1. health and fitness
2. exercise

CLINICAL OBSTETRICS AND GYNECOLOGY
Volume 46, Number 2, 379–389
© 2003, Lippincott Williams & Wilkins, Inc.
Physiologic Aspects of
Exercise in Pregnancy
MARY L. O’TOOLE, PhD
Department of Obstetrics, Gynecology & Women’s Health and
Women’s Exercise Research Laboratory, Saint Louis University,
St. Louis, Missouri
Exercise, regardless of type, intensity, or duration, requires an energy expenditure that is
greater than resting energy expenditure. To
increase energy expenditure, oxygen is
needed by the working muscles to transform
stored chemical energy (mainly fat and carbohydrate) into the mechanical energy of
movement. Physiologic responses (ie, simultaneous cardiovascular and respiratory
responses) occur at the onset of exercise to
match energy expenditure and to ensure adequate oxygen availability to exercising
muscle while maintaining the viability of
other tissues. These cardiovascular and
respiratory responses are mediated by hormonal influences and result in metabolic
changes that provide energy for continued
exercise. During pregnancy, endocrine
changes occur that alter the regulation of
metabolic and cardiopulmonary function
and contribute to maternal responses to exercise. This chapter will describe the physiologic responses to acute exercise and exercise training during pregnancy.
Correspondence: Mary L. O’Toole, PhD, Department of
Obstetrics, Gynecology & Women’s Health, 6420 Clayton Rd., Suite 290, St. Louis, MO 63117. E-mail:
[email protected]
CLINICAL OBSTETRICS AND GYNECOLOGY
/
Physiologic Variables Altered
by Exercise
A number of physiologic variables are altered by exercise and are used to quantify
various aspects of exercise-induced
physi.
ologic stress. Oxygen uptake (VO2) is used
to quantify energy expenditure. Oxygen
up.
take can be reported as absolute VO2 reported in liters per minute (L/min) or as oxy.
gen uptake indexed to body weight (ie, VO2
mL/kg/min). .Respiratory responses (minute
ventilation [VE], tidal volume, and breathing frequency) are useful to describe the response of the respiratory system to exercise.
Cardiovascular responses (heart rate, stroke
volume, cardiac output, blood pressure, and
systemic vascular resistance) can all be measured or calculated and are useful for quantifying the response of the cardiovascular
system to exercise. Relationships between
physiologic variables are used to describe
systemic interactions during exercise. These
include
. . the ventilatory equivalent of oxygen
equivalent of carbon
(VE/VO2),. ventilatory
.
dioxide (VE/VCO2), respiratory exchange
ratio (RER), ventilatory threshold, the respiratory. compensation point, and the heart
rate–VO2 relationship.
VOLUME 46
/
NUMBER 2
/
JUNE 2003
379
380
O’TOOLE
Factors Influencing the
Physiologic Responses to
Exercise
Factors that influence the physiologic responses to exercise include exercise mode,
exercise intensity, whether the exercise
stimulus is acute or chronic, and during
pregnancy time of gestation.
EXERCISE MODE
Exercise can be broadly categorized as either weight-bearing or weight-supported.
During weight-bearing exercise (eg, walking), energy expenditure includes that necessary to counteract the pull of gravity.
Thus, body weight contributes to the intensity of the exercise. For example, the energy
cost for a woman to walk a mile is increased
from early to late pregnancy because her
body weight is increased. During weightsupported exercise (eg, cycling or swimming), energy expenditure is independent of
body weight. Energy expenditure for exercise in which body weight is supported is
based on the amount of external work being
done and the mechanical efficiency of the
exerciser. For example, the energy cost to
perform 100 Watts of work on a cycle ergometer is the same regardless of the exerciser’s weight. A special case of weightsupported exercise is swimming. In swimming, not only is body weight supported, but
also external hydrostatic pressure is increased and may contribute to facilitated venous return, thus altering cardiodynamic responses to exercise. Body posture is also important, particularly during pregnancy.
Supine exercise is not recommended after
the second trimester because the gravid
uterus may mechanically restrict venous return and resultant cardiac output.
