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OCR for page 195
Body Composition and Physical Performance 1992.
Pp. 195-206. Washington, D.C.
National Academy Press
1 ~
Body Composition and Performance
in Relation to Environment
Roy ;J. Shephar`1
DEFINITION OF OBESITY
Clinicians commonly define obesity as a body mass that exceeds the
actuarial "ideal" value by a specified margin such as 10 or 15 pounds (5 to
7 kg). However, this is not an entirely appropriate basis of assessment from
an ergonomic point of view. First, the goal of the clinician when diagnos-
ing obesity is to detect an increased vulnerability to chronic disease (Table
12-11. Thus, groups such as the Society of Actuaries (1959) have expressed
the mortality from a variety of chronic conditions and diseases as a percent-
age of the "standard" mortality values observed in subjects of the same age
and sex who had an "ideal" body mass relative to their height. Notice that a
substantial increase of vulnerability develops only when there is a major
excess of body mass (and by inference a major excess of body fat). More-
over, loss of production from morbid obesity and the resultant chronic dis-
orders has a relatively minor impact upon the performance in the young
adults of a military labor force. In contrast to the threshold of fatness
required for a clinical diagnosis of obesity, most aspects of occupational
performance tend to be continuous functions of body composition. Further-
more, the energy cost of most tasks depends not only on total body mass,
but also on the nature of this mass (muscle or fat) and if the person is
burdened by an excess of fat on its distribution (deep or superficial).
A possible method for defining body fatness is to compare hydrostatic
estimates of body fat content with those observed in a person of ideal body
195
OCR for page 196
96
ROY J. SHEPHARD
TABLE 12-1 Mortality of Obese Subjects, Classified by
Condition or Disease, and Expressed as a Percentage of Standard
Values for Subjects of the Same Gender, Aged 15 to 69 years.
Percent of Standard Values of Mortality
of Matched Controls
Men
Women
Excess Weight Excess Weight
Condition >+24 kg 2 +33 kg >+28 kg 2 +37kg
Diabetes 179 385 270 242
Cerebrovascular disease 136 183 143 142
Heart/circulation 131 155 175 178
Pneumonia/flu 128 193 148 110
Digestive diseases 147 197 140 200
Kidney diseases 146 230 93 122
Accident/homicide 109 126 85 98
Suicide 71 104 47
All causes 123 145 130 138
SOURCE: Based on data of the Society of Actuaries (1959). Previously
published and reprinted by permission of Greenwood Publishing Group, Inc.,
Westport, CT, from Physiology and Biochemistry of Exercise by Roy J. Shephard
(1982).
mass. One study of military recruits arbitrarily set the upper limit of ideal
values at 14 percent body fat in men (Amor, 1978), although the same
author's data apparently suggested that 16.8 percent fat would correspond
with the upper limit of the actuarial ideal body mass. Taking the 14 percent
ideal figure for the men, and the 18 percent ideal body fat for the women,
this would imply respective ideal fat masses of 10 and 11 kg in the two
genders. Applying the clinical criterion of obesity, those men with a 5 kg
excess of fat (the obese) would have 21 percent or more fat, and the women
would have 27 percent or more fat. Some 50 percent of the male soldiers
studied by Amor (1978) exceeded the actuarial ideal of body mass. The
proportion of those who exceeded his arbitrary criterion of obesity (18
percent of body mass in men) increased with age (Table 12-2) but was
unrelated to the physical demands of employment (Table 12-31. Those
whom he classified as obese nevertheless tended to have a poor maximal
oxygen transport, particularly if this was expressed in ml/kg x minute
(Table 12-41. Thus, for military purposes an obesity threshold of 18 per-
cent fat in men and perhaps 24 percent fat in women seems preferable to
the clinical criteria of 21 percent and 27 percent fat in the two genders.
A second potential method of identifying the obese is to measure skin
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PERFORMANCE IN RELATION TO ENVIRONMENT
TABLE 12-2 Relation of
Chronological Age of Military Personnel
to Obesity in the British Army
Age in Years (n)*
Percentage Obese
(> 18 percent body fat)
17-19 (702)
20-24 (1,190)
25-29 (805)
30-34 (255)
35-39 ( 1 1 8)
32
39
46
56
62
*(n) = number of personnel surveyed.
SOURCE: Amor ( 1978).
