Effect of resistance training volume on heart rate variability in young adults

Abstract

BACKGROUND:

The volume in resistance training (RT) perhaps improve the autonomic modulation cardiac in untrained adults.

OBJECTIVE:

The aim was to analyze the effect of RT volume on heart rate variability (HRV) in young adults.

METHODS:

The intervention order was randomized and counterbalanced. Participants ( $n=$ 27) performed 1, 3 or 5 sets of the same exercises with equalized intensity (loading zones) and rested for eight weeks following eight weeks of washout between each experimental condition (1 vs. 3 vs. 5 sets). The researchers assessed HRV by cardiac monitoring seventy-two hours, both before (pre) and after (post) each experimental RT condition (1 vs. 3 vs. 5 sets). Factorial repeated measures ANOVA 2 $\times$ 3 were used to analyze the interaction between time (pre vs. post) and intervention (1 set vs. 3 sets vs. 5 sets) for the HRV index (RMSSD, SDNN, and pNN50).

RESULTS:

An interaction was identified between time and condition for RMSSD ( $F_{(5,22)}=$ 37.02, $p<$ 0.01), SDNN ( $F_{(5,22)}=$ 32.80, $p<$ 0.01), and pNN50 ( $F_{(5,22)}=$ 29.92, $p<$ 0.02). Five set conditions ( $p=$ 0.01) showed improvement in HRV indicators when compared to one set and three set conditions.

CONCLUSION:

The study concluded that 5 set conditions improved HRV in young untrained adults.

1. Introduction

Heart rate variability (HRV) refers to consecutive variations in heart rate [1]. It provides information concerning sympathetic and parasympathetic heart autonomic modulation. Noteworthy, increased HRV is inversely associated with cardiovascular diseases [2]. Moreover, HRV is considered an indicator of cardiovascular fitness, that is, it is possible to evaluate how well-trained the subject is [3]. Moreover, HRV distinguishes the competitive levels of professional athletes [3] and indicates enhanced neuromuscular recovery between training sessions [4].

Exercise modalities that increase HRV long term are important to indicate the performance levels of the athletes as well as prevent from cardiovascular disease. Resistance training (RT), a type of physical exercise, uses resistance generated by equipment, free weights, body mass, and/or elastic bands against the movement of specific muscle groups [5]. The main variables of RT are volume (frequency $\times$ sets $\times$ number of repetitions), intensity (percentage of maximum voluntary strength or zone of repetitions maximum), and rest between sets, exercises, and training sessions [6, 7, 8]. Thereby, it is crucial to understand the effect of the manipulation of RT variables on HRV in healthy adults.

In fact, scientific literature is controversial about the effects of RT intensity on HRV. A study showed HRV improvement with high-intensity RT (8RM) in healthy male adults [9], whereas some other studies do not indicate any difference independently of what intensity (60 to 80% with one repetition maximum) is adopted in healthy male adults [10, 11]. According to rest intervals, two minutes or more between sets and exercises presented the best results [9], indicating that before the next stimulus the sympathovagal system might need to return baseline or next to baseline levels to reach out long-term improvements. Considering the RT volume, HRV changes were not observed after six [12] or eight weeks [13] in the studies that adopted three sets or less of RT, with eight to twelve repetitions. However, the complexity of the prior studies designs that lacks of washouts periods might have compromised the results, additionally, there is not any evidence regarding the effect of greater RT volume ( $>$ 3 sets per resistance exercise) on HRV.

In fact, past studies have suggested that RT volume is the most important RT variable for neuromuscular adaptations [14, 15]. Evidence showed RT volume positively affected strength [16] and muscular hypertrophy [14, 15]. Chen et al. [17] demonstrated reductions in strength performance following a decrease in HRV index in seven weightlifters following a RT session. Strength and HRV remained reduced until 72 h after training, that is, HRV might have an influence in strength and consequently in performance. Considering the fact that RT volume induces improvement in muscular strength [14], it is possible that RT volume could also increase HRV. Thereby, assuming that higher RT volume induces greater autonomic cardiovascular stress (higher reductions in HRV) after a single RT session [18], it is reasonable to postulate that higher RT volume might stimulate superior adaptations on HRV in healthy male adults. To confirm this hypothesis, investigations should be performed to determine the influence of RT volume on HRV.

