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EXAMINING THE VALIDITY OF EXERCISE GUIDELINES FOR THE PREVENTION
OF MORBIDITY AND ALL-CAUSE MORTALITY 1,2
Richard A. Winett, Ph.D.
Center for Research in Health Behavior, Virginia Tech
Ralph N. Carpinelli, Ed.D.
Human Performance Laboratory, Adelphi University
ABSTRACT
Public health guidelines focus on increasing low to moderate
physical activity levels in a largely sedentary population. While
there is some evidence that inactivity is associated with increased
risk of morbidity and mortality, there appears to be much stronger
and consistent evidence for a graded inverse relationship between
physical fitness and morbidity and mortality, However, epidemio-
logical studies investigating physical fitness have often not directly
measured aerobic capacity. This calls into question the specific
recommendations that assume a direct relationship between aero-
bic capacity and risk. Performance on some test protocols can be
favorably affected by increases in strength and musculoskeletal
changes, in addition to aerobic capacity. Other public health
recommendations assume that the volume of training, the total
amount of work, or caloric expenditure is the key stimulus for
health-protective adaptations. However, there is little evidence to
support this long-held axiom. A balance of resistance training and
aerobic training is recommended for decreasing morbidity and
mortality. A threshold theory of adaptation and training is
proposed that potentially can efficiently and effectively enhance
aerobic capacity and strength in minimal time.
(Ann
BehavMed
2000, 22(3):237-245)
OVERVIEW
Epidemiological studies suggest that both physical activity
and aerobic capacity are associated with decreased morbidity and
all-cause mortality (1-3). These basic findings have served as the
cornerstone for public health guidelines that are used worldwide to
increase physical activity and prescribe exercise (4-6). Despite
widespread acceptance, some of the theoretical assumptions,
empirical data, and direct applications of this large body of work
are subject to questioning and alternative interpretations.
One theory underlying this work, the volume theory, proposes
that health outcomes are largely attributable to the amount of work
done (caloric expenditure) and that more work is better than less
work. This article addresses the volume theory and attendant issues
behind much of the epidemiological research and examines how
1 Preparation of this manuscript was supported in part by an ASPIRES
grant from Virginia Tech to the first author.
2 We thank Bernard Gutin, Ph.D., Robert Otto, Ph.D., and Janet Wojcik,
Ph.D. for their comments and feedback on an earlier version of this paper.
Reprint Address:
R. A. Winett, Ph.D., CRHB, Department of Psychology,
Virginia Tech, Blacksburg, VA 24060.
9 2000 by The Society of Behavioral Medicine.
that theory has influenced research as well as public health
applications.
Surprisingly, there is at best only limited data to support the
volume theory. An alternative threshold theory is proposed, which
appears to better fit the prevailing data and suggests different
specific public health intervention prescriptions. The threshold
theory points toward the feasibility of increasing aerobic capacity
and strength with a limited duration of exercise in contrast to the
necessity for a great amount of exercise inherent in the volume
theory of training. The methods used to assess aerobic capacity and
the limited role given to strength training in public health
guidelines are also questioned.
ISSUES
Physical activity is any voluntary bodily movement produced
by the contraction of skeletal muscle that elicits an increase in
energy expenditure. Exercise is planned, structured, repetitive
bodily movement that is performed for the purpose of maintaining
or improving some component(s) of physical fitness such as
muscular strength and aerobic capacity (7). Aerobic capacity is
defined as maximal oxygen consumption, which represents a
person's greatest capacity for aerobic energy transfer (8). It is also
known as aerobic power or maximal oxygen uptake (VO2max).
Tolerance to the Balke multistage treadmill protocol or to similar
protocols is often used in prospective investigations to estimate
aerobic capacity (9,10). The Balke protocol incorporates a brief
2-minute warm-up at 3.3 mph with a 1% increase in grade for each
successive minute. The test duration has been assumed to be
indicative of aerobic capacity.
Low levels of aerobic capacity and a sedentary life-style have
both emerged from almost 50 years of epidemiological and
prospective studies as independent risk factors for cardiovascular
diseases, cancer, and all-cause mortality, and are of prime interest
from the perspective of population attributable risk (10-15).
Aerobic capacity and physical activity are inversely related to
many diseases and disabilities, but few people in developed
countries are sufficiently physically active or consistently engage
in exercise to derive health benefits (5,16). In fact, the increasing
prevalence of overweight and obesity suggests that a greater
percentage of the population is becoming sedentary (17).
Recent consensus statements and health policy guidelines
have focused on increasing physical activity in sedentary popula-
tions (4-6). Recommendations call for engagement in moderate
physical activity for about 30 minutes each day or expending about
150 calories per day in leisure time activity for most days (5,6).
