater levels of adiposity.

Relative Aerobic Effort

The relative effort (%o2max/kg) at the preferred walking speed was greater for the obese compared with the normal weight women. The obese women's strategy of selecting preferred speed based on minimizing energy cost per distance required a moderate aerobic effort (51% of o2max/kg). This aerobic effort is similar to the 56% of o2max /kg reported by Mattsson et al. (17) for moderately obese women walking at 1.18 m/s. These results show that walking at their preferred speed is mildly taxing for young obese women. It may be that during a sustained walking task, these obese women would slow to a speed that required a similar, more comfortable, relative intensity as normal weight women. A slower sustained preferred walking speed would suggest that obese women do not walk at the minimum energy cost per distance but choose to reduce the rate of energy expenditure to reduce the relative aerobic effort. Although the speed that minimizes energy cost per distance does not change with external backpack load, individuals do prefer to walk more slowly as load increases, suggesting they no longer walk at the minimum energy cost per distance speed (40).

Relevance for Exercise Prescription

A common exercise prescription for obese adults is moderate intensity physical activity (e.g., brisk walking) for a set duration (30 minutes) (41, 42). Our data invite a review of that prescription if the primary goal is safe, effective weight loss. The U-shaped energy cost per distance vs. speed relationship (Figure 2) shows that by walking at a slower speed, the energy cost/distance increases. Practically, this means that walking a set distance more slowly requires more energy. Table 4 shows an example of two exercise prescriptions that are equal in distance but differ in time and walking speed. Walking slowly for a longer period of time increases energy expenditure slightly.

Table 4. . Energy expenditure of two exercise prescriptions for an obese adult
    Energy expenditure
TaskDuration (min)Speed (m/s)Distance (m)KilojoulesKilocalories
  1. Data based on mean obese data, mass = 95.4 kg. Energy expenditure for normal walk based on interpolation of energy cost vs. speed regression to 1.4 m/s.

Normal walk301.42520741177
Slow walk421.02520762182

A strategy of prescribing slow walking for a set distance for obese persons may also provide biomechanical benefits. Obese adults have a greater risk of developing knee osteoarthritis (43) than normal weight adults. Slower walking speeds result in reduced biomechanical loads on the lower extremities (44), which may reduce the risk of chronic injuries and osteoarthritis, which are associated with walking. Annually, ∼8% of habitual walkers with BMI >25 kg/m2 abandon their exercise program because of musculoskeletal injury (1). A drawback to the slow walking approach is that the activity may not reach the level of moderate intensity, thus reducing the cardiovascular benefits of the exercise.

Summary

Our results show that walking demands a greater metabolic rate and a greater relative aerobic effort for obese vs. normal weight women. The acute preferred walking speed is not significantly slower for obese women. Obese and normal weight women use a similar strategy of minimizing the energy cost per distance to select preferred speed. Our results suggest that walking slower for a set distance may be an appropriate exercise recommendation for a weight management prescription in obese adults.

Acknowledgement

  1. Top of page
  2. Abstract
  3. Introduction
  4. Research Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgement
  8. References

This study was supported by NIH Grants AR44688 and M01-RR00051. The suggestions of Dr. Ed Melanson and the assistance of the General Clinical Research Center staff and the Locomotion laboratory (Boulder, CO) are gratefully acknowledged.

