Pregnancy has important anatomical, metabolic, and cardiorespiratory adaptations, resulting in physiological changes that are very similar to the cardiovascular and respiratory adaptations, which occur during exercise (19). These adaptations may develop into a circulatory stress, which will continue throughout pregnancy (19).
Published guidelines for prescription of exercise during pregnancy (1,2,7,9,24,28) recommend resistance exercise using 1–3 sets, with 10–15 repetitions and relative load ranging from 50 to 70% of 1 repetition maximum (RM). This recommendation is very conservative because the literature does not report the acute responses of resistance exercise during pregnancy, and little is known about the cardiorespiratory responses of this kind of exercise for pregnant women.
It is understood that the practice of resistance exercise during pregnancy can be better tolerated compared with aerobic exercise because of lower cardiovascular stress and heat production (5). However, there is a lack of knowledge on the effects of maternal acute cardiorespiratory variables during the execution of strength exercises, mainly because few exercises and strategies of the training sessions were studied. The cardiorespiratory responses to various forms of exercise engaging the upper and lower body are different and understanding the responses of these variables during the execution of resistance exercise is of fundamental importance, as they are modified by pregnancy.
This study aims to determine the cardiorespiratory responses of pregnant and nonpregnant women while performing resistance exercise for the upper and lower body at 2 different volumes.
Experimental Approach to the Problem
This experiment used an acute set of resistance testing protocols to determine the cardiovascular responses of women based on two experimental groupings (i.e., pregnant or non-pregnant). Four different resistance exercise protocols were examined on different days. Subjects performed each of the workouts.
Twenty women, 10 pregnant and 10 nonpregnant volunteered from Porto Alegre, Rio Grande do Sul, Brazil.
The inclusion criteria of the pregnant group included: age over 18 years, gestational age between 22nd and 24th weeks and confirmed by ultrasound, not practicing regular exercise in the last 3 months, without orthopedic limitations or medical constraints (consented by a physician) (1), without medication intake that could alter the cardiorespiratory variables and medical authorization for exercise. Gestational age (between 22nd and 24th weeks) was selected because almost all hormonal, metabolic, and structure modifications occur at this time. Pregnancy risk also is attenuated in the second trimester. Nonpregnant women were individually paired to each pregnant woman, by age and prepregnancy body mass index. The physical characteristics of both groups are shown in Table 1. The experimental design was previously approved by the Ethics Committee in Research of the Federal University of Rio Grande do Sul, Brazil (2008185) and by the Ethics Committee in Research of the Municipal Health Department of Porto Alegre (log 449; process 001.000397.10.5). All participants were informed about the study purpose and protocol and signed a written informed consent.
The tests consisted of performing 2 resistance exercises preceded by a rest period. Such exercise protocols were selected by respecting the guidelines and recommendations published in relation to resistance training in pregnancy (1,2,9,24,28). The exercises chosen for this study were pec-deck fly for the upper body and bilateral leg extension for the lower body, both representing major muscle mass (4). All participants were aware that they could not perform other exercises during the period in which data collection occurred, and they were advised to eat between 3 and 4 hours before arrival at the test site without the ingestion of stimulants.
Body height and weight were measured using a stadiometer and a scale with a resolution of 0.5 cm and 100 g, respectively (Filizola, São Paulo, Brazil). Uterine height was measured with an anthropometric tape to 0.1 mm (Gulick model; Mabbis, Parana, Brazil) by fixing the tape from the middle of the upper margin of the pubic symphysis to the height of the fundus of the uterus.
Estimated Maximal Dynamic Strength
The estimated maximum dynamic strength for lower and upper limbs was evaluated by 1 estimated maximum repetition test (1RM) using a bilateral leg extension and a pec-deck fly exercise machine, respectively (Taurus, Rio Grande do Sul, Brazil) with a randomized order. The muscles mainly activated during bilateral leg extension performance are the quadriceps femoris and during pec-deck fly performance are the pectoralis major and anterior deltoid muscles.
The execution stages were established as the initial position and concentric phase. The eccentric phase was the return to initial position. Women were seated with a supported back seat for both the bilateral leg extension and pec-deck exercises. When performing the pec-deck fly exercise, the women also had foot hold support.
Subjects were familiarized with the procedures, followed by a specific warm-up (10 repetitions with a low load) and by a 1RM test. The 1RM test was estimated from the highest possible number of repetitions in the resistance exercise, up to a maximum of 10 repetitions. Each subject's maximal load (maximum of 10 repetitions) was determined with no more than 5 attempts with a 5-minute recovery between each attempt and 40-minute recovery between exercise to avoid the influence of fatigue in the 1RM values. Performance time for each contraction (concentric and eccentric) was 2 seconds, controlled by an electronic metronome (Korg, New York, New York, USA). Apnea was avoided.
