Strength training (ST) has been recommended as a component of a nonpharmacologic intervention to prevent and treat cardiovascular disorders (1,2). Strength training has been shown to promote acute reductions in postexercise blood pressure, a phenomenon called postexercise hypotension (PEH), which may play a key role in chronic blood pressure and cardiovascular risk reduction (1). A small reduction in blood pressure (i.e., 3 mm Hg) has been shown to reduce the probability of having a stroke and coronary arterial disease in normotensive or hypertensive subjects (1,2). In addition, a few numbers of studies have investigated the physiological pathways of blood pressure control derived from an ST session, and the mechanisms were not clear (7,8,13,14), whereas other studies have focused on the manipulation of methodological variables of ST such as training intensity, training volume, or exercise order (7,8,19).
Previous studies have examined PEH responses after ST sessions performed in different formats such as a circuit approach or a more traditional approach with consecutive sets performed for each exercise (18), different numbers of sets (i.e., 1 vs. 2 sets) in a circuit format (16) or in a set repetition format (8), different rest intervals between sets and exercises (5), and exercises performed in opposite sequences (7). However, few studies to date have compared the effect of different load intensities (percentage of 1 repetition maximum [1RM]) on blood pressure responses and heart rate variability (HRV) after an ST session with strict control of other training variables (i.e., number of sets and exercises, rest interval between sets, exercise order, and total volume load); all of which may potentially influence the postexercise blood pressure response and HRV (3,8,13,16,18,19).
Simão et al. (18) analyzed the effect of 2 different load intensities (6RM to failure vs. 12 repetitions with 50% of a 6RM load) in normotensive trained men and found no significant differences in the duration of the PEH response between experimental sessions, independent of the total volume load, suggesting that either low- or high-intensity loads may induce a PEH response (16). Conversely, Brown et al. (3) found an increase in systolic blood pressure (SBP) and a reduction on diastolic blood pressure (DBP) following an ST protocol performed at different load intensities (i.e., 40% vs. 70% of 1RM loads) and short rest intervals between sets (i.e., 30 seconds). In addition, Niemelä et al. (13) analyzed blood pressure responses after 3 sets of 12 repetitions at 80% of a 1RM or 3 sets of 20 repetitions at 30% of a 1RM in trained men. The results did not show significant differences in blood pressure responses between sessions. Presently, there is no consensus in the literature regarding the effects of ST load intensity on postexercise blood pressure response and HRV. Continued research related to this topic has significant professional value as we attempt to define the most appropriate ST prescription for individuals with, or at risk for, cardiovascular disease.
Heart rate variability is a key cardiovascular variable that has been related to the functionality of the autonomic nervous system and cardiovascular mortality. Decreased HRV after myocardial infarction is a risk factor for mortality (21). Previous studies that analyzed PEH and HRV simultaneously after ST are limited (7,8,14), and the study of this variable can contribute to the physiological interpretation of PEH following an ST protocol (21). In addition, no previous study has analyzed the effect of 3 different ST load intensities with the same rest interval between sets and exercises on blood pressure responses and HRV in trained men.
The current literature provides little information concerning the physiological significance of ST and the impact on blood pressure and HRV during the period after exercise. Continued research related to ST load intensity has important implications for professionals in the prescription of exercise for individuals with, or at risk for, cardiovascular disease. Therefore, the purpose of this study was to compare blood pressure and HRV responses in trained men after ST experimental sessions with loads of 60, 70, or 80% of a 1RM. It was hypothesized that as load intensity increased, a progressively longer hypotensive response would ensue in conjunction with a reduction in HRV.
Experimental Approach to the Problem
The subjects underwent a total of 7 laboratory visits; the initial 4 visits (conducted 72 hours apart) involved assessment of 1RM loads during which each exercise in the following sequence was tested twice: bench press (BP), lat pull-down (LPD), shoulder press (SP), biceps curl (BC), triceps extension (TE), leg press (LP), leg extension (LE), and leg curl (LC). The next 3 visits (conducted 72 hours apart) involved performance of 3 sets of 8–10 repetitions for each exercise in the sequence at 60, 70, or 80% of a 1RM and with 2-minute rest intervals between sets and exercises in a random order. During each experimental session, blood pressure and HRV were assessed after a 10-minute passive rest period on arrival at the laboratory, immediately after each experimental session and then at 10-minute intervals for 60 minutes after each experimental session.