EXERCISE INTENSITY
Exercise is also categorized by the amount
of effort necessary to perform a given exercise task. The term “submaximal” is used to
describe exercise that is performed with less
than maximal effort. Submaximal exercise
can be quantified either in absolute or relative terms. Absolute submaximal exercise
reflects the amount of work being done. For
example, walking at a pace of 20 minutes to
the mile and cycling at a power output of 50
Watts are examples of absolute exercise intensities. These activities require submaximal effort from most healthy women of
childbearing age. Absolute exercise intensities require a set amount of energy to perform. Many physiologic responses are responsive to changes in energy demands. For
example, for a given energy demand, expected minute ventilation and cardiac output
can be predicted. Submaximal exercise can
also be expressed in relative terms as a percentage of maximal capacity (eg, 70% of
peak walking speed or 70% of peak Watts).
Exercise at the same relative intensity may
require different absolute amounts of work
and therefore a different magnitude of respiratory and cardiovascular response. The
physiologic stress of exercise (ie, perception
of effort) is more closely related to relative
than absolute intensity. The duration of exercise also has an important effect on physiologic responses. For example, prolonged
exercise often elicits a drift in response from
what was seen during a short (10-minute)
exercise bout. Maximal exercise refers to
all-out exercise to exhaustion (ie, the highest-intensity, greatest-load, or longestduration exercise of which an individual is
capable). It is important to understand the
specifics of the exercise stimulus to fully interpret the physiologic responses and in particular to interpret differences in response
that are attributable to pregnancy.
ACUTE VERSUS CHRONIC EXERCISE
Acute exercise refers to a single exercise
session as a stimulus. The specific physiologic responses to acute exercise depend on
the characteristics of the stimulus, such as
mode, intensity, and duration. The responses may also be dependent on the characteristics of the exerciser, including fitness
level. Chronic exercise refers to exercise
training that results in physiologic changes
Physiologic Aspects of Exercise in Pregnancy
or adaptations that optimize response to an
exercise stimulus. An individual may begin
a pregnancy as a trained individual or as a
sedentary person. Similarly, a pregnant
woman may undertake an exercise training
program during pregnancy.
LENGTH OF GESTATION
Pregnancy causes profound anatomic and
physiologic changes. Most notably, weight
is gained, resting energy expenditure is increased, and cardiovascular and respiratory
homeostasis is disturbed. Most of these
changes occur progressively but at varying
rates from early gestation through term.
Each of these may affect the physiologic responses to exercise in varying amounts at
different times during pregnancy. Therefore, to fully interpret exercise responses,
one should consider the length of gestation
along with other exercise-related factors.
Physiologic Responses to
Acute Exercise
Pregnancy results in physiologic changes
that alter energy expenditure as well as cardiovascular and respiratory homeostasis
during rest and exercise.1
ENERGY EXPENDITURE
With the onset of exercise, energy expendi.
ture (quantified as oxygen uptake or VO2)
increases above resting energy expenditure
in direct proportion to the work being done,
such that physical activity and energy expenditure are linearly related. Total energy
expenditure for a given task can be estimated as resting energy expenditure plus the
energy expenditure necessary to perform a
given amount of external work. During
pregnancy, resting energy expenditure is
significantly increased. The increased resting energy expenditure occurs early in pregnancy and is related to the metabolic demands of the uteroplacental unit as well as to
the increased maternal body weight. There is
also a slightly augmented work of breathing
381
during exercise while pregnant that contributes to an overall increase in energy expenditure. Therefore, a given
amount of exer.
cise requires a higher VO2 during pregnancy
compared with that required during the nonpregnant state. The magnitude of the difference in energy expenditure between pregnancy and nonpregnancy depends on the exercise mode and intensity, the amount of
maternal weight gain, and the age of gestation.
During weight-supported exercise
. such
as stationary cycling, the increased VO2 for
a given amount of submaximal exercise will
be similar to the increase in resting energy
expenditure. Khodiguian
et al2 reported an
.
8% increase in VO2 during cycle ergometry
performed at 25 W and 50 W. A greater increase (11%) was observed when exercise
intensity was higher (75 W). The comparison in this study was between responses at
33 weeks’ gestation and those measured in
the same women at 12.5 weeks postpartum.