Footnotes
  • 1

    Nonstandard abbreviations: o2, oxygen consumption; o2max, maximal oxygen output; V˙CO2, carbon dioxide production.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Research Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgement
  8. References
  • 1
    Hootman, J. M., Macera, C. A., Ainsworth, B. E., Addy, C. L., Martin, M., Blair, S. N.. (2002) Epidemiology of musculoskeletal injuries among sedentary and physically active adults. Med Sci Sports Exerc. 34: 838844.
  • 2
    Hagan, R. D., Upton, S. J., Wong, L., Whittam, J.. (1986) The effects of aerobic conditioning and/or caloric restriction in overweight men and women. Med Sci Sports Exerc. 18: 8794.
  • 3
    Hill, J. O., Peters, J. C.. (1998) Environmental contributions to the obesity epidemic. Science. 280: 13711374.
  • 4
    Jakicic, J. M., Winters, C., Lang, W., Wing, R. R.. (1999) Effects of intermittent exercise and use of home exercise equipment on adherence, weight loss, and fitness in overweight women: a randomized trial. JAMA. 282: 15541560.
  • 5
    Ralston, H. J.. (1958) Energy-speed relation and optimal speed during level walking. Int Z Angew Physiol. 17: 277283.
  • 6
    Margaria, R.. (1976) Biomechanics and Energetics of Muscular Exercise. Clarendon Press: Oxford.
  • 7
    DiPrampero, P. E.. (1986) The energy cost of human locomotion on land and water. Int J Sports Med. 7: 5572.
  • 8
    Zarrugh, M. Y., Todd, F. N., Ralston, H. J.. (1974) Optimization of energy expenditure during level walking. Eur J Appl Physiol Occup Physiol. 33: 293306.
  • 9
    Astrand, P. O., Rodahl, K.. (1977) Textbook of Work Physiology: Physiological Bases of Exercise 2nd ed. McGraw Hill: New York.
  • 10
    Inman, V. T., Ralston, H. J., Todd, B.. (1981) Human walking. In: Human Walking Williams and Wilkins: Baltimore, MD. 6277.
  • 11
    Martin, P. E., Rothstein, D. E., Larish, D. D.. (1992) Effects of age and physical activity status on the speed-aerobic demand relationship of walking. J Appl Physiol. 73: 200206.
  • 12
    Tolani, N. A., Kram, R.. (1999) Biomechanics of backward walking. International Society of Biomechanics Congress. International Society of Biomechanics: Calgary, Alberta, Canada. 254.
  • 13
    Bloom, W. L., Marshall, F. E.. (1967) The comparison of energy expenditure in the obese and lean. Metabolism. 16: 685692.
  • 14
    Freyschuss, U., Melcher, A.. (1978) Exercise energy expenditure in extreme obesity: influence of ergometry type and weight loss. Scand J Clin Lab Invest. 38: 753759.
  • 15
    Melanson, E. L., Bell, M. L., Knoll, J. R., et al (2003) Body mass index and sex influence the energy cost of walking at self-selected speeds. Med Sci Sports. 35(5 Suppl): S183
  • 16
    Ohrstrom, M., Hedenbro, J., Ekelund, M.. (2001) Energy expenditure during treadmill walking before and after vertical banded gastroplasty: a one-year follow-up study in 11 obese women. Eur J Surg. 167: 845850.
  • 17
    Mattsson, E., Larsson, U. E., Rossner, S.. (1997) Is walking for exercise too exhausting for obese women? Int J Obes Relat Metab Disord. 21: 380386.
  • 18
    Melanson, E. L., Sharp, T. A., Seagle, H. M., et al (2002) Effect of exercise intensity on 24-h energy expenditure and nutrient oxidation. J Appl Physiol. 92: 10451052.
  • 19
    Spyropoulos, P., Pisciotta, J. C., Pavlou, K. N., Cairns, M. A., Simon, S. R.. (1991) Biomechanical gait analysis in obese men. Arch Phys Med Rehabil. 72: 10651070.
  • 20
    Dempsey, J. A., Reddan, W., Balke, B., Rankin, J.. (1966) Work capacity determinants and physiologic cost of weight-supported work in obesity. J Appl Physiol. 21: 18151820.
  • 21
    Byrne, N. M., Hills, A. P.. (2002) Relationships between HR and V˙o2 in the obese. Med Sci Sports Exerc. 34: 14191427.
  • 22
    Franklin, B. H., Whaley, G. P., Howley, E. T.. (2002) ACSM's Guidelines for Exercise Testing and Prescription 6th ed. Lippincott Williams & Wilkins: Philadelphia, PA.
  • 23