Eleven men with at least 6-month previous ST experience participated in this study (age: 26.1 ± 3.6 years; body mass: 74.1 ± 8.1 kg; height: 172.0 ± 4.0 cm; body mass index: 25 ± 1.96 kg·m−2; %G: 18.3 ± 6.4). Subjects were recruited according to the criteria established by the Seventh Joint National Committee (4) and included the following characteristics: (a) nonsmokers, (b) absence of any kind of metabolic disease, (c) no articular or bone injury, and (d) absence of any medication that could influence the cardiovascular response. The subjects were informed about the study procedures, possible risks, and benefits and signed an informed consent form. This study was approved by the Ethics Committee of the Rio de Janeiro Federal University. Before the start of each experimental session, subjects were instructed not to consume any caffeinated or alcoholic beverage while maintaining their usual activities and eating habits throughout the study period.
One Repetition Maximum Testing
During the first laboratory visit, height and body mass were measured by means of an analogical scale and a stadiometer (Toledo, Brazil) followed by 1RM testing. The 1RM testing began with a warm-up at 50% of an estimated 1RM. The load was then increased to the predicted 1RM for the first attempt. If that attempt was successful, 5 minutes of rest was given after which another 1RM was attempted; this sequence was repeated until the 1RM attempt was unsuccessful or the subject refused to continue; the highest load successfully lifted was recorded as the 1RM value. Testing was repeated 72 hours later to establish reliability (19). The 1RM assessments were divided over a 4-day period. On the first and third days, BP, LE, BC, and LC were tested and retested, and on the second and fourth days, LPD, LP, SP, and TE were tested and retested.
To minimize the error during 1RM tests, the following strategies were adopted: (a) standardized instructions concerning the testing procedure were given to subjects before the test, (b) subjects received standardized instructions on exercise technique, (c) verbal encouragement was provided during the testing procedure, and (d) the mass of all weights and bars used was determined using a precision scale. The 1RM was determined in fewer than 3 attempts with a rest interval of 5 minutes between 1RM attempts and 10 minutes before starting the 1RM assessment for the next exercise. The range of motion of exercises used was similar to that by Simão et al. (19).
Seventy-two hours after the last 1RM testing session, subjects performed the first of 3 experimental sessions in a random order. The 3 experimental protocols involved the same sequence of exercises (BP, LPD, SP, BC, TE, LP, LE, and LC) performed at 60, 70, and 80% of 1RM loads. The BP, SP, and BC were performed using free weights, and the LPD, TE, LP, LE, and LC were performed using resistance machines (Life Fitness, Chicago, IL, USA).
The warm-up procedure for each experimental session consisted of 1 set of 10 repetitions of the BP and LP at 40% of a 1RM. A 2-minute rest interval followed the warm-up before the assigned experimental session. All experimental sessions involved performance of 3 sets for each exercise in the sequence with 2-minute rest intervals between sets and exercises. During all exercise sessions, irrespective of the load intensity, subjects perform 8–10 repetitions per set through a complete range of motion. The loads were held constant for all exercise sets during each experimental session. No pause was permitted between the eccentric and concentric phases of each repetition, and repetition velocity was volitionally controlled. The total number of repetitions for each set of each exercise was recorded to calculate the total volume load performed (load × total number of repetitions performed).