Heenan et. al3 reported a similar increase
(6%) in VO2 when responses of pregnant
women were compared with a nonpregnant
control group during submaximal cycle. ergometry. Pivarnik et al 4 reported V O 2
(L/min) increases of 10–15% for absolute
work rates of 50 W and 75 W during the
third trimester. Although a decrease in mechanical. efficiency could theoretically increase VO2 even during weight-supported
cycle ergometry, the decrease in efficiency
has been reported to be negligible.3
A related consideration is the apparent effect pregnancy has on substrate utilization.
A preferential use of carbohydrate has been
reported to occur during submaximal,
weight-supported exercise.5 Also of interest
is the report by Artal et al6 that pregnant
women may safely engage in submaximal
exercise at an altitude of 6,000 ft or less with
no adverse effects.
During weight-bearing exercise, energy
expenditure during pregnancy can be expected to increase continually from early
gestation to term in proportion to gains in
body weight. Pivarnik et al4 estimated that
O’TOOLE
382
.
V O2 (L/min) for a given walking speed
would increase by approximately 10% over
the course of a pregnancy. During weightbearing activities, additional energy expenditure can be expected for activities that require agility, balance, or stabilization
throughout the course of pregnancy.
These differences in energy cost
. during
pregnancy are evident only when VO2 is reported in absolute units (ie, L/min) for absolute amounts
of external work (eg, 25 W).
.
When V O 2 is. indexed to body weight
(mL/kg/min), VO2 at absolute external work
rates is expected to decrease as pregnancy
progresses. For example,
Artal et al7 re.
ported a 17% lower VO2 mL/kg/min during
moderate-intensity treadmill walking for
pregnant women in comparison with nonpregnant controls.
.
Absolute VO2max (L/min) appears to be
unaffected by pregnancy itself for most activities. Lotgering et al8 reported no difference
in either treadmill or cycle ergometer
.
VO2max at 16, 25, and 35 weeks’ gestation
in comparison with 7-week postpartum val9–11
ues. Sady
reported no change in
. et al
cycle V O2max when midpregnancy (26
weeks’ gestation) maximal capacity was
compared with values at up to 7 months
3
postpartum. Heenan
. et al reported no difference in cycle VO2max between a group
of pregnant women (35 weeks’ gestation)
and a group of nonpregnant control subjects
matched for physical and demographic char12
acteristics. Spinnewijn et
. al reported on
cycling and swimming VO2max in a small
group of women at 30 to 35 weeks’ gestation
and
. again at 8 to 12 weeks postpartum.
VO2max was not different for either swimming or cycling when comparing pregnancy
and postpartum values, but cycling values
were significantly higher than swimming
values during both pregnancy and the postpartum. This is consistent with findings in
other nonpregnant
populations.
.
Although VO2max appears to be unaffected by pregnancy, the amount of external
work that can be done during pregnancy is
reduced when exercise is weight-bearing
and unchanged for weight-supported activity. Thus, physiologic capacity to increase
metabolism is unchanged, but the resultant
mechanical output is reduced. When energy
expenditure is reported indexed to body
weight (mL/kg/min) during pregnancy,
.
there is a significant decrease in V O2 at
maximal effort regardless of whether the
exercise is weight-bearing or weightsupported.
. Others have reported a decrease in
VO2max during pregnancy and have postulated that the decrease may be the result of
decreases in physical activity during the
course of gestation. In habitually active
women, Clapp et al13 reported a decrease in
usual intensity of running and walking
. from
preconceptual intensities of 74% VO2max
to 57% by gestational week 20 and 47%
by
. gestational week 32. The decrease in
V O2max with advancing pregnancy may
also be the result of factors associated with
the type of activity. McMurray
et al14 re.
ported that swimming VO2max was 17%
lower during pregnancy than postpartum.
They suggested that maternal ventilation
may have been the limiting factor secondary
to limited diaphragm movement caused by
cranial displacement of. the fetal mass. Thus,
reported decreases in VO2max during pregnancy may result from the combination of a
decrease in the intensity and amount of
physical activity as well as from maternal
weight gain.