197
fold thicknesses, again comparing the actual readings with the values ob-
served in a person meeting actuarial standards of ideal body mass (Shep-
hard, 1982~. Average readings are about 10 mm in a male and 14 mm in a
woman of ideal mass. Assuming that a double fold of the skin per se
accounts for 4 mm of the total skinfold reading (Shepherd, 1991), there is a
superficial layer of some 5,400 cm3 of fat in the ideal man with a body
surface of 1.8 m2 (4.8 kg of fat, assuming a density of 0.9) and 8,000 cm3 of
fat in the ideal woman with a body surface of 1.6 m2 (7.2 kg of fat).
Assuming also a 50 percent increase of subcutaneous fat in a person who
is clinically obese, the clinical threshold of obesity would be an average
skinfold reading of 13 mm in a man and 19 mm in a woman. However, if
the military threshold of obesity were to be set at the ideal body mass,
TABLE 12-3 Relation of Job Intensity
to Prevalence of Obesity among Military
Personnel in the British Army
Percentage Obese
Job Intensity (n)* (>18 percent fat)
Sedentary (353) 53
Light ( 1,389) 42
Moderate ( 1,269) 38
Heavy (59) 51
*(n) = number of personnel surveyed.
SOURCE: Amor ( 1978).
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98
ROY J. SHEPHARD
TABLE 12-4 Relation of Percentage of Body Fat to Percentage
of British Army Personnel with Poor Aerobic Fitness
Percentage with Poor Aerobic Fitness
Body Fat Percent (n)* (maximal oxygen intake <35 ml/kg x min)
<10 (154)
10-14 (728)
14-18 (912)
18-22 (642)
22-26 (390)
>26 (244)
* (n) = number of personnel surveyed.
SOURCE: Amor (1978).
2
5
9
21
32
51
as proposed by Amor (1978), the ceiling of militarily acceptable skinfold
reading would average about 11 mm in a man and 14 mm in a woman.
In summary, the standards for judging obesity in military personnel
should be more rigorous than those adopted for clinical purposes; both body
fat (limits of 18 percent in men and 24 percent in women) and skinfold
readings (limits of 11 mm in men and 14 mm in women) should correspond
to those observed in a person of ideal body mass.
PERFORMANCE IN COMFORTABLE CLIMATES
Physical Performance
Physical performance may be classified simply into endurance activi-
ties, well exemplified by prolonged marching with a backpack, and lifting
tasks that are commonly the limiting factor in the front-line employment of
military personnel (for example, the ability to lift a mass of 36 kg from the
ground to a height of 1 10 cm; Nottrodt and Celentano, 1984~. The metabol-
ic load imposed by any given task reflects the sum of resting metabolism
plus the energy cost of the required activity (Shepherd, 19741.
Resting Metabolism
Because of the effects of body surface upon heat loss, resting metabo-
lism is a power function of body mass M (Shepherd, 19821:
VO2 = (Mid 7s
However, a large part of the fat cell is occupied by metabolically inert
stored triglyceride. Thus, the resting metabolism per unit of body mass is
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PERFORMANCE IN RELATION TO ENVIRONMENT
199
greater in a muscular than in a fat individual. Obesity also affects respira-
tion through mass loading of the chest; the obese person shows decreases of
lung volume, chest wall, and lung compliance that can precipitate a classi-
cal Pickwickian syndrome of hypercapnia and hypoxia (Burwell et al., 19564.
In partial compensation for the added respiratory work, the respiratory cen-
ters of the obese individual may show an increased sensitivity to hypoxia
(Burki and Baker, 1984) and in moderate but not in severe obesity there is
an increased sensitivity to carbon dioxide (Emirgil and Sobel, 1973; Nish-
ibayashi et al., 19871.
Endurance Activities
Givoni and Goldman (1971) and Pandolf et al. (1977) developed vari-
ous equations for the prediction of the energy cost of marching in fit young
recruits. In general, these authors found that the metabolic cost is a linear
function of body mass and the mass of any backpack that is being carried.
Thus, a heavier person will spend more energy when marching, regardless
of whether the added body mass is attributable to muscle or fat. If the
normal expectation is that a 70-kg recruit will carry 30 kg of equipment,
then a person who is 10 kg heavier will immediately have a 10 percent
handicap of endurance performance (Givoni and Goldman, 19711.