Measures of Heart Rate and Heart Rate Variability
A heart rate (HR) monitor (Polar RS800cx; Kempele, Finland) was used for 10 minutes before and for 60 minutes after each experimental session. Heart rate and HRV data were collected with subjects in a seated position with palms facing up in a quiet room with temperature maintained between 20 and 22° C. Data were recorded and subsequently downloaded to a computer for analysis. The data were digitized in MATLAB (MATLAB version 6.0; Mathworks, MA, USA) and analyzed for time domain and frequency. The spectral analysis in the frequency domain was performed by the Fourier transform algorithm. The HRV parameters were analyzed according to the components of low frequency in normalized units (LF-nu), which provides information about the activity of the sympathetic nervous system; high frequency in normalized units (HF-nu), which provides information about the activity of the parasympathetic nervous system; and the SD of differences between adjacent normal R-R intervals (RMSSD), which provides information about the predominance of sympathetic or parasympathetic nervous system, after Fourier transformation and noise filtering (21).
Arterial Blood Pressure Assessment
Systolic blood pressure, DBP, and mean arterial pressure (MAP) were measured using an automatic oscillometric device (PM50 NIBP/Spo2; CONTEC, USA). The equipment was autocalibrated before each use. During each experimental session, blood pressure was assessed after a 10-minute passive rest period on arrival at the laboratory. The resting blood pressure values were averaged over 2 consecutive measurements with 5 minutes between measurements. Blood pressure was then assessed immediately after each experimental session and then at 10-minute intervals for 60 minutes after each experimental session, resulting in a total of 6 measurements after each experimental session. Measurements were performed from the left arm, consistent with the recommendations of the American Heart Association (2).
Data for all variables were analyzed using the Shapiro-Wilk normality test and homoscedasticity (Bartlett criterion). A one-way analysis of variance was used to compare the following parameters: 1RM test and retest, the total repetitions performed in the 3 experimental sessions, and the resting values of SBP, DBP, MAP, HR, RMSSD, LF-nu, and HF-nu. Subsequently, the resting values and postexercise values were compared within and between sessions using a multivariate analysis of variance with repeated measures. In all cases, Tukey's post hoc comparisons were used to identify statistically significant differences. Additionally, to determine the magnitude of differences, effect size (ES) statistics (the difference between pretest and posttest scores divided by the pretest SD) were calculated for the SBP, DBP, MAP, and RMSSD for all exercise sequences. The scale proposed by Rhea (15) was used to determine the magnitude of the ES. The alpha was set at p ≤ 0.05, and all tests were performed using SPSS software (v. 19.0; GraphPad, IBM Corp., Armonk, NY, USA).
All tested variables followed a normal distribution. The data for the 1RM test and retest were analyzed using intraclass correlation coefficients (BP, r = 0.99; LPD, r = 0.96; SP, r = 0.98; BC, r = 0.94; TE, r = 0.97; LP, r = 0.97; LE, r = 0.90; and LC, r = 0.97) and showed high reliability. The total volume load for the 60% of 1RM experimental session was 12,711.22 ± 974.88 kg; for the 70% of 1RM experimental session was 14,822.34 ± 1,168.65 kg; and for the 80% of 1RM experimental session was 15,594.02 ± 1,647.92 kg. Significant differences were found between the 60% of 1RM experimental session and the 70 and 80% experimental sessions, respectively (p ≤ 0.05). The total volume load and the total number of repetitions performed for each set or each exercise did not significantly differ between 70% of 1RM and 80% of 1RM experimental session; therefore, the work performed during the 70% of 1RM and 80% of 1RM experimental sessions was similar (p = 0.354).
For SBP, there was a significant difference between the 60% of 1RM and 70% of 1RM experimental session at the 20-minute time point postexercise (p ≤ 0.05). Additionally, for the 70% of 1RM experimental session, significant reductions in postexercise SBP were found from 30- to 60-minute postexercise (Figure 1 and Table 1).
For DBP, there was a significant difference between the 60% of 1RM and 70% of 1RM experimental session at the 50-minute time point postexercise (p ≤ 0.05). Additionally, for the 70 and 80% of 1RM experimental session, significant reductions in postexercise DBP were found across all time points after exercise (Figure 2). The ES data showed a large reduction in DBP for the 70 and 80% of 1RM experimental sessions from 10 to 30 minutes postexercise and for the 70% of 1RM experimental session from 40- to 60-minute postexercise (Figure 2 and Table 1).
