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Development and validation of exercise target heart rate zones for overweight and obese pregnant women Margie H. Davenport, Sarah Charlesworth, Dana Vanderspank, Maggie M. Sopper, and Michelle F. Mottola
Abstract: Validated target heart rate (THR) zones for exercise prescription for overweight and obese pregnant women have not been developed. The purposes of this study were to determine if heart rate reserve (HRreserve) is best described by aerobic capacity at peak exercise or by aerobic capacity reserve (VO2 reserve) and to develop and validate THR zones for light-intensity exercise (20%–39%VO2 reserve) in sedentary overweight and obese pregnant women. One hundred six women between 16 and 20 weeks gestation with medical clearance performed a progressive treadmill test to volitional fatigue (peak). Data from every 4th subject were used for cross-validation. Two linear regression equations were performed for each subject, then pooled to obtain mean group values (± SD): %HRreserve vs. %VO2 peak and %HRreserve vs. %VO2 reserve. THR zones equivalent to 20%–39%VO2 reserve were developed and validated based on the strongest relationship. %HRreserve had a stronger linear relationship with %VO2 reserve (y = 1.046x –7.561; R2 = 0.741) than %VO2 peak (y = 1.259x –28.795; R2 = 0.604). Validated THR ranges for sedentary overweight and obese pregnant women are 102–124 beatsmin–1 (20–29 years of age) and 101–120 beatsmin–1 (30–39 years of age), representing an exercise intensity of 20%–39%VO2 reserve as recommended by the American College of Sports Medicine for previously sedentary pregnant women. Overweight and obese women who are medically prescreened can exercise during pregnancy within our validated THR zones. The relationship between HR and VO2 remains strong, but the two are not equivalent in this population group. Key words: exercise prescription, pregnancy, target heart rate, validation, treadmill, overweight and obese. Re´sume´ : Il n’y a pas encore de plages valide´es de fre´quence cardiaque cible (THR) pour la prescription d’exercice physique a` l’intention des femmes enceintes obe`ses ou pre´sentant un surpoids. Le but de cette e´tude est de ve´rifier si la fre´quence cardiaque de re´serve (HRreserve) est mieux corre´le´e a` la consommation d’oxyge`ne pour un effort de pointe ou a` la consommation d’oxyge`ne de re´serve (VO2 reserve) et d’e´laborer et de valider des plages de THR pour un effort de le´ge`re intensite´ (20–39 % VO2 reserve) chez des femmes enceintes se´dentaires et obe`ses ou pre´sentant un surpoids. Cent-six femmes enceintes depuis 16 a` 20 semaines et pre´sentant un certificat de sante´ participent a` une e´preuve d’effort croissant sur tapis roulant jusqu’a` l’e´puisement volontaire (pointe). Les re´sultats de tous les quatrie`mes sujets sont utilise´s a` des fins de validation croise´e. On de´veloppe deux e´quations de re´gression line´aire pour chaque sujet puis on les regroupe pour obtenir des valeurs moyennes plus ou moins l’e´cart-type : le % HRreserve par rapport au % consommation d’oxygene de pointe et le % HRreserve par rapport au % VO2 reserve. C’est a` la lumie`re des meilleures corre´lations qu’on e´tablit et valide les plages de THR correspondant a` 20–39 % VO2 reserve. La relation line´aire entre le % HRreserve et le % VO2 reserve (y = 1,046, x –7,561; R2 = 0,741) est meilleure qu’entre le % HRreserve et le % consommation d’oxygene de pointe (y = 1,259, x –28,795; R2 = 0,604). Les plages valide´es de THR a` l’intention des femmes enceintes obe`ses ou pre´sentant un surpoids et aˆge´es de 20 a` 29 ans sont 102–124 bpm et 101–120 bpm pour les femmes aˆge´es de 30 a` 39 ans; cette plage correspond a` une intensite´ d’effort e´quivalent a` 20–39 %, ce qui est recommande´ par le Colle`ge ame´ricain de me´decine du sport pour des femmes enceintes se´dentaires. Les femmes obe`ses ou pre´sentant un surpoids et ayant obtenu un certificat de sante´ peuvent durant leur grossesse faire de l’exercice physique a` l’inte´rieur des plages valide´es de THR. On observe une tre`s bonne corre´lation entre la HR et le consommation