If the added burden is muscle, a heavy person may show some compen-
satory increase in their absolute maximal oxygen transport, and because the
active muscles are stronger, it may also be possible for the individual con-
cerned to operate at close to their maximal oxygen intake for a sustained
period, so that their endurance performance may approach that of a lighter
person. However, if the extra body mass is fat, there is certainly no com-
pensatory development of maximal oxygen intake; indeed, oxygen transport
is often poorer than in a lighter person, so that endurance performance is at
least correspondingly limited. In moderate obesity, there is no change of
mechanical efficiency, so that the oxygen cost of walking per kg of body
mass is unchanged. However, if the obesity is extreme, a combination of
heavier limbs, awkward or impeded body movements, and increased respi-
ratory loading may give rise to a low mechanical efficiency, with a further
restriction of potential performance (Dempsey et al., 1966~.
Lifting and Carrying
The current Canadian military requirement is that recruits be able to lift
18 kg regularly and 36 kg occasionally to shoulder height (Nottrodt and
Celentano, 19844. The load normally lifted is thus about 20 percent of body
mass, and if objects are to be picked up from the ground, the resultant
displacement of body mass is a major fraction of the overall task. Brown
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200
ROY J. SHEPHARD
(1966) suggested that the oxygen cost of most occupational tasks could be
described by an equation of the type
VO2 = A + B(M)n
where A and B are constants, and M is body mass raised to an exponent n
that varied from 0.75 to 1.0. Godin and Shephard (1973) suggested that
there might be merit in a three-term equation of the type:
VO2 = A + B(M)0 75 + C(M)n
where A, B. and C are constants, M is body mass, and n is an exponent
varying from 0.1 to 0.2 in very light arm work to near 1.0 in heavy physical
tasks involving displacement of much of the body mass. The second term
in this last equation distinguishes the influence of body mass on resting
metabolism, a particular advantage in situations where a heavy, muscular
Person is performing relatively light industrial work.
Another consideration is that the obese person is often characterized by
insulin resistance and difficulty in mobilizing fatty acids (Pacy et al., 1986;
Scheen et al., 19831. If an endurance task must be sustained for a long
period, the function of such an individual may be impaired by a depletion of
glycogen reserves.
With a lifting task, the factor limiting performance is usually muscular
strength rather than maximal oxygen intake; thus, if body mass is increased,
it becomes critical whether the added load is due to muscle that can provide
a compensating increase of strength or to fat, which merely increases the
overall mass of the system.
Underwater Activity
Underwater activity is a special case where it is an advantage to be
somewhat obese, both from the viewpoint of thermal insulation and also
because of the resultant increase in buoyancy. Heavy, muscular individuals
often have substantial difficulty swimming over long distances, because
they must exert much greater effort to remain afloat, and a less horizontal
leg position also decreases the mechanical efficiency of their swimming
(Shepherd et al., 1973~. However, the person who lacks a normal amount of
body fat can compensate for this handicap by keeping the lungs relatively
well filled with air while swimming.
Size Problems
A final consideration is that many military work stations, such as air-
craft cockpits, tanks, or submarines, have limited space for the human oper
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PERFORMANCE IN RELATION TO ENVIRONMENT
201
ator. A grossly obese person may be handicapped when working in such a
situation because body size exceeds the available clearances.
Poor Health
Minor sickness and absenteeism are common sources of poor perfor-
mance in all kinds of occupation. Among government employees, 10 of
220 working days are commonly lost through absenteeism each year, and an
unpredictable need for well-trained replacements adds some 8 percent to
payroll costs (Shepherd, 19861. Given the well-recognized actuarial associ-
ation between obesity, chronic disease, and premature death, possible rela-
tionships between obesity, absenteeism and increased illness should be ex-
amined in military personnel.