From a practical point of view, the present study might reveal the effect of RT volume on HRV in young untrained adults. Once the previous studies have revealed inconsistence of showing similar results, a strong and controlled design as a long-term crossover might assist to understand the effect of RT volume on HRV. Wheter the findings of this study reveal a positive effect of the higher RT volume on HRV indicators, both strength and conditioning professionals might consider the increment of greater RT volume for healthy untrained adults given the prospect of increasing HRV might indicate an improvement of the performance and reduce cardiovascular disease risks. Therefore, the findings could be extremely important for strength and conditioning professionals. Thus, the study aimed to analyse RT volume effect on HRV in young untrained adults. The study hypothesized that a high RT volume during eight weeks improves HRV.

2. Materials and methods

2.1 Participants

The study recruited participants using a non- probabilistic sampling method. A total of thirty-one male volunteers aged from 18 to 30 years old (23.5 $\pm$ 4.7 years old; 78.4 $\pm$ 9.3 kg, 1.76 $\pm$ 0.08 m, 19.1 $\pm$ 7.7 % body fat) were recruited. The sample size of 31, alpha of 5%, and effect size of 0.5 produced statistical power greater than 90% according the software G*Power 3.1.9.2. The participants were recruited by word of mouth and brochures around the university. As inclusion criteria, the participants should not have performed RT for at least two years, present no history of any muscular or joint injury, present no history of smoking, and should not have ingested any ergogenic substance for strength, and muscle mass that could alter HRV in the last 6 months. The participants that either failed to complete the training program of the present study or engaged in another physical exercise program were excluded. Four participants dropped out of the study because personal reasons right before starting the investigation.

Figure 1.

Experimental design of investigation. Note. HRV $=$ heart rate variability; RM $=$ repetition maximum; DXA $=$ dual-energy x-ray absorptiometry.

The procedures of this study were approved by the Institutional Review Board at the Federal University of Pernambuco (59787716.9.0000.5208) in compliance with the Brazilian National Research Ethics System Guidelines and Helsinki Declaration. Written informed consent was obtained from each participant before they participated in the study.

2.2 Experimental design

This is a controlled, randomized, and cross-over investigation that lasted forty-weeks (http://dx.doi.org10. 17504/protocols.io.skdecs6). The participants were male young untrained adults to test the effect of long-term RT in combination with different volumes on HRV. This study was conducted with healthy untrained participants to investigate the effects of RT on HRV without pathological influences. They underwent a forty-week intervention that was divided into three RT segments (8 weeks each) and two washouts (8 weeks each) according to Fig. 1. The study randomly allocated participants into one of the three experimental conditions. After the eight-week training period, the participants were submitted to an eight-week washout period before the next condition started. This procedure was repeated until all three conditions were performed by all the participants. The intervention order was randomized and counterbalanced. A simple randomization directed the three experimental conditions (1, 3, or 5 sets). The random number table was generated on the www.randomizer.org site.

The participants performed 1, 3, or 5 sets of the same exercises (bench press, leg press 45 ${}^{\circ }$ , seated row, leg curl) with equalized intensity (loading zones) and rest (Table 1). It is important to note that the researchers were not blind to the experimental conditions.

Table 1

Resistance training program

Week	1 set	3 sets	5 sets
Note. RM $=$ repetition maximum.
1 to 8	1 set of 10RM;	3 sets of 10RM;	5 sets of 10RM;
	180 s between sets and exercises	180 s between sets and exercises	180 s between sets and exercises

One repetition maximum (1RM) test in the bench press and leg press (45 ${}^{\circ }$ ) determined the maximum muscular strength. The ten-repetition maximum (10-RM) test was performed to define the training intensity [8, 9] throughout the study, and if necessary the load was adjusted to maintain 10-RM intensity.

HRV was assessed by cardiac monitoring seventy-two hours before (pre-conditions) and after (post-conditions) each experimental RT condition (1 vs. 3 vs. 5 sets). Moreover, the participants were instructed to not perform any physical exercise forty-eight hours before the HRV baseline and the condition evaluations.

2.3 Resistance training program

For the training program, the study followed recommendations for RT in healthy untrained adults [5]. The program consisted of four popular exercises in RT programs (bench press, leg press 45 ${}^{\circ }$ , seated row, leg curl). Those exercises were performed three times per week (Monday, Wednesday, and Friday) during an eight-week period for each experimental approach (1 vs. 3 vs. 5 sets). All the sessions were performed during the afternoon (4 p.m.) to avoid circadian rhythm effect and each one lasted between twenty to sixty minutes, depending on the protocol. Experienced RT researchers, supervised all sessions. It is important to highlight that during the eight weeks of washout the participants were monitored via social network and cell phone callings. They were weekly advised to not engage in any physical exercise.