d’oxygene, mais en valeur relative au maximum, les deux variables ne sont pas e´quivalentes dans cette tranche de la population. Mots-cle´s : prescription d’exercice physique, grossesse, fre´quence cardiaque cible, validation, tapis roulant, surpoids, obe´site´. [Traduit par la Re´daction] Received 28 February 2008. Accepted 22 May 2008. Published on the NRC Research Press Web site at apnm.nrc.ca on 24 September 2008. M.H. Davenport, S. Charlesworth, D. Vanderspank, and M.M. Sopper. Exercise and Pregnancy Laboratory, School of Kinesiology, Faculty of Health Sciences, University of Western Ontario, London, ON N6A 3K7, Canada. M.F. Mottola.1 Exercise and Pregnancy Laboratory, School of Kinesiology, Faculty of Health Sciences, University of Western Ontario, London, ON N6A 3K7, Canada; Deptartment of Anatomy and Cell Biology, Schulich School of Medicine and Dentistry, University of Western Ontario, London, ON N6A 3K7, Canada. 1
Corresponding author (e-mail:
[email protected]).
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doi:10.1139/H08-086
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Introduction Women of childbearing age are at an increased risk of obesity due to excessive weight gain during pregnancy and excessive weight retention after delivery (Kim et al. 2007). Current US population estimates of maternal obesity (prepregnant BMI ‡ 30 kgm–2) range from 29% (Flegal et al. 2002) to 39% (LaCoursiere et al. 2005). Considering that epidemiological and animal studies indicate that maternal obesity during pregnancy leads to metabolic programming, which increases the risk of obesity in offspring (Danielzik et al. 2002; Shankar et al. 2008; Wu and Suzuki 2006), prevention of excessive weight gain during pregnancy may be an effective strategy to curb the rise in obesity in this population group. Research over the past 20 years has demonstrated the safety and benefits of aerobic exercise throughout pregnancy along a wide range of exercise intensities (Davies et al. 2003; Mottola et al. 2006; Weissgerber et al. 2006; Wolfe et al. 1994). Exercise has many benefits for pregnant women, including maintenance of aerobic fitness and increase in total daily caloric expenditure. Therefore, exercise could play a role in the prevention of excessive weight gain during pregnancy. The Physical Activity Readiness Medical Examination for Pregnancy (PARmed-X for pregnancy) (Wolfe and Mottola 2002) was developed in Canada to provide age-specific target heart rate (THR) zones for aerobic exercise equivalents of 60%–80% of maximal aerobic capacity (VO2 max) and guidelines for exercise during pregnancy. However, the PARmed-X for pregnancy document does not provide heart rate zones for exercise intensities below 60% VO2 max. This is particularly important, as overweight and obese pregnant women may be incapable of exercising at 60%–80% of aerobic capacity. According to the American College of Sports Medicine (ACSM), 20%–39%, 40%–59%, and 60%–84% VO2 reserve correspond to light, moderate, and high intensities of exercise, respectively, for males and nonpregnant females (American College of Sports Medicine 2005). ACSM currently suggests that previously sedentary overweight and obese pregnant women initiate an aerobic exercise program at an intensity equivalent to 20%–39% VO2 reserve (American College of Sports Medicine 2005). The relationship between heart rate and aerobic capacity (VO2) in the non-pregnant population is linear at submaximal exercise intensities; based on this relationship, heart rate is often prescribed to indicate exercise intensity. During pregnancy, resting heart rate is elevated by 15– 20 beatsmin–1 (Wolfe and Davies 2003) and maximal heart rate is blunted (Wolfe and Mottola 1993). Karvonen et al. (1957) was the first to address this issue by using heart rate reserve (HRreserve = HRmax – HRrest) for exercise prescription in healthy adults. Although never actually described by Karvonen or any subsequent study (Swain and Leutholtz 1997), the reigning dogma for many years was that a given percentage of HRreserve was equivalent to the same percentage of maximal aerobic capacity (VO2 max) (American College of Sports Medicine 2005). However, recent studies in healthy individuals (Swain and Leutholtz 1997; Swain et al. 1998), obese individuals (Byrne and Hills 2002), elite cyclists (Lounana et al. 2007), and cardiac patients (Brawner et al. 2002) indicate that this is not the case and that %HRreserve is actually more closely related to percent aerobic capacity