RESPIRATORY RESPONSES
In general, respiratory activity is precisely
matched to oxygen uptake and controlled by
carbon dioxide. levels. During pregnancy,
the increased VO2 at rest causes respiratory
responses that are in the same direction as
those seen during mild exercise in nonpregnant women—that is, minute ventilation increases mainly as a result of increased tidal
volume with only small increases in breathing frequency until tidal volume reaches its
highest point. The increased minute ventilation at rest continues to rise throughout pregnancy. Resting minute ventilations at the
Physiologic Aspects of Exercise in Pregnancy
end of the second and third trimesters have
been reported to be 21% and 50% higher,
respectively, than those postpartum. 7,14
During pregnancy, there is an increased ventilatory sensitivity to carbon dioxide (CO2)
that is mediated by higher circulating progesterone. The increased sensitivity to CO2
is reflected in the increased
ventilatory
. .
equivalent of CO2 (VE/VCO2) that is observed throughout gestation at rest and during both submaximal and maximal exercise.
Dyspnea is also common in pregnant
women both at rest and during exercise.
At light to moderate exercise intensities,
the increase in minute ventilation is linearly
related to oxygen uptake. During pregnancy,
minute ventilation is increased by 23–26%
during a given level of submaximal exercise
in comparison to nonpregnant controls.2
With advancing gestational age, minute
ventilation for a given level of submaximal
cycle ergometry continues to increase.
Wolfe et al15 reported that minute ventilation at the end of the third trimester was significantly higher than at the end of the second trimester for intensities of 30, 60, and 90
W (Fig. 1). At the end of the third trimester,
the minute ventilation during submaximal
exercise (30, 60, and 90 W) was greater
(36%, 29%, and 27%, respectively) than
postpartum values.15 As exercise intensity
increases to maximal effort, the augmentation in minute ventilation is reduced.
In the nonpregnant individual,
. the
. ventilatory equivalent for oxygen (VE/VO2) during light exercise is expected.to be
. less than
26:1, but during pregnancy VE/VO2 is frequently greater than 32:1 (Fig. 2).7
FIG. 1. Minute ventilation at the end of
the second and third trimesters and postpartum during light, moderate, and peak
exercise intensities. (Wolfe LA, Walker
RMC, Bonen A, et al. Effects of pregnancy and chronic exercise on respiratory responses to graded exercise. J Appl
Physiol. 1994;76:1928–1936)
383
Another respiratory response of interest
is the respiratory exchange
ratio (RER = ra.
tio of CO
. 2 produced [VCO2] to O2 metabolized [VO2]). Below the ventilatory threshold, RER can be used as an estimate of the
relative utilization of fat and carbohydrate in
nonpregnant individuals. During moderate
exercise, RER is not different at different
times during pregnancy, nor are pregnancy
values different from those during the postpartum period.15,16 However, the relationship of RER to substrate utilization during
pregnancy is unclear.
During
vigorous exercise, the slope
. more
.
of the VE/VO2 relationship is increased at a
point designated as the
. ventilatory threshold. At this point,. V CO2 increases more
steeply relative to VO2, mainly as a result of
bicarbonate buffering
of lactic acid. Above
.
this
threshold,
V
E
is
closely coupled to
.
VCO2. As exercise intensity continues to increase, a respiratory compensation point is
reached that marks the onset of metabolic
acidosis. In sedentary individuals, the ventilatory threshold and the respiratory compensation point occur at exercise intensities of
approximately
50–60% and 80–90% of peak
.
VO2, respectively. Pregnancy does not appear to have a marked effect on the identification of. these points despite its marked effect on VE and on ventilatory .sensitivity to
CO
. 2. However, the slope of VCO2 versus
VO2 above the ventilatory threshold is more
shallow during pregnancy than postpartum,
suggesting that the buffering of lactic acid is
reduced.17
At maximal exercise, minute ventilation
during cycle ergometry has been reported to
384
O’TOOLE
FIG. 2.
. .Ventilatory equivalent of oxygen (VE/VO2) before and during symptom-limited treadmill walking to maximal effort in pregnant versus nonpregnant controls. (Artal R, Wiswell R,
Romem Y, et al. Pulmonary responses to
exercise in pregnancy. Am J Obstet Gynecol. 1986;154:378–383, with permission)
be higher8 or not different12,15 during pregnancy than
postpartum. Lotgering et al8
.
found VEmax to be 7–11% higher during
both cycle ergometry and treadmill exercise
during pregnancy compared with the postpartum. In contrast, others have reported
no
.
change in cycling or swimming VEmax in
12,15
pregnancy compared
. with postpartum.