Physical activity and aerobic capacity are thought to be protective
from different morbidities through multiple pathways and mecha-
nisms, such as increased oxygen supply to the heart, collateral
coronary artery formation, electrical changes to the cardiovascular
237
238 ANNALS OF BEHAVIORAL MEDICINE Winett and Carpinelli
system, enhanced lipid profile and various components of the
immune system, increased insulin sensitivity, greater resting
metabolic rate, increased muscular mass and strength, and en-
hanced bone mineral density (1,6,8). Indeed, it is often noted that
lower to moderate intensity activity may be health-protective
through one set of pathways and mechanisms while higher
intensity activity may operate through other pathways and mecha-
nisms (1,2). Or, it may be that activity is primarily health-
protective through its effects on aerobic capacity (18).
If evidence supports the position that physical activity is
primarily health-protective through its effects on aerobic capacity,
this would have profound effects on current recommendations.
That is, physical activity would have to be of sufficient intensity to
cause adaptive changes in the cardiorespiratory and neuromuscular
systems. In contrast, providing the means for sedentary people to
do almost any low to moderate intensity activity is the current
public health policy emphasis (5,19). This approach makes sense
for achieving minimal health behavior changes and health policy
goals if merely performing almost any activity is health-protective
(6). While it is likely that involvement in some physical activity is
health-protective and that involvement in vigorous activity is more
beneficial for increasing aerobic capacity, recent data open this
common interpretation of the epidemiological data to some
question in relation to risk for heart disease, some cancers,
diabetes, and all-cause mortality. For example, it is believed that
physical activity alone can reduce the risks for obesity and
diabetes, but the key protective factor may be maintaining a
moderate level of fitness and not engagement in activity per se
(20,21). Moreover, the inverse association of morbidity and
all-cause mortality with aerobic capacity is strikingly consistent
with high relative risk ratios (i.e. the ratio of the presence of the
risk factor to the absence of the risk factor). That is, a greater
aerobic capacity is related to a lower risk of morbidity and
mortality. The association between physical activity and morbidity
and mortality is only moderately consistent with moderate relative
risk ratios (1,2).
There is little scientific basis for assuming that all physical
activity will increase aerobic capacity (22). Nor, is there much
evidence to support the recommendation in current exercise
guidelines (4) that increased aerobic capacity is related primarily to
the volume or duration of exercise--the volume theory. The
recommendations for specific durations of training either through
continuous exercise/physical activity or its accumulation in shorter
bouts during the day reveal little evidence to support longer
duration (4). We propose a different theory for adaptation involv-
ing a time-efficient threshold theory. Aerobic capacity may be
enhanced with very short duration exercise in either structured
exercise programs or life-style interventions as long as an adequate
stimulus is applied (23).
In order to address the points raised in this introduction,
several issues need to be examined: (a) the epidemiological basis
for activity and exercise guidelines; (b) the relationship between
the changes in time on treadmill tests used in studies that estimate
changes in aerobic capacity and the actual changes in aerobic
capacity (VO2max); (c)
the validity of the theory behind physical
activity and exercise guidelines; (d) the time-efficiency of the
exercise prescriptions for enhancing VO2max; (e) the effectiveness
of low to moderate intensity physical activity for increasing
aerobic capacity; and (f) the importance of resistance exercise.
These issues are examined throughout the following sections of
this paper.
EPIDEMIOLOGICAL BASIS
Well-known longitudinal studies (10,13,24-31), which are
extensively reviewed elsewhere (1-3), have generally used two
methods to examine the relationship of both physical activity and
aerobic capacity with morbidity and mortality (2). Epidemiologists
have tended to rely on standardized self-reported activity. The
reports are then used to derive estimates of caloric expenditure in
work, leisure, and recreational activities for a specific period, such
as a week. Caloric expenditure for different intensities, for
example, vigorous or nonvigorous, using different criteria (e.g. <6
METs = nonvigorous) have also been used. A MET is a multiple
of the resting metabolic rate of oxygen consumed per kilogram of
body mass per minute (~3.5 ml 9 kg -1 9 min-~). For example, a
6-MET activity is 6 times the resting oxygen consumption. People
are classified at "time one" based on caloric expenditure in
different categories of physical activity. Any association with
morbidity and mortality at "time two" constitutes the major
findings of the aforementioned longitudinal studies (10,13,24-31).
Given the often discussed problems of the specificity, reliability,
and validity of self-reported data and different criteria used in
different studies and different populations (1,2), it is not surprising
that there is some inconsistency in the results of the epidemiologi-
cal studies that use activity measures (1,2). However, there is more
support for the protective effect of engagement in vigorous activity
as contrasted with the mere expenditure of calories in any activity,
which suggests that physical activity may be protective through its
effects on aerobic capacity (2,26,29,30). In studies using fitness
and activity measures, aerobic capacity is a stronger predictor of
risk (2,26).