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reserve (VO2 reserve = VO2 max – VO2 rest), particularly at lower exercise intensities. In 1998, the ACSM revised exercise guidelines for healthy adults to reflect these findings (American College of Sports Medicine 1998). According to these guidelines, %VO2 reserve and %HRreserve are equivalent (American College of Sports Medicine 2005) in non-pregnant individuals. However, this relationship has not been demonstrated in an overweight or obese pregnant population. THRs based on %HRreserve or %VO2 reserve have not been developed for exercise prescription in an obese pregnant population. In addition, the relationship between aerobic capacity and heart rate has not been described for overweight or obese pregnant women along a full spectrum of exercise intensities. In obese women, and during pregnancy, it has not been established if this relationship holds or is uncoupled. Therefore, the purposes of the present study were to determine if HRreserve is best described by peak aerobic capacity (VO2 peak) or VO2 reserve to ensure precise exercise prescription for this population and to develop and validate age-related THR zones equivalent to light-intensity exercise in previously sedentary overweight and obese pregnant women based on a graded exercise treadmill test to volitional fatigue (peak). We hypothesize that in this population of overweight and obese pregnant women, VO2 reserve will best describe HRreserve.
Materials and methods The Human Research Ethics Board for Health Sciences at the University of Western Ontario approved the protocol. One hundred six women with a pre-pregnancy BMI between 25.0 and 29.9 kgm–2 (overweight, OW) or a BMI ‡ 30 kgm–2 (obese, OB), between 16 and 20 weeks gestation, were recruited through physician and midwife referrals, posters and advertisements in newspapers in London, Ont. Self-reported mass and height were used to calculate prepregnancy BMI. Before being enrolled in the study, all women were medically pre-screened (PARmed-X for Pregnancy; Wolfe and Mottola 2002) by their health care provider. Written informed consent was obtained from each participant. Because of the risks and discomfort associated with the first trimester of pregnancy, women in their 16th– 20th week of gestation (second trimester) was chosen for testing (Wolfe and Mottola 2002). Following calibration of a metabolic cart (SensorMedics, VOmax 29c) as described elsewhere (Mottola et al. 2006), pre-exercise respiratory gases were collected for 5 min. To accurately measure differences in metabolism and ventilatory characteristics between rest and peak exercise without the confounding factor of a non-weight-bearing position, we measured resting metabolism while the subject was standing quietly on the treadmill. Heart rate was recorded via 4 electrocardiogram (ECG) leads attached to the metabolic cart. The test began with a 5 min warm-up at 3 milesh–1 (4.8 kmh–1) and a 0% grade. During the test, the treadmill speed was held constant at 3 milesh–1 while the incline was increased by 2% every 2 min until volitional fatigue was achieved (Fig. 1). If fatigue was not reached by 12% grade, the speed was increased by 0.2 milesh–1 (0.3 kmh–1) every 2 min until volitional fatigue was attained (Fig. 1). At the start of each stage, the subject rated her perceived exertion using the Borg scale (Borg 1982). Once volitional fatigue was reached #
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Fig. 1. Modified Balke treadmill exercise protocol. Following a 5 min warm-up at 3 milesh–1, 0% grade, the grade was increased by 2% every 2 min. Beyond 12 min, grade was maintained at 12% and speed was increased 0.2 milesh–1 every 2 min until volitional fatigue. Dashed line indicates incline; solid line indicates speed.