No differences in VEmax based on length of
gestation have been reported for treadmill,
cycle, or swimming exercise. At maximal
exercise, neither the ventilatory equivalent
of oxygen or carbon dioxide is different during pregnancy in comparison with values for
matched controls, but RER is significantly
lower (1.19 vs. 1.25).3 This is consistent
with a lower peak lactate concentration and
may reflect a reduced carbohydrate oxidation at high-intensity exercise.
CARDIOVASCULAR RESPONSES
Cardiovascular responses to exercise during
pregnancy follow the same general pattern
as nonpregnant responses. In both cases,
cardiovascular responses are most meaningful when related to changes
in absolute ex.
ercise intensity
(ie,
V
O
L/min).
Cardiac
2
.
output (Q) increases in a linear and
fixed
re.
lationship with oxygen uptake (VO2). Dur-.
ing pregnancy, however,
the increase in Q
.
per unit increase. in. VO2 is greater than when
not pregnant. Q/VO2 during pregnancy is
approximately 25% higher than that expected at rest when not pregnant. Both
stroke volume .and heart rate contribute to
the increase in Q at rest and during exercise.
During exercise, stroke volume increases in
untrained individuals until exercise intensity
reaches 40–50% of maximal capacity and
then plateaus at that level. Highly trained individuals may have the capacity to continue
to increase stroke volume. Heart rate increases linearly with oxygen uptake up to
maximal capacity. With increasing exercise
intensity, systolic blood pressure rises,
mainly the result of the increased cardiac
output. Diastolic blood pressure may
slightly increase, decrease, or stay the same.
Systemic vascular resistance decreases because of generalized vasodilation, thereby
preventing an exaggerated rise in blood
pressure in response to the increased cardiac
output. Distribution of cardiac output is a
function of both neural and hormonal input
such that blood is selectively directed toward areas of higher metabolic activity and
away from splanchnic and renal vascular
beds.
Most studies have shown cardiac output,
heart rate, and stroke volume to be increased
at submaximal exercise intensities during
pregnancy in comparison with nonpregnant
responses. For example, during cycle ergometry at 25, 50, and 75 W, heart rates have
been reported to be 18%, 16%, and 12%
higher, respectively, during pregnancy than
during the postpartum period (Fig. 3).2
Physiologic Aspects of Exercise in Pregnancy
385
FIG. 3. Submaximal exercise heart
rates during cycle ergometry during
pregnancy versus nonpregnancy.
(Khodiguian N, Jaque-Fortunato SV,
Wiswell RA, et al. A comparison of
cross-sectional and longitudinal methods
of assessing the influence of pregnancy
on cardiac function during exercise. Semin Perinatol. 1996;20:232–241)
During cycle ergometry with exercise intensities up to 100 W, Sady et al9–11 reported
that cardiac outputs were 2.2 to 2.8 L/m in
higher during pregnancy than postpartum.
Interestingly, the cardiac output–oxygen uptake relationship was unchanged, suggesting that cardiac output response to exercise
was not altered by pregnancy. Using direct
Fick methodology, Pivarnik et al18 reported
similar findings in response to both cycle ergometry and treadmill walking. Differences
in cardiac output for treadmill walking during pregnancy in comparison to responses
postpartum are greater than those for cycle
ergometry. This is to be expected since postpartum weight loss markedly reduces the exercise load for weight-bearing exercise in
comparison with that during pregnancy.
Blood pressure responses to exercise during
pregnancy are unchanged or slightly decreased in comparison to those in the nonpregnant state. The same linear relationship
between systolic blood pressure and exercise intensity has been observed during
pregnancy as expected in the nonpregnant
state. Since cardiac output is increased to a
greater extent during submaximal exercise
during pregnancy, systemic vascular resistance during exercise must decrease more
than during the nonpregnant state. There appears to be a dose-response effect of maternal exercise intensity on uteroplacental
blood flow. Although limits have been extrapolated from animal studies, Mottola suggested that a maternal. exercise intensity of
approximately 80% VO2max seems to be
the threshold above which fetal blood flow
is significantly diminished.19
The magnitude of cardiovascular responses to exercise during pregnancy is affected by length of gestation as well as by
exercise mode and intensity. Several studies
have demonstrated that stroke volumes and
cardiac outputs are reduced as pregnancy
advances, with little change in exercise heart
rates. McMurray et al14 demonstrated higher
stroke volumes when exercise was done in
water rather than on land. They also demonstrated that stroke volumes during water exercise did not decrease as gestation advanced. These data support the hypothesis
that decreases in stroke volume as pregnancy advances can be attributed to increased venous pooling and to decreased venous return. Veille et al20 studied maternal
cardiodynamics via m-mode and twodimensional Doppler echocardiography and
concluded that the increased stroke volumes
seen during exercise early in pregnancy are
the result of increased ventricular contractility, with increased preload becoming more
important as gestation advances. This work
lends support to the hypothesis of a blunted
response to catecholamines in the third trimester.