For studies using an actual measure of aerobic capacity, there
is more consistency in the findings. Exercise physiologists have
used standardized tests of physical fitness such as the Balke
treadmill protocol at time one and observed the association of
treadmill time with morbidity and mortality at time two (10-13,24-
28,31). Thus, in these studies, fitness is defined as aerobic capacity,
not merely the expenditure of calories. Men and women in the
lowest fitness categories in these studies have high relative risk for
death from cardiovascular diseases, cancer, and all causes (10-
13,24-28,31). The relationship is an inverse gradient of risk from
least-fit to most-fit categories with most of the studies showing
some additional benefits for the highest levels of fitness ( 10,13,31).
While the mean relative risk for coronary artery disease in
sedentary people compared to active people is approximately 1.9,
the mean relative risk for cardiovascular or coronary artery disease
mortality for the least fit people compared to the most fit people has
been reported to be 6.0 (1). In addition, increasing or decreasing
fitness over time has a profound effect on risk, suggesting that the
associations are not simply explainable by genetic factors (25,30-
32). Fitness also appears health protective in the face of other
traditional risk factors such as smoking, obesity, high blood
cholesterol, hypertension, and a family history of disease (14). A
reasonable conclusion from the entire epidemiological base is that
there is some evidence for the benefits of nonvigorous activity,
stronger evidence for the risk reduction benefits of vigorous
activity, and much stronger evidence for the benefits of a greater
aerobic capacity.
While the next section reviews a number of issues involved
with fitness testing, it is important to understand that fitness testing
and aerobic capacity do not revolve around long duration perfor-
mance. Rather, different protocols bring individuals relatively
quickly to the point where they are working very hard (8). Within
Examining Exercise Guidelines VOLUME 22,
NUMBER 3,
2000 239
these protocols, aerobic capacity is defined as how hard--how
intensely--a person can exercise. Despite its promotion by the
scientific community, long duration, low intensity activity or
exercise as exemplified by slow jogging and particularly the
prescription of simply doing more of it, may not be the most
effective way to increase aerobic capacity.
FITNESS TESTING
Although the widely-used Balke treadmill protocol has been
shown to be highly correlated (r = .92) with VO2max (9), there is no
evidence to support the validity of a relationship between an
increase or decrease in treadmill time and an increase or decrease
in VO2max. An increase in treadmill time (or time on other
modalities and protocols) has been associated with a decreased
chance of dying from all causes (25,31), but no study has reported
a similar increase in actual VOzmax. The data simply indicate that a
change in performance on these protocols is associated with a
change in risk. In fact, there is some evidence that refutes the
assumption that there is a linear relationship between changes in
treadmill time and VOzmax. DeBusk and colleagues (33) reported
similar increases (12%) in multistage protocol treadmill time
(--1.5 minutes) for two training groups (described later in this
paper); however, one group increased VO2max (13.9%) almost
twice as much as the other (7.6%).
These are not trivial points, because it has been assumed that
the mechanism for cardiovascular disease risk reduction is primar-
ily attributable to central adaptations of the cardiorespiratory
system. However, performance on a treadmill protocol may be
enhanced through peripheral adaptation of the musculoskeletal
system, such as increased lower body strength. In other words, it
may be easier to walk on a treadmill set at a high incline as in the
Balke protocol because a person's lower body has become
stronger, not necessarily because there have been changes in the
cardiorespiratory system. Blair and colleagues (31) claim that a
4-minute increase in treadmill time (Balke protocol) is equivalent
to an increase in maximum oxygen consumption of approximately
2 METs (-7.0 ml.kg -1. min-1), and hence, substantial risk
reduction. However, there is no evidence to suggest that the
increased time on the treadmill was produced by an enhanced
VO2max
rather than from stronger muscles.
There is evidence that an increased time on a treadmill
protocol may be attributed to enhanced muscular strength. It has
been shown that 12 weeks of resistance exercise increased
endurance time on the treadmill by 38% at an intensity of 80%
VO2max, even though there was no significant increase in VOzmax
(34). The increase in treadmill time was significantly related to an
increase in lower body strength, which may be accomplished with
only a few minutes a week of resistance training for each muscle
group. Epidemiologists have rarely addressed the possibility of this
safe, time-efficient resistance exercise. Simply because most
people who are modestly fit currently choose low to moderate
intensity steady-state activities (35) such as walking, it does not
prove that increased performance on treadmill and other protocols
depends on those kinds of activities. If time on the treadmill is an
important factor, perhaps resistance exercise is an effective training
modality. The intensity of resistance training and cardiovascular
training may be more important than duration.