For MAP, a significant difference was noted between the 60% of 1RM and 80% of 1RM experimental sessions at the 30-minute time point postexercise (p ≤ 0.05). Additionally, for the 70% of 1RM experimental session, significant reductions in postexercise MAP were noted across all time points after exercise (Figure 3). The ES data showed moderate and large reductions in MAP for the 70% of 1RM and 80% of 1RM sessions across all time points after exercise (Table 1).
No significant postexercise differences (vs. baseline values) were found in LF-nu or HF-nu across all time points after exercise. The postexercise RMSSD value was reduced throughout the 60-minute postexercise monitoring period after the 70 and 80% of 1RM experimental sessions, with a significant difference (vs. immediately postexercise) at the 60-minute time point (p ≤ 0.05) (Figure 4). The ES data showed a large reduction in RMSSD at the 10-minute time point postexercise in all sessions, especially for the 70% of 1RM experimental session (Table 1).
The purpose of this study was to compare blood pressure and HRV responses in trained men after ST experimental sessions with loads of 60, 70, or 80% of a 1RM. The key findings of this study were that ST reduced SBP, DBP, and MAP after exercise independently of the exercise intensity, and the duration of the PEH response was significantly related to the load intensity used. All ST protocols showed a PEH response; however, the 70% of 1RM condition elicited a longer response vs. the 60 and 80% of 1RM conditions, respectively. Only after the 70% of 1RM experimental session, did blood pressure values remain significantly below baseline for the entire 60-minute postexercise. Therefore, our hypothesis was partially confirmed in that the 70% of 1RM condition elicited a longer PEH response, but there were no significant differences in HRV between conditions.
In this study, subjects performed 3 different ST experimental sessions consisting of the same exercises, number of sets and repetitions per set, and rest interval between sets and exercises but differed only in the load intensity used during each experimental session. A possible mechanism that may account for the longer PEH response after the 70% of 1RM condition might be that repetition failure did not occur as frequently (compared with the 80% of 1RM condition), given that 8–10 repetitions were performed for all sets. With fewer sets performed to repetition failure, there might have been less predominance of the sympathetic nervous system during and after the 70% of 1RM condition (Figure 4 and Table 1, index RMSSD). The ES data showed large and a moderate reduction of sympathetic nervous system predominance after 70% of 1RM condition, and it do not occur in 80% of 1RM condition (Table 1). Conversely, the 60% of 1RM condition might not have involved a sufficient volume load to stimulate a long-duration PEH response. Thus, it seems from this study that the 70% of 1RM condition involved the right trade-off between less sympathetic nervous system activation (through fewer sets to failure vs. the 80% of 1RM condition) and a greater load volume completed (vs. the 60% of 1RM condition) to stimulate the longest duration reduction in postexercise blood pressure in trained men.
In relation to SBP, DBP, and MAP, this study demonstrated that the load intensity influenced both the magnitude and duration of the response during the 60-minute postexercise. The ES data demonstrated a longer duration of SBP, DBP, and MAP decrease after the 70% of 1RM session vs. the 60 and 80% of 1RM sessions, respectively (Table 1). Previous studies demonstrated a hypotensive response after single bouts of ST in normotensive and hypertensive subjects (8,11,14,18). In this study, a possible explanation for the longer PEH response for the 70% 1RM session might be the load being at the optimal level to promote moderate but not excessive muscular fatigue with 8–10 repetitions per set. Thus, a dose-response seems to be evident when other prescriptive factors are controlled (8,14).
The blood pressure results of this study are partially in agreement with previous studies (18,20). These studies showed greater and longer PEH responses with higher load intensities (i.e., 6RM vs. 12 repetitions with 50% of 6RM and 30 repetitions with 23% of 1RM vs. 16 repetitions with 43% of 1RM), although the latter is not common in ST prescription.
One study to date analyzed the influence of load intensity on blood pressure and HRV (14). Rezk et al. (14) investigated the effect of 2 different load intensities (20 repetitions with 40% of 1RM vs. 10 repetitions with 80% of 1RM) on PEH and HRV in young normotensive untrained men and women. Different from this study, Rezk et al. (14) showed a longer duration PEH response for the 40% of 1RM session. These divergent results might be attributed to different rest intervals between sets and exercises (45 seconds for 40% of 1RM and 90 seconds for 80% of 1RM) and the training status of the sample (i.e., untrained men and women). A recent study found that different rest intervals between sets can influence the PEH response due to variation in training intensity and the longer time of the ST stimulus (5).