    Borg, G. A., Linderholm, H.. (1967) Perceived exertion and pulse rate during graded exercise in various age groups. Acta Med Scand. 472(suppl): 194206.
  • 24
    Brockway, J. M.. (1987) Derivation of formulae used to calculate energy expenditure in man. Hum Nutr Clin Nutr. 41: 463471.
  • 25
    World Health Organization (1998) Obesity. Preventing and Managing the Global Epidemic. World Health Organization: Geneva, Switzerland.
  • 26
    Pandolf, K. B., Givoni, B., Goldman, R. F.. (1977) Predicting energy expenditure with loads while standing or walking very slowly. J Appl Physiol. 43: 577581.
  • 27
    Strydom, N. B., van Graan, C. H., Morrison, J. F., Viljoen, J. H., Heyns, A. J.. (1968) The influence of boot weight on the energy expenditure of men walking on a treadmill and climbing steps. Int Z Angew Physiol. 25: 191197.
  • 28
    Ralston, H. J., Lukin, L.. (1969) Energy levels of human body segments during level walking. Ergonomics. 12: 3946.
  • 29
    McGraw, B., McClenaghan, B. A., Williams, H. G., Dickerson, J., Ward, D. S.. (2000) Gait and postural stability in obese and nonobese prepubertal boys. Arch Phys Med Rehabil. 81: 484489.
  • 30
    Hoffman, M. D., Sheldahl, L. M., Buley, K. J., Sandford, P. R.. (1997) Physiological comparison of walking among bilateral above-knee amputee and able-bodied subjects, and a model to account for differences in metabolic cost. Arch Phys Med Rehabil. 78: 385392.
  • 31
    Donelan, J. M., Shipman, D. W., Kram, R., Kuo, A. D.. (2004) Mechanical and metabolic requirements for active lateral stabilization in human walking. J Biomech. 37: 827835.
  • 32
    Donelan, J. M., Kram, R., Kuo, A. D.. (2001) Mechanical and metabolic determinants of the preferred step width in human walking. Proc R Soc Lond B Biol Sci. 268: 19851992.
  • 33
    Shipman, D. W., Donelan, J. M., Kram, R., Kuo, A. D.. (2002) Metabolic cost of lateral leg swing in human walking. World Congress of Biomechanics. American Society of Biomechanics: Calgary, Alberta, Canada.
  • 34
    Heglund, N. C., Willems, P. A., Penta, M., Cavagna, G. A.. (1995) Energy-saving gait mechanics with head-supported loads. Nature. 375: 5254.
  • 35
    DeVita, P., Hortobagyi, T.. (2003) Obesity is not associated with increased knee joint torque and power during levesl walking. J Biomech. 36: 13551362.
  • 36
    Malatesta, D., Simar, D., Dauvilliers, Y., et al (2003) Energy cost of walking and gait instability in healthy 65 and 80 yr olds. J Appl Physiol. 95: 22482256.
  • 37
    Mills, P. M., Barrett, R. S.. (2001) Swing phase mechanics of healthy young and elderly men. Human Movement Sci. 20: 427446.
  • 38
    Nene, A., Bryne, C., Hermens, H.. (2004) Is rectus femoris really a part of quadriceps? Assessment of rectus femoris function during gait in able-bodied adults. Gait Posture. 20: 113.
  • 39
    Minetti, A. E., Boldrini, L., Brusamolin, L., Zamparo, P., McKee, T.. (2003) A feedback controlled treadmill (treadmill-on-demand) and the spontaneous speed of walking and running in humans. J Appl Physiol. 95: 838843.
  • 40
    Hughes, A. L., Goldman, R. F.. (1970) Energy cost of “hard work.”. J Appl Physiol. 29: 570572.
  • 41
    Pate, R. R., Pratt, S. N., Blair, S. N.. (1995) Physical activity and public health: a recommendation from the Centers for Disease Control and Prevention and the American College of Sports Medicine. JAMA. 273: 402407.
  • 42
    National Institutes of Health (1998) Clinical Guidelines on the Identification, Evaluation and Treatment of Overweight and Obesity in Adults. National Heart Lung and Blood Institute: Bethesda, MD.
  • 43
    Felson, D. T., Anderson, J. J., Naimark, A., Walker, A. M., Meenan, R. F.. (1988) Obesity and knee osteoarthritis: the Framingham study. Ann Intern Med. 109: 1824.
  • 44
    Lelas, J. L., Merriman, G. J., Riley, P. O., Kerrigan, D. C.. (2003) Predicting peak kinematic and kinetic parameters from gait speed. Gait Posture. 17: 106112.