Introduction

Body dimensions and body composition are known to influence the capacity for physical performance. Taller stature, for example, is associated with longer muscle length, which in turn is associated with proportionally greater muscle cross-sectional area and muscle mass (Astrand and Rodahl, 1986). The greater muscle area and mass of the taller individual is related to proportionally greater force development; for example, strength and aerobic capacity are proportional to the cube of height, with aerobic capacity also proportional to the two-thirds power of body weight (Astrand and Rodahl, 1986; Hebbelnick and Ross, 1974; see also Malina, 1975).

Body composition associations with exercise capacity are less well defined mathematically but nevertheless are quite evident. For example, it is apparent that there is a relationship between marathon running performance and a body type characterized by leanness and modest muscle mass, or between football defensive linemen and a large muscle mass and modest-to-high levels of body fat (BF). Thus in athletic performance, particularly in elite athletes, the influence of body dimensions and composition are readily evident (McArdle et al., 1985).

In contrast, the association of body composition with the capacity for occupational task performance has received little attention. One exception to this may be the military services who use on-the-job body weight or BF standards or both. At least in the case of the U.S. Army, these standards are said to be based in part on the requirements for physical job performance. In recent years the Army has become increasingly concerned with excess body weight and BF, although this concern appears to be focused as much on appearance as it is on performance. The relationship between military appearance and BF has been addressed earlier (Hodgdon et al., 1990).

Physical fitness or the capacity for physical performance is not a single entity but is composed of several components, each representing a separate source or pathway of energy for muscular activity. Although all energy for muscular contraction is derived initially from muscle, the size of these energy systems or fitness components are not equally influenced by the size of the muscle mass or the fat-free component. Likewise, the relatively metabolically inactive fat mass also does not influence these fitness components in similar ways. Therefore our consideration of body composition on physical performance must differentiate between these components of fitness capacity.

The purpose of this report is to address the relationship of the two major components of body composition—fat and fat-free mass (FFM)—with the major components of physical performance capacity—aerobic power and strength—and present new data on these relationships in a large Army population. Emphasis is placed on how these relationships might be used to establish BF standards for the U.S. Army.

Design and Methods

The data presented here were collected as part of a larger project to validate BF standards based on objective criteria, including physical performance. Measurements were made on an unselected population of soldiers at Fort Hood, Texas, and Carlisle Barracks, Pennsylvania. The sample obtained at Carlisle Barracks, which provided most of the 40+ age group, consisted of students from the Army War College who were likely to be more physically fit relative to the rest of the sample. The total sample consisted of 1,126 men and 265 women. Age and racial distributions of the sample are given in Table 6-1.

Table 6-1

Distribution of Sample by Gender, Age and Racial Grouping.

Body composition was determined from hydrostatic weighing (Fitzgerald et al., 1987; Goldman and Buskirk, 1961) using the Siri equation (Siri, 1961) to estimate BF from density; residual lung volume was measured by oxygen dilution (Wilmore et al., 1980). Aerobic capacity was assessed as maximal oxygen uptake () determined from a treadmill progressive running procedure (Maksud and Coutts, 1971) that measured oxygen uptake by the open circuit procedure with Douglas bags, and maximal lift capacity (MLC) by an incremental maximal lifting test to a height of 152 cm (McDaniel et al., 1983). Scores on two items of the U.S. Army's physical fitness test (2-mile run and sit-ups) were also collected by self-report. A preliminary description of this study was previously reported (Fitzgerald et al., 1986).

Results

Body Composition and Performance Capacity Related To Age

The U.S. Army's BF standards are established according to age, using arbitrary age groupings set some years ago. Table 6-2 presents the mean plus or minus standard deviation (± SD) of the body composition variables, and Table 6-3 presents the corresponding values for performance variables for these established age groups. In this sample, percent BF and fat mass of men increased with age across all age groups while FFM was stable. Women's BF was not different between the first two age groups (17 to 20 and 21 to 27 years) but did increase in the third age grouping (28 to 39 years).

Table 6-2

Body Composition Variables by Gender and Age Groupings, Mean ± SD.

Table 6-3

Performance Variables by Gender and Age Groupings, Mean ± SD.

Maximal oxygen uptake decreased through the first three age groups in men, on an absolute basis, relative to body weight and relative to fat-free weight. In women, the decrease was clearly evident only on a body weight basis. Two-mile run time followed the same pattern as (per kg body weight). MLC also decreased as a function of increasing age in men, most prominently when expressed relative to body weight, but it was largely unaffected by age in the women's sample.