Decreased cardiovascular risk may in part be attributed to
increased muscular strength, which may contribute to decreased
stress on the cardiorespiratory system. That is, a given activity
becomes less strenuous and reduces cardiovascular responses, such
as rate pressure product (heart rate x systolic blood pressure),
which is an estimate of myocardial oxygen demand. Consider, for
example, a higher risk task such as shoveling wet snow. It may
become less taxing on the cardiorespiratory system, not through
aerobic conditioning, but because the relative intensity is less as a
result of increased muscular strength. This alternative pathway for
risk reduction needs much more research and development (36).
INCREASING AEROBIC CAPACITY
One of the primary public health goals is to increase the
aerobic capacity of the least fit segment of the population
(10,13,31). Examination of studies classifying men and women by
aerobic capacity suggests that reaching the goal entails increasing
aerobic capacity by 1 to 2 METs. ff this were accomplished, unfit
people would sufficiently increase fitness and markedly decrease
risk. Based on current data, it appears unlikely that following the
current activity recommendations will promote a 1 to 2 MET
change in aerobic capacity. That is, there is little evidence that
simply performing such activities as gardening, yard work, house-
hold chores, washing a car, leisurely walking and biking, or
occasionally walking a few blocks from a parking garage to an
office will stimulate the necessary increase in aerobic capacity,
except for a population segment that is extremely infirm (37). Nor
does there appear to be clear evidence that engagement in activity
alone appreciably modifies risk factors, except perhaps for se-
verely obese or infirm groups of people (22,37). Enhancing aerobic
capacity generally requires some structured exercise program or
more challenging activity (8). Indeed, the result of one life-style
intervention where participants presumably engaged in the diverse
activities recommended by the current guidelines (5) showed that
for a sedentary sample of men and women, the program only
increased aerobic power by .75 METs after 6 months (38). This
falls short of the criterion for a 1 to 2 MET increase (31).
Increasing aerobic capacity is generally thought to require at
least three 20- to 30-minute sessions a week at 60% to 90% of
maximum heart rate, or 50% to 85 % of VOzmax or heart rate reserve
(4). Inconvenient access to exercise programs and a lack of
sufficient time are frequently cited barriers for engagement in
structured exercise. Is it the case that 20 to 30 minutes are
required?
MODELS OF TRAINING
Epidemiological research has tended to focus on the volume
of energy expenditure as the key risk reduction stimulus despite
early (39) and more contemporary (22,29) evidence that intensity
of activity and exercise may be the primary protective stimulus.
This volume perspective also has apparently influenced exercise
science's approach to both cardiovascular and resistance training
despite evidence that training intensity, not volume, appears to
provide the stimulus promoting adaptive responses. For example,
with continuous training or accumulated short bout training, the
exercise training literature has explicitly held to the axiom that the
total volume of training is the critical stimulus for adaptation (4).
The belief in this axiom seems to cloud the interpretation of
seminal studies, with those studies then cited in public health
guidelines to support the axiom (1). For example, one frequently
cited study (40) with premenopausal women assessed the impact of
walking 3 miles, 5 days per week, for 24 weeks on aerobic
capacity, percent body fat, and high density lipoprotein (HDL)
cholesterol. Women walked at either a 20-minute, a 15-minute, or a
12-minute per mile pace. The study is noted to demonstrate that the
total volume of exercise is critical for risk reduction; presumably,
because about the same caloric expenditure occurred across the
240 ANNALS OF BEHAVIORAL MEDICINE Winett and Carpineili
walking conditions and the results across the outcome measures
were favorable for all three groups of walkers. An examination of
the actual data from the study suggests other interpretations.
Outcomes across conditions for changes in percent body fat and
HDL cholesterol showed small effect sizes (approximately .20 to
.25), suggesting that walking at any speed is only a marginally
effective intervention for reducing body fat and increasing HDL
cholesterol. However, the differences in outcomes across condi-
tions for aerobic capacity were striking. Walking 20-minute and
15-minute miles increased aerobic capacity .60 METs and .86
METs, respectively, while walking 12-minute miles increased
aerobic capacity by 1.43 METs (40). A more accurate interpreta-
tion of this study is that walking pace--intensity--has a profound
effect on aerobic capacity.
A recent study by Manson and colleagues (30) further
illustrates how much the volume theory influences the assump-
tions, measurement strategies, results, and recommendations de-
rived from research. In this large prospective study using data from
the Nurse's Health Study data base, the investigators developed a
measure of activity or exercise relative to intensity and duration.