Bardsley (1978) has commented on a substantial cost to the armed
services from "diseases of choice," where risk is influenced by lifestyle-
conditions such as myocardial infarction, bronchitis, emphysema and alco-
holism (Table 12-51. As shown in this table, in 1973, the Canadian forces
expended $5.9M for replacement of the dead, $5.8M for replacement of the
released, $12.4M for those who were hospitalized, and $1.5M for those
who were on sick leave due to "diseases of choice." However, much of this
expense is related to the adverse health effects of smoking and alcohol
abuse rather than to the adverse consequences of obesity; a substantial ex-
cess body mass (20 to 30 kg) is needed for an appreciable increase of
morbidity from back problems and of deaths from such diseases as coronary
atherosclerosis and diabetes (Society of Actuaries, 19591. Moreover, the
economic impact of obesity-related morbidity and mortality would be great-
est in older members of the labor force, after the normal time of retirement
TABLE 12-5 Estimated Monetary Costs
Incurred by the Canadian Forces in 1973 (1973
Canadian dollars) Due to Diseases Associated
with Choice of an Adverse Personal Lifestyle
Cost ($ million)
Replacement of the dead
Replacement of the released
Hospitalization + lost wages
Wages of those on sick leave
Total
$ 5.9
5.8
12.4
1.5
25.6*
*This cost is spent on a labor force of about 80,000
military personnel.
SOURCE: Bardsley (1978).
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202
ROY J. SHEPHARD
from the armed services. Finally, many of the absences from work among
younger individuals are attributable to causes other than organic disease,
and such personnel would be unlikely to respond to the correction of obesi-
ty or indeed to any other form of medical treatment (Williamson and Van
Nieuwenhuijzen, 19741. While there remains scope for more detailed anal-
yses of the economic impact of "diseases of choice," at first inspection it
thus seems much more important to correct smoking and an excessive con-
sumption of alcohol than to attempt a reduction of body fat in military
personnel.
OTHER IMPLICATIONS OF OBESITY
Image is an important aspect of effectiveness in many organizations,
including the armed services (Shepherd, 1986~. Obese personnel do not fit
the public image of a soldier, and it seems logical that for this reason they
will weaken the military effectiveness of a unit, although there has been no
experimental examination of this point.
Baun et al. (1986) have further commented on an association between
achievement orientation and personal fitness. By selecting personnel who
meet specified standards of body composition and physical fitness, a unit
may be enriched by the recruitment of premium personnel.
HOT AND COLD ENVIRONMENTS
Because heat exchange is proportional to body surface area, tolerance
of hot and cold environments may be influenced somewhat by the differenc-
es of body surface area between a tall, thin ectomorph and a short, fat
endomorph. Indeed, at one time anthropologists sought to explain the colo-
nization of hot and cold regions in terms of body linearity, the so-called
"rules" of Bergmann (1847) and Allen (1877~. However, the effects of
body form are at most of secondary importance in a normal working popu-
lation.
The main impact of obesity on thermal balance comes from the added
insulation of subcutaneous fat (Shepherd, 1985), although an increased con-
striction of subcutaneous blood vessels may further augment the insulation
of a fat person (Jequier et al., 1974), and there are also effects of body fat
stores upon cold-induced thermogenesis. It has been shown that a 5 kg
accumulation of fat may add 1.5 mm to the subcutaneous fat layer of a man
and 2.5 mm to that of a woman. The functional value of this thin layer of
fat can be gauged from the importance that distance swimmers attach to
covering their skins with a few mm of grease. At rest, the thermal gradient
across a layer of superficial fat is only about 0.15°C/mm, but the insulating
effect is proportional to heat flux. Thus, when a person is working at 10
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PERFORMANCE IN RELATION TO ENVIRONMENT
203
times the basal metabolic rate, there will be a 10-fold increase of thermal
gradient, to about 1.5°C/mm of superficial fat (Pugh et al., 1960~. Assum-
ing an energy expenditure of 10 METS (the ratio of observed to basal
metabolism) and a uniform pattern of heat loss, a woman who had accumu-
lated 5 kg of body fat, with a resultant 2.5 mm increase of the subcutaneous
fat layer, would have a subcutaneous temperature some 3.8°C higher than a
person of ideal body composition.
It might be thought that a thick layer of subcutaneous fat would be an
advantage when adapting to a very cold climate. Certainly skin tempera-
tures drop to a lower level in the obese before their metabolism is stimulat-
ed (Wyndham et al., 1968), perhaps because of an altered setpoint in the
hypothalamic thermal regulators (Zahorska-Markiewicz and Straszkiewicz,
1987), although this change has adverse consequences for manual dexterity.
