Introduction

There is little experimental data describing the effects of altered body composition on physical performance. This is because body composition is difficult and time-consuming to change in human volunteers. In the late 1970s, three studies were conducted that were designed to investigate the effects of experimental alterations in excess weight on physiological responses to exercise and on physical performance capabilities. The research objective was to use an experimental model to simulate the effects of different levels of body fatness in order to determine whether the cross-sectional data available describing relationships between percent body fat (BF) and physical performance reflected cause and effect.

Excess Weight, Aerobic Capacity and Running Performance

The first study (Cureton et al., 1978) involved investigating the effects of experimental manipulation of excess weight on aerobic capacity and distance running performance. It was known from cross-sectional data that percent BF is inversely related to aerobic capacity (?B?) expressed relative to body weight (BW) and to distance running performance (Figure 5-1), but the magnitude of changes in ?B? and distance running performance that result from altered percent BF had not been established.

Figure 5-1

Diagram of relationships among percent body fat, (ml/kg body weight × minute) and distance run performance reported in cross-sectional studies.

Six recreational runners, four men and two women, 20 to 30 years of age, were used as subjects. They were relatively lean and had above average (ml per kg body weight per minute) (Table 5-1). Body composition was estimated from body density, which was determined using hydrostatic weighing. A maximal, graded, running treadmill test and the 12-minute run were administered under four added-weight (AW) conditions: 0, 5, 10, and 15 percent AW. Weight was added to the trunk of the subjects using a weight belt and shoulder harness.

Table 5-1

Physical Characteristics of Subjects in Cureton and Coworkers (1978) Study.

During submaximal running on the treadmill at 188 meters per minute (7 miles/hour), addition of excess weight significantly and systematically increased ventilation, oxygen uptake in liters per minute, and heart rate but did not significantly alter the oxygen uptake expressed relative to the total weight carried (TW). This latter measure tended to decrease slightly (Table 5-2). During maximal running, addition of excess weight did not significantly affect ventilation, oxygen uptake in liters per minute, or heart rate but systematically decreased (ml/kg TW × minute), treadmill run time, and 12-minute run performance. Under the 15 percent AW condition, these three measures were reduced 6.9 ml/kg/minute (12 percent), 1.5 minutes (10 percent), and 277 meters (8 percent), respectively, compared to the normal weight condition (Table 5-3). The changes by individual subjects for (ml/kg TW × minute) and 12-minute run performance were very consistent (Figure 5-2). The average reductions in and 12-minute run distance per 1 percent added weight were 0.5 ml/kg TW × minute and 18 meters, respectively.

Table 5-2

Means ± SD and F Ratios for Physiological Variables Measured During Sub-Maximal Treadmill Running (7 mph) for the Four Added Weight (AW) Conditions.

Table 5-3

Means ± SD and F Ratios for Physiological Variables Measured During Maximal Treadmill Running and 12-Minute Run Performance for the Four Added Weight (AW) Conditions.

Figure 5-2

Individual values for the 12-minute run performance and (ml/kg total weight × minute) for the four added-weight conditions. Source: Cureton et al. (1978) by permission.

Comparison of the (liters per minute) during submaximal and maximal running clearly indicated that the primary metabolic effects of addition of excess weight were to increase the energy requirement of running at submaximal speeds without affecting the absolute (Figure 5-3). Any submaximal speed of running therefore required a higher percentage of and was reached at a lower speed of running, which in turn, resulted in a reduction in treadmill time. The mechanism by which added weight affected the 12-minute run performance should have been the same, assuming an individual ran at the same fraction of the across AW conditions.

Figure 5-3

Treadmill (TM) run time estimated from mean submaximal and maximal (liters per minute) values for the 0 percent and 15 percent added-weight (AW) conditions. Source: Cureton et al. (1978) by permission.

A measure of the total excess weight (EW) carried during the treadmill and track runs can be computed by adding the fat weight of each subject to the AW. This increases the dispersion of excess weight compared to that when just AW is considered and substantially strengthens the relationship of EW to (ml/kg TW × minute) and 12-minute run performance (Figure 5-4). The changes in run performance associated with the variation in percent BF closely paralleled the changes that resulted from added weight, which indicates that the relationship of percent BF to these measures was similar to the effects of added weight.

Figure 5-4

Relationships of percent excess weight (EW) to 12-minute run performance and (ml/kg total weight × minute). AW = added weight. Source: Cureton et al. (1978) by permission.