The measure was MET-hours per week. It was derived by taking
the MET value of an activity or exercise and multiplying it by the
time participants reported engaging in that activity. Thus, to derive
their volume indicator, Manson et al. (30) combined the MET
level, which is a measure of intensity relative to resting oxygen
consumption, with duration. Thus, a person could engage in a
higher intensity activity and exercise for 20 minutes with a MET
value of 15 and receive a 5 MET-hours score (15 MET • % hour =
5 MET-hours). Another person could receive an equivalent MET-
hours score by engaging in an activity or exercise at 5 METs for 1
hour (5 MET • 1 hour = 5 MET-hours). This approach assumes
that multiples of intensity and duration are essentially equal, but
this position is not supported in overall reviews of the exercise
science literature (8).
Using multivariate analyses controlling for time spent walk-
ing, that is, MET-hours per week, Manson et al. (30) found that
walking pace (intensity) was inversely associated with risk from
heart disease, with parallel findings for diabetes (41). They did
emphasize that it was not merely caloric expenditure that was
critical for risk reduction but rather some moderate to vigorous
activity. Indeed, while more intensive exercise may be necessary
for increasing fitness, it could be that activity or exercise at
approximately 5-6 METs is a sufficient stimulus to reduce risk.
However, what remains unclear is whether recommendations for
risk reduction from Manson et al. can justify a specific duration of
activity or exercise (30). Perhaps, reaching the 5- to 6-MET
threshold for a limited time is all that is required. Thus, the total
caloric expenditure "axiom"--an axiom without much support--
influences the assumptions, measurement strategies, interpreta-
tions of results, and ultimately the recommendations emanating
from otherwise soundly conducted research.
Most contemporary reviews of exercise training research, as
noted, sharply contradict the caloric expenditure axiom (8), yet it is
repeated in recommendations and position stands (4). Clearly,
specific exercise stimuli have very specific effects in terms of
modifying either aerobic capacity or strength (8,40,42,43). The
exact physiological mechanisms for the stimulus/adaptation pro-
cess in aerobic exercise and strength training are not entirely clear.
However, intensity--not frequency or duration--seems to be the
critical dimension for imposing demands that require adaptations
across specific biological systems (23). The total volume of
activity or exercise appears to be relatively less important. Indeed,
one overall review of aerobic exercise training research concluded:
"Training-induced physiologic changes depend primarily on the
intensity of the overload" (8, p. 402).
The threshold theory proposes that in order for adaptation to
occur, a stimulus may only need to surpass a specific threshold of
intensity for a minimum period of time (23). Intensity in steady
state aerobic exercise is defined as the percent of aerobic capacity.
With resistance training, intensity is percent of momentary ability
to perform a specific exercise. This certainly seems to be the
critical stimulus causing adaptations in the musculoskeletal system
and may also be the case for enhancing aerobic capacity (8). There
may be little requirement for long duration of training as specified
in current guidelines (4). In fact, there is very little evidence, and
no new substantive evidence in over 20 years, that long duration is
required to enhance aerobic capacity.
An examination of the citations in the current American
College of Sport Medicine's (ACSM) Position Stand (4) support-
ing the importance of specific durations of training (20-60
minutes) to increase aerobic capacity indicates that most of these
citations simply cite prior reports and position stands or were
conducted decades ago. One frequently cited and rigorous study
(44) does support the importance of duration, but that study could
be conceptualized within the threshold model because of the
specific interval running protocol that was used. It entailed long
rest periods between intervals and thus may have simply required
more time for fatigue to create higher levels of intensity. That level
of intensity could have been quickly achieved with very short rests
between intervals.
For the last 30 years, relatively few studies have compared the
effects of different exercise duration on aerobic capacity. Three
studies (44--46) showed that longer durations elicit a greater
increase in aerobic capacity, while three studies (47-49) reported
no significant difference among groups performing different dura-
tions. The most recently cited study in the 1998 ACSM Position
Stand (4) is a nonexperimental study that shows some association
between duration of training and increases in aerobic capacity (50).
Patients with coronary disease and/or hypertension, who were not
randomly assigned to any specific exercise protocol, self-reported
the duration and intensity of their calisthenics, stretching, cycling,
walking, or aerobic dancing for 4 months. Aerobic capacity
increased approximately 1 MET, which was correlated (r = .58)
with exercise duration.
In contrast to duration, however, the evidence for the impor-
tance of frequency of training (number of times per week) and
especially intensity of training (percent of maximal aerobic
capacity) is current and strong (4). If duration or accumulated
duration is not important and the threshold theory is viable, aerobic
capacity may be increased in a matter of a few minutes in ways that
are compatible with either structured training programs or life-
style interventions.