In this study, no significant differences were found in HRV between sessions, with an increase in sympathetic modulation and a decrease in parasympathetic modulation after all sessions, irrespective of the load intensity. This absence of significant differences can be attributed to the large variability in responses among subjects (21). In addition, the results for HRV showed that all experimental sessions elicited significant cardiac stress (Figure 4 and Table 1). Lima et al. (12) demonstrated that cardiac sympathetic activation remains higher than resting values following upper-body ST protocol. Their results partially corroborate the results of this study. For example, after all sessions, there was an increase in sympathetic activation and a reduction of parasympathetic activation observed by the RMSSD index. This finding may have an important clinical implication; an increase in sympathetic activation combined with a reduction of parasympathetic activation may increase the risk of cardiovascular events in patients with cardiovascular disease (21).
Conversely, the ES data showed a large increase in sympathetic tonus and a large decrease in parasympathetic tonus as indicated by RMSSD results (Table 1), after the 80% of 1RM session when compared with the 60 and 70% of 1RM sessions, respectively. Sets for the 80% of 1RM session would have been closer to a RM given that the repetitions per set were controlled at 8–10 for each load intensity session. Thus, the 80% of 1RM session may have activated a greater number of motor units with greater activation of the sympathetic nervous system to maintain consistent repetitions on each set (Figure 4 and Table 1) (17). The higher occurrence of repetition failure during the 80% of 1RM session may have prompted a greater reduction in plasma volume, and consequently, a greater reduction in cardiac output and end-systolic volume, which may have led to a greater imbalance in cardiovascular neural control after the session (9,14). Furthermore, the 80% of 1RM condition may have involved greater activation of metaboreceptors, mechanoreceptors, and the arterial baroreflex due to a reduction in blood flow (due to a reduction in plasma volume) to the active muscles. Finally, an increase in peripheral vascular resistance induced by the mechanical occlusion of blood flow promoted by muscular contraction may have resulted in less blood pressure reduction (14).
Beyond the findings presented here, it is important to consider some limitations of this study. Blood pressure can be affected during a supine position after an ST session, because of the enhanced venous return, which stimulates baroreceptors, increases cardiac filling and stroke volume, and reduces HR for the same cardiac output (6). Conversely, the HRV can be affected during a prolonged seated position due to a reduction in venous return and increased baroreflex activity (10). Because of the lack of studies reporting the influence of load intensity on HRV after an ST session and how to perform the 2 measurements together, a seated position was adopted. We could not analyze variables closely related to PEH, such as cardiac output and blood flow. Additionally, because this study tested systolic isolate prehypertensive young subjects, the results presented here may not be applicable to other populations, such as hypertensive individuals who use medication. Further research is needed to examine the PEH response with the manipulation of other ST methodological variables like rest interval between sets, training method, and exercise order.
The findings of this study demonstrated the longest duration of PEH response in normotensive trained men at a load intensity of 70% of 1RM when compared with 60 or 80% of 1RM loads intensities. Therefore, it is possible to assume that ST composed of upper-and lower-body exercises performed at a 70% of 1RM load intensity for a fixed number of repetitions per set (i.e., 8–10) and for multiple sets per exercise may elicit a longer duration of PEH vs. routines using lower or higher intensity and total volume load. In conclusion, strength and conditioning professionals can prescribe exercises at moderate to high intensity (from 60 to 80% of 1RM) if the goal is to acutely reduce blood pressure after training. Consistent PEH may have positive impacts on early phases of high blood pressure development. However, the results of this study are likely applicable only to systolic isolated prehypertensive, trained, male adults, and further research testing other populations is warranted.
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Keywords:Copyright © 2015 by the National Strength & Conditioning Association.
resistance exercises; autonomic control; blood pressure; resistance training