After this procedure, the load was scaled using the coefficients proposed by Lombardi (15) to estimate the corresponding 1RM value. From the values of 1RM for each exercise, a value corresponding to 50% of this load was calculated (4), and this value was used to perform 15 repetitions of the following stage. Heart rate (HR), systolic (SBP), and diastolic (DBP) blood pressure (BP) were monitored before and after the execution of the exercises (frequencymeter model F6TM; Polar, Kempele, Finland; BP monitor ABPM-04 recorder with MAP optical interface, Meditech, Budapest, Hungary, respectively). The mean BP (MBP) was calculated using the formula MBP = DBP + (0.333 × [SBP − DBP]).
Each subject visited the laboratory 4 times on 4 different days, with a minimum interval of 48 hours between them, to complete 4 randomized resistance exercises protocols—experimental session 1: 1 set of 15 repetitions of the pec-deck fly, experimental session 2: 1 set of 15 repetitions of bilateral leg extension, experimental session 3: 3 sets of 15 repetitions of the pec-deck fly, and experimental session 4: 3 sets of 15 repetitions of bilateral leg extension. A passive interval of 2 minutes between sets was allowed and a relative load of 50% of 1RM was used.
To assess the HR, BP, Ventilation (Ve), and oxygen consumption (
; portable gas analyzer
000, MedGraphics, Ann Arbor, MI, USA) at rest, the subjects always started from the same metabolic state and were monitored for 30 minutes, in a recumbent position (45°) with legs extended on a gurney, before performing the exercises. HR and
were collected every 10 seconds and BP every 5 minutes during the entire period of rest. Before each session, the portable gas analyzer was calibrated according to manufacturer's instructions. The exercise was performed after rest, without a warm-up, so that warm-up did not influence the exercise bouts.
The acquisition of
and HR was done every 10 seconds throughout the execution of exercise, and BP was measured as recommended by Polito and Farinatti (25), which indicates the cuff had to be inflated at the 12th repetition of the set so that the measure of BP can be performed immediately after the 15th repetition.
Normal distribution and homogeneity parameters were checked with Shapiro-Wilk and Levene tests, respectively. Results were reported as mean ± SD. Sample characterization between the 2 groups was compared through Student's t-test. The comparison of dependent variables between the situations was assessed using a 2-way analysis of variance (ANOVA) with repeated measures (group × exercise) and 3-way ANOVA with repeated measures (group × exercise × volume) with Bonferroni. Statistical analysis was performed with the Statistical Package for Social Sciences software (version 13.0 for Windows; SPSS Inc., Chicago, IL, USA), and the significance value was set to p ≤ 0.05.
The results of the SBP, DBP, MBP, Ve, HR, and
at rest showed no significant difference within each group among the 4 experimental sessions (Figure 1) demonstrating that the subjects started from the same metabolic and cardiovascular status before performing the exercise protocol. Thus, it can be inferred that the magnitude of the changes found in these variables during the exercise bouts can be attributed to the effort of each group. It was observed that SBP, DBP, MBP,
, and Ve differed between the groups at rest, with the pregnant group showing lower BP values and higher values of
For the analysis of the variables in the exercise situation, comparisons of groups and exercises in the performance of a single set (Figure 2) and in the third set of multiple sets (Figure 3) were made. Table 2 shows the comparison of a single set with the third set of multiple sets for each exercise and group.
The interaction exercise × volume showed significance only for
(p = 0.006). Thus, the main factors exercise and volume were tested again using the F test. The result of the comparison between exercises showed that for only 1 set, there was no significant difference between them for both groups (pregnant women: p = 0.271; nonpregnant women: p = 0.905). For the third set of multiple sets, there were significant differences between exercises for the nonpregnant women (p = 0.020), with the mean values of bilateral leg extension exercise being higher compared with pec-deck fly exercise. This was not observed in the pregnant group (p = 0.112).
Regarding the comparison between exercise volume, a significant difference for both groups in the bilateral leg extension exercise was found (pregnant women: p = 0.035, nonpregnant women: p = 0.007), and in both groups, the third set of multiple sets showed higher
mean values when compared with a single set. No significant differences were observed between the volumes in the pec-deck fly exercise for both pregnant and nonpregnant women (pregnant: p = 0.111; nonpregnant women: p = 0.933). The interactions exercise × group, volume × group, and exercise × volume × group showed no significance for any of the variables.
The purpose of this study was to determine the cardiorespiratory responses of pregnant women while performing resistance exercise for upper and lower body, with single and multiple sets. The protocol design was selected in agreement with the literature, which recommends resistance training for nonpregnant individuals and beginners in this modality, with emphasis on improving muscular endurance, to perform 1–3 sets with 10–15 repetitions and relative load ranging from 50 to 70% of 1RM (13,26), which are similar recommendations for women during pregnancy (9,24).