ACCUMULATED
SHORT-BOUT ACTIVITY
AND THE
VOLUME THEORY
Attempts to make exercise training more acceptable have
included breaking sessions into shorter intervals so that, for
example, people exercise for three 10-minute sessions on each
training day instead of 30 minutes of continuous exercise. The
assumption with the short-bout, interrupted training is that accumu-
lating time across a day up to a specific duration (the volume
theory) is necessary for increasing aerobic capacity and decreasing
risks. Thus, if the public health goal of increasing the aerobic
capacity of the least fit segment is to be reached, the problem seems
Examining Exercise Guidelines VOLUME 22, NUMBER 3, 2000 241
to be that at least three 20-30 minute (or equivalent accumulated
10-minute sessions) exercise training sessions a week are thought
to be required. But, as noted, there is very little evidence to support
the axiom in the guidelines (4) that increasing aerobic capacity is
essentially time-dependent or that 20-30 minutes is necessary.
In the previous section, it was shown that for increasing
aerobic capacity, there is little evidence that a specific duration is
necessary. This may be because a different mechanism, not
dependent upon long duration, is involved. These same guidelines
(4) note that activity can be accumulated throughout a day to reach
30 minutes and that the accumulation of activity and caloric
expenditure is the critical dimension. Indeed, a frequently cited
study investigating the comparable effects of continuous long-bout
versus accumulated short-bout exercise training has been summa-
rized in several reviews (1,4-6) as indicating that the effects of the
two approaches are the same for increasing aerobic capacity (33).
The ACSM guidelines (4) further indicate that either continuous
long-bout or multiple short-bout training can be used, and finally,
that the study (33) demonstrates there are equal outcomes with
equal durations. That is, the prevalent belief is that there are similar
results with continuous or accumulated time, as long as caloric
expenditure is about the same. Because this idea has become a
health policy centerpiece and is derived from the volume theory, it
is important to show that the usual conclusions about this study are
not supported by the study's data.
The aforementioned study by DeBusk and colleagues (33)
involved 36 unfit, sedentary men from 40-60 years. Half were
randomly assigned to unsupervised exercise for 30 continuous
minutes at home or work, 5 days a week, for 8 weeks. The intensity
was 65% to 75% of peak treadmill heart rate, which was based on a
multistage treadmill protocol. The other half was assigned to
unsupervised exercise for three 10-minute sessions, 5 days a week,
for 8 weeks, also at an intensity of 65% to 75%. The 10-minute
sessions were separated by at least 4 hours. Based on exercise logs
kept by the men and from some limited monitoring that recorded
heart rate and movement throughout the day, it was ascertained that
adherence (frequency of training, heart rate, duration) to the
regimes was over 90%. Both types of protocols were comparable
on a number of measures of acceptability (e.g. enjoyment) and on
some measures of physiological adaptation from training. How-
ever, there was a large difference in the increase in aerobic
capacity. The short-bout group increased 7.4% (0.69 METs), from
32.1 mi 9 kg -1 9 min -1 to 34.5 ml 9 kg -1 9 min-L The continuous
long-bout group increased 13.8% (1.31 METs), from 33.3
ml. kg -1- rain -1 to 37.9 ml. kg -1. rain -1, which was signifi-
cantly greater than the short-bout group. Thus, the data from this
study do not support the recommendation that short bouts of
exercise of moderate intensity can be accumulated if the goal is to
increase aerobic capacity by 1 to 2 METs. Certainly, the data do not
indicate that the results were comparable between the two condi-
tions. Indeed, at first glance, these data appear to support some
derivative of the volume theory that would suggest that exercise
must be accumulated at one time. But, if duration of exercise is
relatively unimportant for increasing aerobic capacity, how can the
results be explained?
One possible explanation consistent with the threshold theory
is found in the results of the aforementioned study (33). The report
describes the mean time spent above, within, and below the target
heart rate range for the long-bout and short-bout exercise sessions.
The men in the long-bout group exceeded their target heart rate
(116 to 133 bpm) for about one-third of their exercise time with a
heart rate range of about 135 to 152 bpm (81% to 92%; 19 bpm
over the target), while the men in the short-bout condition
exceeded the same target heart rate range 17% of the time with a
range of about 135 to 145 bpm (81% to 87%; 12 bpm over the
target). One explanation of the outcomes consistent with threshold
theory is that the continuous-exercise group showed better out-
comes than the accumulated short-bout group not because of the
duration of each session, but because men in this group simply
were more likely to reach a higher level of intensity. Thus, even
though the mean exercise heart rate was about the same in each
condition (127 bpm in the long bout, 124 bpm in the short bout),
the mean heart rate achieved may not be critical. Indeed, from the
perspective of the threshold theory, the mean heart rate in the
exercise session is almost meaningless. Conceivably, as illustrated
later, if participants perform only one 10-minute bout and reach a
given threshold, they too could show a large increase in aerobic
capacity.