Table 1. Predicted target heart rate for overweight or obese pregnant women based on age using percent heart rate reserve (HRreserve) vs. percent aerobic capacity reserve (VO2 reserve) regression equation. Heart rate (beatsmin–1) %VO2 reserve 20 30 40 50 60 70 80
%HRreserve 13a 24a 34a 45a 55a 66 76
Age: 20–29 y 102 113 125 136 147 158 169
Age: 30–39 y 101 111 121 131 142 152 162
Note: Mean resting heart rate (n = 80) = 88 beatsmin–1; Table 2. a
Differs significantly from ACSM non-pregnant guidelines (p < 0.01).
(Borg rating = 9 or 10; maximal on a 10-point scale; Borg 1982), a 5 min cool-down at 3 milesh–1 and 0% grade was immediately initiated. VO2 peak was determined from the average of the previous 30 s of breath-by-breath analysis recorded by the computer software, once volitional fatigue was reached (Lotgering et al. 1991). An initial analysis comparing the overweight (BMI = 25.0–29.9 kgm–2; n = 47) and obese women (BMI ‡ 30.0 kgm–2; n = 59) found that, with the exception of BMI, there were no significant differences in subject characteristics, regression equations, or heart rate zones (results not reported). Therefore, all subjects were combined for all subsequent analyses (n = 106). The 106 women were randomly assigned into 1 of 2 groups (Mottola et al. 2006): the first to develop the equations (n = 80) and the second to cross-validate the equation (n = 26). Statistical analysis Statistical analysis included subject characteristics (mean ± SD) and Mann–Whitney U tests to measure the relationship between the equation and cross-validation groups. VO2 and HR data were collected as an average of the last 30 s of rest, of each stage, and the 30 s prior to peak, and were used to construct linear regression equations. %HRreserve, %VO2 peak, and %VO2 reserve were determined from the last 30 s of each 2 min stage (Fig. 1) using the following equations: HRreserve ¼
HRworkload HRrest 100 ðHRpeak HRrest Þ
(Karvonen et al. 1957) %VO2
peak
¼
VO2 workload 100 VO2 peak
(Lounana et al. 2007) %VO2
reserve
¼
ðVO2 workload VO2 rest Þ 100 ðVO2 peak VO2 rest Þ
(Lounana et al. 2007) Two linear regression equations and a coefficient of deter-
mination (R2) were performed for each participant in the exercise and cross-validation groups using %HRreserve vs. %VO2 peak and %HRreserve vs. %VO2 reserve. The regression equations and correlations for %HRreserve vs. %VO2 peak and %HRreserve vs. %VO2 reserve were then pooled (equation: n = 80; cross-validation: n = 26) to obtain mean group values (±SD) for each equation. Non-parametric Mann–Whitney U tests were used to determine if the mean intercepts and slopes differed from 0 and 1 (line of identity: y = x), respectively, to determine if %HRreserve and %VO2 reserve /%VO2 peak are equivalent. A non-parametric Mann–Whitney U test and an independent samples t test were used to determine significant differences between equation and cross-validation groups for each of the pooled regression equations. Using the %HRreserve vs. %VO2 reserve regression equations described above, the percentage HRreserve corresponding to 20%, 30%, 40%, 50%, 60%, 70%, and 80% VO2 reserve was determined and compared with ACSM values (%HRreserve = %VO2 reserve). Significance was accepted at p < 0.05. THR zones equivalent to light-intensity exercise (20%–39%VO2 reserve) were determined using the equation of Karvonen (1957): HR ¼ ½ð220 age HRrest Þ%HRreserve þHRrest where the %HRreserve is equivalent to 20%–39%VO2 reserve from the %HRreserve vs. %VO2 reserve regression equation (Table 1). All analyses were performed using SPSS software (version 13).