The BP was reduced in the pregnant group during exercise with single and multiple sets, in agreement with many studies that showed that pregnancy is responsible for a reduction in BP due to decreased peripheral vascular resistance (6,8,12,18,29). This attenuation also occurred during exercise, representing a protective effect of cardiovascular health in the pregnant woman (10). However, this result was contradictory to other studies (3,16), which found no significant differences between pregnant and nonpregnant women in the leg extension exercise. This may be explained by differences in gestational periods studied (second trimester in this study and third trimester in the studies cited) because BP response vary depending by trimester studied (12).
Another factor that may influence this difference between the results is that in the study by Lotgering et al. (16), the authors compared the pregnancy period (29 and 35 weeks) with the postpartum period of 8 weeks. This postpartum period may not reflect the true nonpregnant state, according to the authors' comments. Many pregnancy changes remain for a period of up to 6 months of postpartum.
The oscillometric method selected to measure BP during exercise may also have contributed to this difference between the results, since the literature is quite clear on the difference in the values obtained when the methods and equipment differ, like the oscillometric, auscultatory, and intra arterial methods or even when the measurements are carried out in different parts of the body, like the left and right arm, or in a different body position, like seated or standing, for example (23).
Regarding the cardiorespiratory responses of the groups to exercise, larger values of HR, SBP, DBP, MBP, Ve, and
were evidenced when the resistance exercise was performed on the bilateral leg extension machine for both groups. These results are consistent with those obtained by Seals et al. (27) that studied the influence of active muscle mass on the responses of
, HR, and MBP during isometric contraction in unilateral handgrip and bilateral knees extension exercises (load 30% of maximal voluntary contraction). The authors described a direct relationship between the size of the active muscle mass and the magnitude of the increases in the analyzed variables, although the relative intensity was the same in both exercises.
MacDougall et al. (17) also found higher values of cardiovascular variables during resistance exercise for the lower limb (unilateral leg press), compared with unilateral elbow flexion exercise, in nonpregnant individuals. The authors justify this effect by mechanical compression of vessel walls that can be proportional to the size of the active muscle mass during exercise. McCartney (21) states that the greater muscle mass needs a larger circulatory and metabolic demand to meet the energy requirements for maintenance of physical exertion. The authors also report that there is a positive relationship, but not linear, between the circulatory responses and active muscle mass in lifting the load. For example, HR and BP are greater during bilateral leg press than the unilateral leg press, but not enough to present the duplication of values.
The SBP response showed a higher value with increasing the number of sets performed, and it was influenced by the volume of exercise. This result can be explained by the existence of a cumulative effect as the increase in cardiovascular response to the number of consecutive sets performed. This cumulative cardiovascular response has been observed in other studies (11,14), which indicated significantly higher BP values, as they increased the number of sets. The possible mechanism is that while performing resistance exercise, there is a tension that is generated during the concentric phase of muscular contraction, which compresses the peripheral arterial vessels supplying the muscles being activated, thereby increasing peripheral vascular resistance and reducing muscle perfusion (17,20). The MBP was not influenced by the volume performed in the resistance exercise, perhaps under the influence of unchanged DBP.
This study seems to be the first to show the acute respiratory responses of 2 resistance exercises performed by pregnant women. The results showed the respiratory variables were higher with increasing exercise volume achieved by both groups, supporting the assertion that exercise resulted in an instantaneous increase in energy demand of exercised muscle (20). It is clear from the literature that pulmonary ventilation and
increase during exercise in direct proportion to the metabolic needs of the body. The magnitude of this is related to the number of sets performed during the resistance exercise response and seems to be directly influenced by the interaction of other resistance exercise variables, such as the intensity of exercise, the muscle mass used, and the interval between repetitions (22).
From the results of this study, it can be concluded that resistance exercise performed on the bilateral leg extension and pec-deck fly machines using either single or multiple sets of 15 repetitions, with a relative load of 50% of 1RM, the BP responses were lower in the pregnant group. When the exercises were performed in a single set, only HR showed different responses between exercises, with higher values for the bilateral leg extension exercise. However, when the exercises were performed with multiple sets, the responses of the HR, SBP, DBP, MBP, Ve, and
were different between exercises, with higher values for the bilateral leg extension exercise in both groups.
The results of this study highlight the safety of the cardiorespiratory responses while pregnant women perform resistance exercise. Considering this, a pregnant woman in the second trimester, who is healthy and wishing to perform resistance exercise, can be safely tested in relation to her strength at 1 estimated maximum repetition test by a fitness professional. She can also start a fitness program and perform resistance exercise using pec-deck fly and bilateral leg extension machines, using 1 or 3 sets, with 15 repetitions of a 50% load of 1 estimated maximum repetition and a 2-minute interval between sets with safe maternal cardiorespiratory responses.
For future research, studies examining the resistance exercise effects of minor active muscle mass should be done, as well as studies regarding the cardiorespiratory responses of a workout with at least 10 resistance exercises. Understanding the fetal responses during maternal resistance exercise is also important.
The authors especially thank the CAPES and CNPq government associations for their support of this project and Dr. Michelle F. Mottola for the revision of this article. The authors are also grateful to all the subjects who participated in this research and made this project possible.
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