What is lacking in this study (33), and more recent ones
(51,52) is a condition where people only exercise for 10 or fewer
minutes but at a similar level of intensity (e.g. 80%) that promotes
appropriate adaptations. It may be that the only purpose of the
longer duration of training is that eventually through fatigue the
necessary threshold is reached as the exercise finally becomes
sufficiently harder. Thus, it may take 20 minutes jogging at a
10-minute mile pace to finally reach a threshold level of intensity,
but that level could have been safely reached in a few minutes.
Perhaps the effect of the protocol in this study (33) was less
attributable to moderate exercise training but rather to occasionally
reaching a more intense level. This is an important point because
the study is frequently cited as supporting the benefits of moderate
exercise (4). However, 80% to 90% of maximum heart rate is
generally not considered moderate exercise (4). Studies to examine
the issues raised by this study seem feasible, that is, carefully
controlling a specific exercise intensity and comparing changes in
VO2rn~x as a result of different durations of exercise. For example,
Otto and colleagues (53) compared the effects of 4 versus 20
minutes of aerobic exercise at 70%-85% maximal heart rate, 4
times per week for 6 weeks. Both groups showed a significant
increase in aerobic capacity (-1 MET), with no significant
difference between the 4-minute (12.5%) and 20-minute (9.6%)
groups.
PUBLIC HEALTH POLICY RECOMMENDATIONS
If duration of training is not a critical requirement for
improving aerobic capacity and the threshold theory has credence,
then there are direct applications for both structured exercise
training programs and for life-style interventions that can be
investigated. In structured exercise programs, long-duration train-
ing can be replaced by graded exercise protocols (GXPs), which
are similar to graded exercise tests (GXTs). Such protocols feature
a brief, graduated warm-up, a peak performance at a desired
intensity for 34 minutes, and a cool-down. The level of the peak
performance depends upon fitness level, supervision, and goals.
Thus, an unfit individual interested in appreciably improving
aerobic capacity could warm up and then attain a level of
60%-70% of heart rate reserve for 34 minutes. Supervision
should only involve adjusting the workload periodically to margin-
ally improve fitness over time. The session could consist of a
3-minute multistage warm-up, 3-4 minutes of exercise at 60%-
70% intensity, and a 3-minute cool-down. Such 10-minute work-
outs would be performed twice a week (8). Once an acceptable
fitness level was attained, one session a week could maintain that
level (8). A fitter and more ambitious person can use the same
242 ANNALS OF BEHAVIORAL MEDICINE Winett and Carpinelli
protocol but simply attain a higher warm-up level and then reach
75%-85% of heart rate reserve for 3--4 minutes, followed by a
cool-down. In either case, the same protocol is used. There is no
difference in duration or frequency of training, but rather in
intensity. Both workouts only require 10 minutes twice a week and
could be performed on any cardiovascular exercise modality or
adapted to walking, jogging, running, stair climbing, biking, and
swimming.
Life-style interventions can essentially follow the same plan.
For example, millions of people are trapped each day in facilities
such as office buildings that appear to offer no time or place for
meaningful exercise. Hov~ever, office buildings have stairs. Using
the threshold model, anyone can increase aerobic capacity by
simply climbing stairs, albeit in a systematic way. After walking
around the halls of an office building for a few minutes as a
warm-up, unfit people can climb stairs slowly for 3-4 minutes and
then gradually increase their pace (intensity) over several months.
Once again, two such workouts in a week may be all that are
required to increase aerobic capacity. Perhaps people have been
reluctant to climb stairs for fitness because they may assume that
they have to spend 20~ minutes climbing stairs--a daunting task
for an unfit person. However, it may be that very brief, but
systematic stair climbing can provide considerable increases in
aerobic capacity. Thus, millions of people in developed countries
have ready access to an excellent aerobic training facility.
Approximate MET levels in GXPs or life-style activities to
achieve minimal levels of risk reduction can be estimated from
analyses of prospective studies (10012,14,20,21,25,27) where
actual fitness was assessed. At present, it appears that a 7-MET
capacity for women and a 9-MET capacity for men may be
health-protective. This suggests that gradually over weeks or
months reaching a point (-75% MET capacity) through exercise
or activity where effort is at about 5 METs for women and 7 METs
for men may confer risk-reduction benefits. These levels equate to
modestly paced stair climbing, for example, but clearly not
strolling, and perhaps more intensive effort than usual brisk
walking, for example, brisk walking on hilly terrain. However,
after a warm-up, it may be that such levels only need to be
sustained for a few minutes.
RESISTANCE TRAINING
In most public health guidelines there is an absence of any
consistent and strong recommendation to perform resistance
exercise training in addition to steady-state aerobic exercise (5),
despite the fact that recent research shows strength inversely
associated with disability and morbidity (36). Studies have also
shown that in a relatively short time, both young and elderly
populations of men and women can markedly increase strength
(54,55), and it has been demonstrated that strength can be doubled
with as little as 2 minutes of training each week for each muscle
group (56).