Based on the results of this study, it was concluded that (a) excess (fat) weight causally affects expressed relative to weight and distance running performance and (b) two alternate metabolic explanations can be given for the detrimental effect of excess weight on distance running performance. One explanation is that excess (fat) weight increases the energy requirement of submaximal exercise without affecting the absolute Therefore, running at any submaximal speed requires a higher percentage of , and the pace that can be maintained for a given duration is reduced. An alternate explanation is that excess (fat) weight reduces the expressed relative to weight without affecting the oxygen requirement of submaximal running per. unit weight. Therefore, as for the other explanation, the percentage of used during running at a submaximal speed is increased, and the pace that can be sustained for a given duration is reduced.

The primary limitation of the AW model is that weight was added to the trunk and not distributed over the limbs and trunk as would be the case for BF. Therefore, the effects of changes in BF might be underestimated by the AW model, because it is known that weight added to the limbs has a bigger effect on the energy requirement of submaximal exercise than weight added to the trunk. Another limitation of the model is that when body weight changes, fat is not the only tissue to change. Gains in BF are typically accompanied by gains in fat-free weight (FFW), and losses in BF are usually accompanied by losses in FFW (Forbes, 1987). Thus, acute changes in the fat-free component of the body that accompany weight loss or gain may have effects not accounted for by the model. The validity of the model is supported by data indicating that the increased oxygen required to walk at a given submaximal speed brought about by adding weight to the trunk using a backpack is the same as that produced by a similar weight gain produced by overeating (Hanson, 1973). A number of other studies have indicated that the oxygen required per unit weight carried to walk or run at a given speed is not related to whether some of the weight is carried externally using a weighted belt, vest, or backpack (Cureton and Sparling, 1980; Goldman and Lampietro, 1962; Hanson, 1973; Miller and Blyth, 1955).

To evaluate whether the effects of added weight were the same as the relationship of BF to performance in cross-sectional data, the regression lines relating percent BF to 12-minute run performance in 34 male and 34 female recreational runners (Sparling and Cureton, 1983) were compared to the regression line indicating the average effect of the AW in this study (Figure 5-5). The slopes of the regression lines were almost identical, which supports the validity of the model and the conclusion that the inverse relationship between BF and distance running performance reported in cross-sectional data is cause and effect.

Figure 5-5

Comparison of regression lines describing the relationship between percent fat and 12-minute run performance in men and women to the regression line describing the effect of added weight on 12-minute run performance.

Running Performance, Metabolic Responses and Gender Differences

The second study also used the AW model (Cureton and Sparling, 1980). The purpose of this study was to investigate the extent to which differences between men and women in distance running performance and metabolic responses during running are due to the gender difference in percent BF. On the average, the percent BF of women is approximately 10 points higher than for men. Women also have lower average (ml/kg BW × minute) and poorer distance running performance (Figure 5-6). Of interest was the determination of the effect of experimentally eliminating the gender difference in percent BF (by adding excess weight to the men) and observing how much the gender differences in (ml/kg TW × minute) and 12-minute run performance were reduced.

Figure 5-6

Diagram of the effects of gender on percent body fat, (ml/kg body weight × minute) and distance run performance based on comparative data in the literature.

The subjects for the study were 10 male and 10 female recreational runners who were matched on running mileage and competitive experience. The expressed relative to fat-free weight (FFW) of the groups was also not significantly different, which indicates that the men and women had similar cardiorespiratory capacity. Both the men and women were relatively lean (Table 5-4). The measurements and procedures were the same as for the earlier study (Cureton et al., 1978). Women were measured only once with normal weight. The men were administered the graded treadmill and 12-minute run test twice, once under a normal-weight (NW) condition and once under an AW condition. The objective of the AW condition was to equate the mean percentage EW carried by the men and women. EW was defined as the sum of fat weight and added external weight. Each man was paired with a woman, and weight was added to the man such that his total percent EW was equal to the percent BF of the woman. Therefore, different percentages of EW were added to individual men, but the average percent EW added was equal to the mean gender difference in percent BF (7.5 percent).

Table 5-4

Physical Characteristics of the Subjects in Cureton and Sparling (1980) Study.

The differences between the men and women during submaximal and maximal running, and on the 12-minute run, were similar to those reported in other studies (Tables 5-5 and 5-6). During running at submaximal speeds, men had higher absolute levels of ventilation and oxygen uptake, but women had higher heart rates and higher oxygen uptake values expressed relative to body weight (BW) or FFW. The higher (ml/kg BW × minute) indicated that the women had poorer running economy than the men, which was an unexpected finding. Most studies of trained runners have reported no gender difference in running economy. The mean expressed in liters per minute and relative to body weight was significantly higher in the men, with the mean gender difference for (ml/kg BW × minute) being 6 ml/kg × minute (11 percent). expressed relative to FFW was not significantly different in the men and women, with the mean gender difference being 1.9 ml/kg × minute (2.8 percent). Mean treadmill run time was 4 minutes (34 percent) longer, and 12-minute run distance was 568 m (20 percent) greater in the men than in the women.