Most studies suggest that maximal cardiac outputs and stroke volumes9–11 are increased during pregnancy, but that maximal
heart rates are unchanged.3 Some have suggested, however, that heart rates during
pregnancy may be reduced because of a
blunted response to sympathetic stimulation.2,8,15 They reported peak heart rate for
cycle ergometry and treadmill exercise to be
slightly (1–7%) but statistically signifi-
386
O’TOOLE
cantly lower during pregnancy than postpartum.
Because of the linear relationship that exists between heart rate and oxygen uptake,
heart rates can be used to quantify exercise.
Recently Pivarnik et al 21 provided data
documenting changes
. in the slope and intercept of heart rate–VO2 regression lines as
pregnancy advances and into the postpartum. Slopes were approximately 10% flatter
during pregnancy.
This means that a greater
.
change in VO2 occurs per change in exercise
heart rate during pregnancy compared with
the postpartum. At a given submaximal
heart rate, energy expenditure is less during
pregnancy than during the postpartum.
Thus, if exercise during pregnancy is to be
used
to control caloric balance, heart rate–
.
VO2 regression lines appropriate for gestational age should be constructed to predict
caloric expenditures.
Physiologic Response to
Exercise in Well-Trained
Pregnant Women
Well-defined physiologic responses are expected in individuals engaged in regular exercise training. Whether these are preserved
during pregnancy in women who continue to
train is unclear. Cross-sectional studies22
suggest that women who continue aerobic
exercise training during pregnancy have
lower resting heart rates and higher stroke
volumes than matched sedentary controls at
25 and 36 weeks’ gestation as well as at 12
weeks postpartum. No differences were observed in resting cardiac output, a-vO2diff,
mean arterial pressure, or systemic vascular
resistance between trained and sedentary
women during gestation or at 12 weeks postpartum.22
During submaximal cycling exercise at a
heart rate of approximately 140 bpm, physiologic responses of trained and sedentary
pregnant women differed in virtually all
measured variables.22 Caution should be
used in interpreting these differences, since
the trained group was exercising at a much
higher absolute exercise intensity. Others
have also compared the responses of active
women (all regularly exercising three to six
times per week) who were pregnant with
those of similarly active nonpregnant
women. In response to a single submaximal
exercise intensity, no significant effect of
pregnancy was evident on heart rate, oxygen
uptake, or minute ventilation. Additionally,
ventilatory thresholds during cycle ergometry were not different between pregnant
and nonpregnant groups, nor were the respiratory compensation points or work efficiency different.3 The ventilatory equivalents for oxygen and carbon dioxide were
significantly increased, and end-tidal and arterial carbon dioxide levels were significantly decreased in the pregnant women.3
Physiologic Response to
Exercise Training
During Pregnancy
A number of studies have investigated
physiologic responses of pregnant women to
exercise training. Although specific training
has varied, most has conformed to the same
general structure: aerobic exercise (cycling,
swimming, or varied aerobic exercise) at
moderate intensity (exercise target heart
rates 140–150 bpm) done three times per
week for 25 to 60 minutes per session has
been the training stimulus. Conditioning
programs have started early in the second
trimester and continued throughout pregnancy. Various outcomes have been used to
assess the effectiveness of the training program. Most have documented an increase in
aerobic capacity. However, some of the
variables used to assess the effectiveness of
training in nonpregnant populations may not
show training-induced changes during pregnancy. For example, in nonpregnant individuals, a decrease in resting heart rate is a
common result of training. When a training
program is begun during pregnancy, resting
bradycardia is usually not observed. An-
Physiologic Aspects of Exercise in Pregnancy
other cardiovascular adaptation to aerobic
conditioning in nonpregnant individuals is
an increase in left ventricular dimension.