Muscle weakness may be a limiting factor not only for
independent functional ability, but also for the ability to perform
some currently recommended cardiovascular exercise (54). In
addition, if the current recommendation (31) to increase aerobic
capacity by 1-2 METs (-3.5 to 7.0 ml. kg -1. min -1) is valid,
perhaps strength training should be considered asan alternative
morality. Messier and Dill (57) reported similar (--11%) signifi-
cant increases in VO2max (5.0 and 6.2 ml. kg -1 9 min -1, respec-
tively) for a running group (30-minute sessions 3 times per week)
and a strength training group using a Nautilus circuit (20-minute
sessions 3 times per week) with limited time between exercise
movements. It should be recognized that the strength training
group performed only about 6 minutes of intermittent lower body
exercise at each session.
A recent review (58) of controlled intervention trials of
walking programs, which are traditionally prescribed for increas-
ing bone mineral density (BMD), showed only modest increases of
0.5% to 0.6% in lumbar spine BMD following 30 to 52 weeks of
brisk walking at 70%-85% of maximal heart rate for 90 to 180
minutes a week. In contrast, studies have shown that one set of
progressive resistance lumbar extension exercise performed once a
week is an adequate stimulus to significantly increase BMD (59).
For example, Pollock and colleagues (60) reported an increase of
16.5% in BMD using one set per week (taking 60 to 120 seconds
for the one set) on the MedX Lumbar Extension machine. Bevier
and colleagues (61) reported that muscular strength was a better
predictor of bone mineral density in older men and women than
VO2max.
It is also speculated that increased muscular strength may
help to prevent falls, fractures, and reduce the cardiorespiratory
stress of everyday tasks (54,55).
In addition to increased muscle strength and bone mineral
density as a result of progressive resistance exercise (62,63),
studies demonstrate numerous health-related benefits such as
decreased resting systolic and diastolic blood pressure (64),
decreased gastrointestinal transit time (65), decreased low-density
lipoprotein cholesterol and total cholesterol/high-density lipopro-
tein cholesterol ratio (66), decreased plasma insulin and increased
insulin action (67), increased lean body mass and muscle size
(68-74), increasing resting metabolic rate (69) and resting energy
expenditure (75), decreased body fat (69,70,72) and intra-
abdominal adipose tissue (76), and decreased cardiovascular
responses (heart rate, systolic blood pressure, and rate pressure
product) to a given intensity of exercise (77). The research
suggests that resistance exercise can potentially reduce the risk of
cardiovascular disease, diabetes, stroke, colon cancer, osteoporo-
sis, and obesity.
Carpinelli and Otto (78) have conducted an extensive review
of resistance training studies and concluded that there is very little
evidence to support the common practice of using multiple sets of
the same exercise in a training session--the volume approach--for
people at any level of training. Instead, performing one set of an
exercise for each body part seems to provide the necessary
stimulus for promoting musculoskeletal adaptation--findings that
support the threshold model. This means that effective total body
resistance training protocols can be performed in 15-20 minutes
twice per week. Indeed, when done in a circuit, such training also
confers cardiorespiratory benefits (57). If resistance training were
combined with GXP training as described previously, enhanced
strength, muscle mass, aerobic capacity, and other attendant
benefits can be gained for less than 1 hour per week of training.
Interestingly, resistance training has become the centerpiece of
recommendations for older adults (79). One implication of this
paper is that resistance training should become a focus of exercise
training for all adults.
CONCLUSIONS
Public health guidelines used worldwide recognize the impor-
tance of physical activity and exercise for reducing the risk of
morbidity and mortality. While questions remain about the relation-
ship of changes in performance on standard protocols to assess
aerobic capacity, the strongest epidemiological evidence shows
Examining Exercise Guidelines VOLUME 22, NUMBER 3, 2000 243
that aerobic capacity is consistently and inversely related to risk.
The evidence for caloric expenditure through the engagement in
almost any physical activity is considerably weaker then the
evidence for increased aerobic capacity, although public health
guidelines currently revolve around activity promotion. Increasing
aerobic capacity generally depends upon engaging in some struc-
tured exercise program. However, there is little evidence that the
duration or volume of training is the critical stimulus for adaptive
changes. Rather, a time-efficient threshold theory suggests that
intensity coupled with minimal frequency and duration of training
may provide the requisite stimulus. Resistance training also needs
to be given more prominence for potential risk-reduction benefits.
Based on the threshold model, specific aerobic training programs
can be combined with resistance training protocols and substantial
risk-reduction benefits can be achieved with less than 1 hour of
combined weekly training.
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