Table 5-5

Means ± SD for Physiological Variables Measured During Maximal Treadmill Exercise and 12-Minute Run Performance.

Table 5-6

Means ± SD for Physiological Variables Measured During Submaximal Treadmill Running (7 mph).

As expected, the effects of adding weight to the men were the same as in the first study. The during running at submaximal speeds expressed in liters per minute or relative to fat-free weight was significantly increased, whereas the expressed relative to the TW was reduced by a small amount. The mean increase of 2.4 ml in (ml/kg FFW × minute) elimi nated 32 percent of the gender difference for this variable. in liters per minute or expressed relative to FFW was not significantly affected by equating excess weight, but expressed relative to body weight was significantly reduced by an average of 3.9 ml, which reduced the mean gender difference by 65 percent. With excess weight equated in the groups of men and women, there was no significant difference between the men and women in expressed relative to TW or FFW, with mean differences being 2.1 ml (3.8 percent) and 2.5 ml (3.6 percent), respectively. Addition of weight to the men reduced the mean gender differences in treadmill run time and 12-minute run distance by 1.2 minutes (32 percent) and 173 m (30 percent), respectively.

Examining the relationships among expressed relative to FFW and TW during submaximal and maximal running, and treadmill run time (Figure 5-7) revealed that the mechanism through which EW contributed to the gender difference in treadmill run time could be explained in either of two complementary ways. First, the greater EW of women increases the energy required per kg of Flew to run at any given speed. without affecting (ml/kg FFW × minute). Thus, the percent used at different speeds is increased, the pace that can be maintained for a given duration is less, and is reached at a lower speed of running. Or second, the greater EW of women reduces the expressed relative to BW without substantially affecting the (ml/kg BW × minute) required to run at submaximal speeds. The percent required to run at submaximal speeds is therefore increased with the same consequences as in the first explanation.

Figure 5-7

Mean submaximal and maximal (ml/kg fat-free weight × minute) and (ml/kg total weight × minute) values during running at various speeds during the treadmill test for women and men under the normal-weight (NW) and added-weight (AW) conditions. (more...)

The conclusions from this experiment were, first, that the greater average gender-specific excess weight (fat) of women causes a portion of the gender differences in (ml/kg BW × min) and distance running performance. About 65 percent of the gender difference in (ml/kg BW × minute) and about 30 percent of the gender difference in distance running performance in the sample studied were eliminated by removing the gender difference in excess weight (BF). A greater percentage of the gender differences in treadmill time and distance running performance (probably closer to 65 percent) would have been eliminated if there had been no difference in running economy. And second, because the additional gender-specific BF of women is not eliminated by diet or physical training, it provides part of a biological justification for separate distance running performance standards and expectations for men and for women.

Physical Performance, Body Fat and Women Athletes

The third study (Johnson, 1978), in which the AW model was used, compared the physical performance changes associated with increased BF based on cross-sectional data with performance changes resulting from added external weight in women athletes. The relationships between percent BF, estimated from body density determined by underwater weighing, to four physical performance tests (50-yd dash, agility run, modified pull-up, and standing long jump) were determined in 44 women varsity athletes at the University of Georgia. A significant negative relationship between percent BF and each of the performances was found, although the correlations were not high, ranging from about 0.4 to 0.6 (Figure 5-8). Six subjects were selected at random from the 44, and the physical performance tests were readministered with 5, 10, and 15 percent AW. Performances on each of the tests decreased consistently and systematically with AW (Figure 5-9). The slopes of the regression lines relating percent BF to the performance scores based on the cross-sectional data were very similar to the regression lines indicating the average effect of the AW (Figure 5-10). Therefore, it was concluded that changes in performance associated with increased BF are similar to changes that result from AW. The results support the validity of the AW model for investigating the effects of differences in BF on performance and provide experimental data indicating that relationships be tween percent BF and different types of physical performance that involve movement of the BW are cause and effect relationships.

Figure 5-8

Scatter diagrams and linear regression lines predicting performances on the standing broad jump (SBJ), 50-yard dash (DASH), agility run (AR), and modified pull-up (MPU) from percent body fat. Source: Johnson (1978) by permission.

Figure 5-9

Individual values on the standing broad jump (SBJ), 50-yard dash (DASH), agility run (AR), and modified pull-up (MPU) for the four excess-weight conditions. Source: Johnson (1978) by permission.

Figure 5-10

Comparison of regression lines predicting the standing broad jump (SBJ), 50-yard dash (DASH), agility run (AR), and modified pull-up (MPU) from percent body fat (———) and from percent excess weight (———). (more...)

References

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