Performance Capacity in Relation To Body Composition

Figures 6-1 and 6-2 illustrate contrasting expressions of aerobic and strength capacity in their relationship to BF and FFM in men. The same patterns exist for women. Figure 6-1 illustrates that absolute aerobic capacity (maximal oxygen uptake in liters per minute) is not related to the percent BF (metabolically inactive tissue) but instead is related to the amount of FFM or, more specifically, to the amount of oxygen-consuming muscle mass. Relative (per kg of body weight), which is typically used in expressing aerobic fitness, is related to BF because increasing fat increases the denominator and thereby lowers the value. This relationship corresponds to the physiological situation where the capacity for body propulsion is decreased as BF or non-energy-producing tissue (''dead weight'') increases. This is also reflected in a similar association with the 2-mile run times. For this reason is expressed relative to body weight when referring to the capacity of moving the body as in running.

Figure 6-1

Relationship between aerobic fitness and body fat, and fat-free mass, by quartiles in men.

Figure 6-2

Relationship between maximal lift capacity (MLC) and body fat, and fat-free mass, by quartiles in men.

Figure 6-2 illustrates that absolute lifting capacity is unrelated to BF but directly related to FFM in men. Absolute lifting capacity is the appropriate measure in relationship to actual job task performance. Relative lift capacity (kg lift per kg of body weight) changes with percent BF because of the changing denominator. The performance of sit-ups is related to changes in BF, not FFM, apparently due to the mechanical interference of the fat. Similar results were observed in the women.

These two primary associations, relative with percent BF, and absolute MLC with FFM, are shown in further detail for men and women in scatter plots in Figures 6-3 and 6-4. The observed correlations in each case are substantial, indicating that BF and FFM account for approximately one-third of the variability in aerobic capacity and MLC, respectively.

Figure 6-3

A, Scatter plot illustrating the relationship between and percent body fat in men. percent body fat; r = -.60; Standard Error of the Estimate (SEE) = 5.02. B, Scatter plot illustrating the relationship between and percent body fat in women. percent (more...)

Figure 6-4

Scatter plots illustrating the relationship between maximal lift capacity (MLC) and fat-free mass (FFM) A in men (MLC = .502 + 2.107 FFM; r = .62; Standard Error of the Estimate (SEE) = 20.55) and B in women (MLC = 23.158 + .945 FFM; r = .38; SEE = 11.75). (more...)

Relationship To Fitness Standards

Although a stated purpose of the U.S. Army's BF standards is to ensure adequate physical performance capacity (U.S. Army, 1986), the standards were not actually based on performance requirements (passing scores on the Amy's physical fitness test) when they were initially established and implemented in 1982 (Friedl et al., 1989). Therefore, the data presented here were used in a retrospective fashion to determine how well the BF standards did in fact correspond to the physical fitness standards. Two analyses were carried out.

The first analysis was preliminary in nature and determined in a general fashion whether aerobic fitness corresponded to the fat standard by matching those physically fit versus those unfit against those meeting and those not meeting the fat standard. This was done with the use of a 2 × 2 contingency table plot (Figure 6-5). A of 45 ml/kg body weight/ minute was used as a cutoff point to represent being aerobically fit. This was an initial attempt to determine if the fat standards were in general agreement with the fitness standards by computing the number of correct and incorrect matches. There were 74 percent correct classifications for men and 84 percent correct matches for women.

Figure 6-5

2 × 2 contingency table for validating body fat standards against aerobic performance by determining the percent of correct matches.

This initial attempt to validate fat standards based on a single level of aerobic fitness did not take into account the actual fitness test scores (2-mile run times) and their adjustment by age. The second analysis (Friedl and Vogel, in press) plotted the passing (minimum) 2-mile run time equivalent to on a histogram of versus percent BF. In this case, the BF value used was that determined by the U.S. Army's circumference measurement procedure as actually applied to soldiers in their units. The procedure was derived from and validated against hydrostatic weighing (Vogel et al., 1988). An example of such a plot for the youngest male age group is shown in Figure 6-6, which identifies the percent BF that corresponds to the 2-mile run score requirement. The figure shows a very good correspondence between the aerobic fitness requirement and the BF standard that had been previously established for this age group, 20 percent BF. The correspondence of these points for all age groups in men is shown in Table 6-4.

Figure 6-6

Histogram of (per kg body weight) versus percent body fat (by anthropometric equations) showing the minimum 2-mile run test score equivalent for 17- to 20-year old men. Source: J.A. Hodgdon, P. I. Fitzgerald, and J. A. Vogel. 1990. Relationships between (more...)

Table 6-4

Correspondence between Aerobic Fitness Requirement (2-Mile Run Time) and Established Body Fat Standard by Age Group in Men.