With exercise training during pregnancy,
cardiac dimensions have been reported to be
unchanged beyond those attributable to
pregnancy itself.23
During submaximal exercise by nonpregnant individuals,
heart rates, minute ventila. .
tion, VE/VO2, and RER are lower in conditioned versus sedentary individuals. During
pregnancy, no differences in these variables
were observed between exercise and control
groups participating in a standard submaximal intensity of 75 W.24 However, during
heavier
. . exercise, submaximal heart rates,
VE/VO2, and perceived exertion were reported to be lower in exercising versus sedentary pregnant women.25 Several investigators have suggested that the conditioning
effects of exercise training normally seen at
rest and during light exercise may be obscured by the physiologic effects of pregnancy (eg, increased blood volume) but are
evident at higher exercise intensities. Wolfe
et al15 reported
. .an increase in ventilatory
sensitivity (VE/VCO2) with advancing gestation in both control and exercise groups at
submaximal levels, but no difference between groups. These data are in agreement
with other reports of a well-documented increase in respiratory sensitivity during pregnancy and demonstrate that the increased
sensitivity continues to be operative during submaximal exercise even in wellconditioned pregnant women. Wolfe et al15
also reported that the exercise intensity that
represented the onset of blood lactate accumulation did not change with advancing
gestation in the sedentary control group, but
increased with exercise training during
pregnancy and then decreased during the
postpartum, when the physical conditioning
program was discontinued. 15 Thus, the
changes in ventilatory thresholds during
pregnancy appear to be affected by exercise
training, but not by pregnancy itself.
Exercise training during pregnancy can
result in improvement of aerobic fitness
387
similar in magnitude to that expected in a
a
nonpregnant group. Wolfe et al15 reported
.
17% increase in oxygen pulse (VO2/HR)
from early in the second trimester to late in
the third trimester in a group that followed
an exercise training program compared with
a control group. Peak heart rates were lower
at the end of the third trimester in comparison with postpartum in both the exercise and
the control group, but there were no differences between groups. Peak postexercise
lactate levels are expected to increase with
exercise training in nonpregnant individuals. No peak postexercise increase was observed in this study,15 perhaps suggesting a
decreased availability of carbohydrate late
in gestation.
Summary and Conclusions
Both pregnancy and exercise alter energy
expenditure as well as cardiovascular and
respiratory homeostasis in comparison with
that expected at rest for a nonpregnant individual. The magnitude of the differences in
physiologic response to exercise between
pregnancy and nonpregnancy depend on the
exercise mode and intensity, the amount of
maternal weight gain, and the age of gestation. During submaximal exercise, energy
expenditure, minute ventilation, cardiac output, and heart rates are greater during pregnancy in comparison with responses during
the nonpregnant state. During maximal exercise, the ability to transform energy appears to be unaffected by pregnancy, but
maximal energy expenditure may decrease
because of decreased regular activity during
pregnancy. Maximal responses of respiratory and cardiovascular systems during pregnancy are less clear. Some studies have reported minute ventilation and cardiac output
to be higher during pregnancy, while others
have reported no change. Most studies suggest that maximal heart rates are unchanged,
while others suggest that maximal heart
rates may be reduced. Physiologic responses
to exercise training during pregnancy appear
to be similar to those expected in a nonpreg-
388
O’TOOLE
nant population. However, caution is suggested in interpreting markers, such as resting heart rate, that are commonly used to assess the efficacy of exercise training. Many
of these markers may be obscured during
rest or light exercise by the physiologic effects of pregnancy (eg, increased blood volume) but will be evident at higher exercise
intensities.
References
1. Artal R, Wiswell RA, Drinkwater BL, eds.
Exercise in Pregnancy, 2nd ed. Baltimore:
Williams & Wilkins, 1991.
2. Khodiguian N, Jaque-Fortunato SV,
Wiswell RA, et al. A comparison of crosssectional and longitudinal methods of assessing the influence of pregnancy on cardiac function during exercise. Semin Perinatol. 1996;20:232–241.
3. Heenan AP, Wolfe LA, Davies GA. Maximal exercise testing in late gestation: maternal responses. Obstet Gynecol. 2001;97:
127–134.
4. Pivarnik JM, Lee W, Miller JF. Physiological and perceptual responses to cycle and
treadmill exercise during pregnancy. Med
Sci Sports Exerc. 1991;23:470–475.