Results Subject characteristics are presented in Table 2. There were no significant differences between the equation and cross-validation groups for the dependent variables (p > 0.05; Table 2). The regression equation for %HRreserve vs. %VO2 peak was significantly different from the line of identity (slope = 1; intercept = 0) (Table 3). In contrast, the slope of the regression line for %HRreserve vs. %VO2 reserve was not significantly different from the slope of the line of identity (slope = 1). However, the intercept was significantly different from zero. The coefficient of determination (R2) explains the proportion of variability in the dataset explained #
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15..3±8.3
Table 3. Intercepts, slopes, and R2 of linear regression analyses for percent heart rate reserve (%HRreserve) predicted from percent aerobic capacity reserve (%VO2 reserve) and percent peak aerobic capacity (%VO2 peak).
22.2±3.9
%HRreserve vs. %VO2 peak %HRreserve vs. %VO2 reserve
Slope 1.259b±0.088
R2 0.604
–7.561a±5.915
1.046±0.073
0.741
Differs significantly from 0 (p < 0.01). Differs significantly from 1 (p < 0.01).
b
3.76±0.79
by the model. Analysis of the %HRreserve vs. %VO2 reserve found that R2 = 0.741. The regression equation of %HRreserve vs. %VO2 peak found that R2 = 0.604. When the two equations were used in the cross-validation group, %HRreserve vs. %VO2 reserve was not significantly different between the equation and cross-validation group (p = 0.65); however, %HRreserve vs. %VO2 peak was significantly different between the equation and cross-validation group (p < 0.01). The results of the prediction analysis for %HRreserve from %VO2 reserve are presented in Table 1. The values were calculated from the mean of the regression lines. Values of %HRreserve from 20% to 70%VO2 reserve were significantly lower than the ACSM guideline that %HRreserve is equivalent to %VO2 reserve (Table 1). However, %VO2 reserve was not significantly different than %HRreserve above 70%HRreserve. Age specific THR zones equivalent to 20%– 39%VO2 reserve (13%–33%HRreserve) were developed from the equation of Karvonen (1957) (where resting heart rate = 88 beatsmin–1; age = 25 for 20–29 and 35 for 30–39 years of age; Table 2). The heart rate zones we developed were 102–124 beatsmin–1 (20–29 years of age) and 101– 120 beatsmin–1 (30–39 years of age).
169±13 89±10 31.9±5.1
Discussion
Note: Values are means ± SD; VO2, aerobic capacity.
83.7±14.2 30.6±4.1
Equation group (n = 80) Cross-validation group (n = 26)
Intercept –28.795a±7.268
a
1.62±0.07
Resting HR (beatsmin–1) 88±9 Pre-pregnancy BMI (kgm–2) 31.7±4.2 Height (m) 1.67±0.07 Pre-pregnancy mass (kg) 88.3±13.0 Age (y) 31.4±4.3
Table 2. Subject characteristics for overweight and obese pregnant women at 16–20 weeks gestation.
Peak HR (beatsmin–1) 166±12
VO2 rest (mLkg–1min–1) 3.71±0.81
VO2 peak (mLkg–1min–1) 21.6±3.8
Time to peak (min) 13..3±+4.6
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The results of the present study indicate that %HRreserve was best described by %VO2 reserve and that validated THR zones equivalent to 20%–39%VO2 reserve for overweight and obese pregnant women are 102–124 beatsmin–1 (20–29 years of age) and 101–120 beatsmin–1 (30–39 years of age) in previously sedentary, medically pre-screened, overweight and obese pregnant women. The PARmed-X for pregnancy was developed and validated for normal-weight pregnant women (Wolfe and Mottola 2000; Wolfe and Mottola 2002). This document recommends that pregnant women exercise at 60%–80% of VO2 max. This is equivalent to 135–150 beatsmin–1 and 130– 145 beatsmin–1 for women 20–29 and 30–39 years of age, respectively. Mottola et al. (2006) further developed these THR zones based on age and fitness level. They indicated that unfit pregnant women exercising at 60%–80%VO2 peak would have heart rate zones of 129–144 beatsmin–1 and 128–144 beatsmin–1 for 20–29 and 30–39 years of age, respectively. Since the exercise intensities prescribed by the PARmed-X (Wolfe and Mottola 2002) may be too high for previously sedentary overweight and obese pregnant women and would likely result in non-compliance, the lower THR zones developed in the present study should #