Thus for men, the established BF standard agrees with the percent BF found in this population for the passing 2-mile run score for the two youngest age groups, which makes up a large share of the U.S. Army, but not for the two older groups. This result suggests that a more liberal BF standard is compatible with the aerobic fitness requirements in these older male groups. It is unlikely, however, that a more liberal BF standard would be acceptable for appearance criterion.

Such an analysis for women is not possible due to the limited size of the sample. In general, the relationship between BF and aerobic fitness is more flat in women than in men, which indicates a weaker relationship (Friedl et al., 1989, Friedl and Vogel, in press).

Another issue is whether the other component of fitness, strength capacity, should be related to a body composition standard, that is, a minimal acceptable level of FFM. Although the relationship between FFM and absolute strength or lifting capacity has been shown (Figure 6-4), the practical problem is what measures should be used to represent strength fitness in a field fitness test. The current U.S. Army fitness test for strength or strength endurance is sit-ups and push-ups. Neither of these items are correlated with any actual Army tasks, such as lifting (Meyers et al., 1984). Thus in attempting to identify a minimal FFM standard, appropriate test item measures of strength would first need to be identified that are suitable for the Army's fitness test battery.

Discussion

The data presented here show a moderate relationship between both aerobic and strength capacity with certain body composition components in a heterogenous population. These relationships are explained by the physiological fact that greater muscle mass will produce greater muscular strength or lift capacity, as well as maximal oxygen uptake, while greater fat mass will increase the required relative amount of oxygen uptake to propel the body that has more dead weight to propel.

These relationships are important in the military and other occupational settings for two reasons: (1) to set body composition standards that will support the level of physical performance capacity that is required and (2) to appropriately express fitness capacity tailored to different occupational activities. In regard to the former, it might be argued that if one displays adequate fitness capacity (passes the fitness test) or can successfully perform the physical demands of his or her job, then a body composition standard is unnecessary. However, a body composition standard (that is, a minimum requirement) at least for BF, is added insurance for achieving the desired level of fitness. Because fitness tests are not perfect measures of capacity, nor is fitness capacity a perfect indicator of job performance ability, a BF standard, in this case percent BF, would be an additional indication of adequate level of physical activity and capacity for a particular level of desired physical performance. Furthermore, even with an adequate capacity level, an inappropriately high BF may be a risk factor for musculoskeletal and heat-related injuries. This risk, along with the added relationships between BF and appearance or health, at least in the military and public safety arenas, seems to justify the desirability of body composition standards in addition to fitness standards.

With respect to the appropriate expressions of physical capacity, body composition is important when contrasting fitness capacities between genders or between individuals of different body size or stature. In such cases, differences in exercise capacity may be largely accounted for simply by differences in body weight, BF, or muscle mass. In comparing strength capacity of men and women, absolute force is a more appropriate expression relative to job performance, while strength (force) per unit of FFM would be advantageous when evaluating the response to a training program or comparing the contractile "quality" of muscle.

expressed in liters per minute, uncorrected for body or muscle mass, provides a measure of the total amount of aerobic power that the body can produce and is positively related to the absolute quantity of muscle present (Buskirk and Taylor, 1957; Welch et al., 1958). For the same level of training and fat mass, muscular individuals are likely to outperform less muscled individuals when significant amounts of external weight are carried or backpacked. This difference is due to the proportionally smaller "dead weight" being carried by the more muscular individual. The greater the external load, the more appropriate is the use of the expression of absolute aerobic capacity ( in liters per minute) as compared to minimal or no-load conditions where adjusted by body weight is more useful.

A final comment is appropriate regarding the question of whether BF content alone is a good indicator of aerobic fitness (Parrish and Gustin, 1986; Slack et al., 1985). Direct measures of aerobic capacity () or aerobic performance (for example, 2-mile run for time) will always be preferable to indirect indications such as BF when assessing an individual's ability to carry out aerobic tasks if there are no measurement constraints. The fact that percent BF is correlated rather well with (an r of about 0.6) suggests that there may be limited applications where fat content could be used as a screening device or indicator of relative fitness in population studies. It is inappropriate as an estimate of aerobic fitness in groups homogenous in terms of fitness or fatness, in highly fit individuals, or in following changes in fitness of individuals during training.

In sum, physical capacity, in the context of occupational task performance, is related to body composition in a heterogenous population, with BF accounting for about one-third of the variability seen in aerobic capacity and FFM accounting for one-third of the variability in muscle endurance/ lifting capacity. The expression of physical capacity, whether uncorrected for body size or composition, depends on the physical activity or comparison of concern. BF content can also be used in some circumstances as an indicator of aerobic fitness. The U.S. Army's BF standards for men correspond to the aerobic standards in the younger age groups but deviate in the older groups due apparently to the influence of an appearance criterion.

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