5. Artal R, Masaki DI, Khodiguian N, et al. Exercise prescription in pregnancy: weightbearing versus non-weight-bearing exercise. Am J Obstet Gynecol. 1989;161:1464–
1469.
6. Artal R, Fortunato V, Welton A, et al. A
comparison of cardiopulmonary adaptations
to exercise in pregnancy at sea level and altitude. Am J Obstet Gynecol. 1995;172:
1170–1180.
7. Artal R, Wiswell R, Romem Y, et al. Pulmonary responses to exercise in pregnancy. Am
J Obstet Gynecol. 1986;154:378–383.
8. Lotgering FK, Van Doorn MB, Strunk PC,
et al. Maximal aerobic exercise in pregnant
women: heart rate, O2 consumption, CO2
production, and ventilation. J Appl Physiol.
1991;70:1016–1023.
9. Sady SP, Carpenter MW, Sady MA, et al.
Prediction of VO2max during cycle exercise
in pregnant women. J Appl Physiol. 1988;
65:657–661.
10. Sady SP, Carpenter MW, Thompson PD, et
al. Cardiovascular response to cycle exercise during and after pregnancy. J Appl
Physiol. 1989;66:336–341.
11. Sady MA, Haydon BB, Sady SP, et al. Cardiovascular response to maximal cycle exercise during pregnancy and at two and
seven months postpartum. Am J Obstet Gynecol. 1990;162:1181–1185.
12. Spinnewijn WE, Wallenburg HCS, Struijk
PC, et al. Peak ventilatory responses during
cycling and swimming in pregnant and nonpregnant women. J Appl Physiol. 1996;81:
738–742.
13. Clapp JF III, Wesley M, Sleamaker RH.
Thermoregulatory and metabolic responses
to jogging prior to and during pregnancy.
Med Sci Sports Exerc. 1987;19:124–130.
14. McMurray RG, Hackney AC, Katz VL, et
al. Pregnancy-induced changes in maximal
oxygen uptake during swimming. J Appl
Physiol. 1991;71:1454–1459.
15. Wolfe LA, Walker RMC, Bonen A, et al. Effects of pregnancy and chronic exercise on
respiratory responses to graded exercise. J
Appl Physiol. 1994;76:1928–1936.
16. Bessinger RC, McMurray RG, Hackney
AC. Substrate utilization and hormonal responses to moderate intensity exercise during pregnancy and after delivery. Am J Obstet Gynecol. 2002;186:757–764.
17. Lotgering FK, Struijk PC, Van Doorn MB,
et al. Anaerobic threshold and respiratory
compensation in pregnant women. J Appl
Physiol. 1995;78:1772–1777.
18. Pivarnik JM, Lee W, Clark SL, et al. Cardiac
output responses of primigravid women
during exercise determined by the direct
Fick technique. Obstet Gynecol. 1990;75:
954–959.
19. Mottola MF. The use of animal models in
exercise and pregnancy research. Semin
Perinatol. 1996;20:222–231.
20. Veille JC, Hellerstein HK, Cherry B, et al.
Maternal left ventricular performance during bicycle exercise. Am J Cardiol. 1994;73:
609–610.
21. Pivarnik JM, Stein AD, Rivera JM. Effect of
pregnancy on heart rate/oxygen consumption calibration curves. Med Sci Sports Exerc. 2002;34:750–755.
Physiologic Aspects of Exercise in Pregnancy
22. Pivarnik JM, Ayres NA, Mauer MB, et al.
Effects of maternal aerobic fitness on cardiorespiratory responses to exercise. Med
Sci Sports Exerc. 1993;25:993–998.
23. Wolfe LA, Preston RJ, Burggraf GW, et al.
Effects of pregnancy and chronic exercise
on maternal cardiac structure and function.
Can J Physiol Pharmacol. 1999;77:909–
917.
389
24. South-Paul JE, Rajagopal KR, Tenholder
MS. The effect of participation in a regular
exercise program upon aerobic capacity
during pregnancy. Obstet Gynecol. 1988;
71:175–179.
25. Ohtake PJ, Wolfe LA, Hall P, et al. Physical
conditioning effects on exercise heart rate
and perception of exertion in pregnancy.
Can J Sport Sci. 1988;13:71P–73P.