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be prescribed to this population. The ACSM currently recommends that since such a large portion of the population is sedentary, exercise prescription should be based on the lowest level of physical activity that could provide health benefits (American College of Sports Medicine 2005). They recommend that women who were sedentary prior to pregnancy should begin with an exercise intensity equivalent to 20%–39%VO2 reserve (American College of Sports Medicine 2005). This corresponds to our THR zones of 102–124 beatsmin–1 and 101–120 beatsmin–1 for overweight and obese pregnant women 20–29 and 30–39 years of age, respectively. Our THR zones have been validated and agree with the exercise intensity recommended by ACSM for previously sedentary pregnant women. This is the first study to provide validated THR zones for lower exercise intensities based on age for overweight and obese pregnant women. We found that in obese and overweight pregnant women, %HRreserve and %VO2 reserve were not identical as suggested by ACSM (American College of Sports Medicine 2005). In the present study, %VO2 reserve was higher than %HRreserve. These results concur with a previous study that examined the relationship between %HRpeak /%HRreserve and %VO2 reserve in obese men and women (Byrne and Hills 2002). This may indicate that if overweight and obese pregnant women follow the ACSM guidelines for non-pregnant individuals, they will be exercising at a higher intensity (%VO2) than intended for a given heart rate. The results of the present study indicate that %HRreserve is best described by %VO2 reserve. This is in agreement with several other studies (Brawner et al. 2002; Byrne and Hills 2002; Lounana et al. 2007; Swain and Leutholtz 1997; Swain et al. 1998). Swain and Leutholtz (1997) were the first to demonstrate that the regression line between %HRreserve and %VO2 reserve was not different from the line of identity in healthy adults using a cycle ergometer. When the protocol was repeated on a treadmill using healthy adults (Swain et al. 1998), the slope of the %HRreserve vs. %VO2 reserve regression equation was not significantly different from the line of identity while the y intercept was significantly lower than 0. Our results in overweight and obese pregnant women correspond with those of Swain et al. (1998) — the slope of %HRreserve vs. %VO2 reserve was not significantly different from the line of identity, but the y intercept was significantly lower. Since HRreserve and VO2 reserve are significantly different from each other below 70%HRreserve, %HRreserve should be used for exercise prescription at lower intensities. Therefore, in our pregnant population, we cannot use the ACSM guideline (Swain et al. 1998) that %HRreserve and %VO2 reserve are equivalent. Heart rate is elevated at rest during pregnancy and may account for the slight but consistent difference between %HRreserve and %VO2 reserve along a range of intensities. Although the %HRreserve was not equal to %VO2 reserve below 70%, the relationship is still stronger than that with %VO2 peak. In light of these findings, exercise prescriptions for overweight and obese pregnant women can be described using %VO2 reserve as a proxy for %HRreserve above 70%, but not below. The results of the present study indicate that %HRreserve is best described by %VO2 reserve; however, they are not equivalent. The THR zones we developed for exercise prescrip-
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tion in previously sedentary overweight and obese pregnant women are 102–124 beatsmin–1 (20–29 years of age) and 101–120 beatsmin–1 (30–39 years of age). This represents 20%–39% of VO2 reserve, which is the lower threshold of a training stimulus. In practical terms, this is similar to a walking speed of approximately 3 milesh–1 with no grade. These results are based on previously sedentary overweight and obese women with low fitness levels and may not apply to a similar population of women with higher levels of fitness (Mottola et al. 2006). It is our recommendation that previously sedentary overweight and obese pregnant women who are medically prescreened can safely exercise using our validated THR zones.
Acknowlegements The authors wish to thank the following funding sources: the Molly Towell Perinatal Research Foundation, the Canadian Institute of Health Research–Institute of Aboriginal People’s Health, and the Government of Ontario Graduate Scholarship Award.
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