The very low skinfold readings of traditional Inuit (Rennie, 1963; Shep-
hard, 1978) is one strong argument against the view that fat accumulation
has any great adaptive value in the cold. It is only with acculturation to
"western" civilization that body fat has accumulated (Rode and Shephard,
19841. The main problem of the fat person who is living in the arctic is that
the extent of insulation cannot be reduced once physical work is begun; in
fact, because of the increased heat flux, the effectiveness of any insulation
is actually increased during work. Body temperature thus rises to the point
where sweating is initiated; this wets the clothing from within, largely de-
stroying its insulation, and the individual rapidly becomes hypothermic when
a rest-break must be taken. In contrast, a traditional Inuit hunter with a
very thin layer of subcutaneous fat is able to conserve body heat by using
skillfully fashioned clothing that provides up to 11 CLOT units of insulation
(Burton and Edholm, 1969~.
One fascinating feature shown by military personnel who have recently
arrived in the arctic is an increased metabolism of fat. Both field observa-
tions and laboratory crossover trials have shown that a given amount of
activity produces a fat loss in the cold that is not mirrored in a warm or
temperate environment (O'Hare et al., 1978, 1979~. Reasons for this fat
loss are still not entirely clear. Contributing factors include energy lost in a
substantial ketosis, small increases in the energy cost of movement due to
the weight and hobbling effect of arctic clothing, and a possible cold-in-
duced stimulation of resting metabolism; it remains unclear whether this
last response occurs through residual brown fat (Huttunen et al., 1981) or
1 The CLO unit was originally defined as the insulation provided by British indoor clothing.
It is now defined in terms of the thermal gradient from the skin to ambient air (Ts - Ta, °C),
the body surface area (m2) and the heat loss (kJ/hour); therefore, CLO units = 0~75 (Ts - Ta)
m2 / kJ per hour.
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204
ROY J. SHEPHARD
TABLE 12-6 Effects of a 2-mm Increase in
Subcutaneous Fat (a 4-mm Increase in
Skinfold Readings) on the Performance of
Physical Work in the Heat
Intensity of Work
(kJ/min)
Core Temperatures Work Rate
(°C)* (percent)"
Light (<14) +0.8 -11
Moderate (<23) 1.2 -17
Heavy (<38) 2.0 -28
*If work-rate is unchanged.
tIf core temperature is unchanged.
NOTE: It is assumed there is an initial thermal
gradient of 7°C from the body core to the environment.
SOURCE: Based on a concept of Pugh et al. (1960).
the initiation of futile metabolic cycles in other tissues. Interestingly, it
seems easier to produce the fat loss in obese men than in those who are
initially slim (O'Hare et al., 1978), and it is also more readily developed in
men than in women (Murray et al., 19861. Perhaps fat stores in women are
more stable, in order to meet the demands of pregnancy and lactation.
Obese subjects have difficulty undertaking vigorous work in a hot envi-
ronment, partly because they must expend more energy to complete a given
task and partly because insulation is increased by the thicker layer of super-
ficial fat. One potential method of restoring thermal balance in the obese
person is a greater relative increase of skin blood flow during exercise.
However, if blood is directed to the skin, it cannot be directed elsewhere-
to the working muscle and the brain. The peak oxygen transport and peak
power output are thus reduced in the heat. The obese person works more
slowly than a slimmer peer, or if pace is maintained, collapse occurs earlier
than in a thin subject. One recent calculation suggested that with an initial
thermal gradient of 7°C from the body core to the environment, the core
temperature would rise by an additional 0.8 to 2.0°C if a person with an
additional 2 mm of subcutaneous fat undertook industrial work (Shepherd,
1987; Table 12-61; conversely, if the rise of core temperature were to be
avoided by a slower rate of working, it would be necessary to reduce the
work-rate by 1 1 to 28 percent.
CONCLUSIONS AND RECOMMENDATIONS
In young adults who make up the bulk of military personnel, the main
argument for controlling the burden of body fat is a deterioration of physi
OCR for page 205
PERFORMANCE IN RELATION TO ENVIRONMENT
205
cat performance rather than the risk of morbid conditions. Deterioration of
function in a temperate environment is almost directly proportional to ex-
cess fat mass, without evidence of a threshold. It is thus recommended that
the target body fat percentage set for military personnel correspond to their
actuarial ideal of body mass. The adverse effect of body fat upon perfor-
mance is exacerbated when personnel must operate in a hot climate, but a
modest excess of fat may contribute to buoyancy and insulation when work-
ing in cold water.
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Representative terms from entire chapter:
body fat