Table 1 shows the RT procedure for each condition (1 vs. 3 vs. 5 sets). Warm-ups [1 $\times$ 20–25 reps with 50% of predicted 10RM and 1 $\times$ 10–15 reps with 80% of predicted 10RM, there were three minute intervals between sets] were performed for the first two exercises (bench press and leg press 45 ${}^{\circ }$ ) and before each RT session [6, 8]. The training intensity was equalized and participants should perform 10-RM in each set, thereby, weekly adjustments were made if necessary (increase/decrease of 2–5% and 5–10% for upper and lower limbs, respectively) as recommended by Ribeiro et al. [8]. That is, the participant should perform 10-RM throughout sessions, in cases the participant performed more than ten repetitions within two subsequently sessions, the load (kg) was increased in the following week, whereas whether the participant was unable to perform 10-RM, the load was decreased in the following week. The training attendance was above 85% for all the participants in each experimental approach (1 vs. 3 vs. 5 sets). Moreover, it is noteworthy to point out that during the washout period the participants did not perform RT.

2.4 Measures

2.4.1 Heart rate variability

All the evaluations were performed under the same conditions. The participants remained in the sitting position for ten minutes before initiating the resting HRV [1]. The R-R intervals were obtained using a portable heart rate monitor (Polar ${}^{\mathrm{\circledR }}$ RS800cx, Kempele, Finland) using a sampling at 1,000 Hz, uninterruptedly for five minutes [1] in a room with a temperature of 24 ${}^{\circ }$ C. Data were visually inspected to identify ectopic beats ( $<$ 2%) that were removed [a long interval, followed by a short interval, while the two points either side were unaffected (1.1%); a short interval, followed by a long interval, while the two points on both sides were unaffected (0.8%); the R-R interval(s) were entirely missed, detectable (1.5%)], as indicated by the scientific literature [19]. The R-R values were transferred to the computer via Polar Software (Polar ${}^{\mathrm{\circledR }}$ ProTrainer, Kempele, Finland) and exported to HRV time-domain analyses using Kubios v2 Software (Polar ${}^{\mathrm{\circledR }}$ Kubios v2, Kuopio, Finland). The analyzed variables were: the standard deviation of all NN intervals (SDNN), the successive percentage of R-R interval differences greater than 50 ms (pNN50), and the difference of the quadratic mean of the successive R-R normal intervals (RMSSD). These variables are most commonly adopted in scientific studies with physical exercise [4, 9, 10, 17, 18]. The SDNN and RMSSD values were presented in milliseconds (ms).

2.4.2 Maximum muscular strength (1RM)

Maximum muscular strength was determined using the 1RM test in the bench press and leg press 45 ${}^{\circ }$ (the most popular exercises in RT’s program 1RM test [6, 7, 8, 9]). First, familiarization of the techniques was used to reduce motor learning influences before the baseline test [7]. Next, using the adopted protocol, the participants were tested under similar conditions and in three distinct sessions within forty-eight hour intervals in baseline and before each experimental condition. The participants were allowed three attempts for each exercise with five-minute intervals between exercises and sets [8]. The intraclass correlation coefficient and standard error of measurement between familiarization and 1RM tests were 0.99 and 4.1 kg for bench press and 0.99 and 6.8 kg for leg press 45 ${}^{\circ }$ , respectively.

Additionally, warm-ups (1 $\times$ 10–15 reps with 50% of predicted 1RM and 1 $\times$ 5–8 reps with 80% of predicted 1RM, with three-minute intervals between sets) for each exercise (bench press and leg press 45 ${}^{\circ }$ ) were performed before each muscular strength assessment [6, 8]. Moreover, verbal encouragement was used throughout the test.

2.4.3 10 repetitions maximum (10RM)

After the 10RM test, the researchers determined the intensity zone for 10RM. The exercises performed were bench press, leg press 45 ${}^{\circ }$ , seated row, and leg curl. All the participants performed the 10RM test in two distinct sections within forty-eight-hour intervals in baseline and each experimental condition. For each exercise, there was two attempts with ten-minute intervals between sets and exercises. Intraclass correlation coefficient and standard error of the measurement between familiarization and 10RM tests were 0.98 and 2.7 kg for bench press, 0.99 and 4.0 kg for leg press 45 ${}^{\circ }$ , 0.97 and 3.6 kg for seated row, and 0.98 and 2.4 kg for leg curl, respectively.

Accordingly, warm-ups (2 $\times$ 15–20 reps with 50% of predicted 1RM, adopting 120-second intervals between sets) for each exercise occurred before performing the 10RM test. Verbal encouragement was given throughout the 10RM test.

2.4.4 Body composition

Body mass (kg – portable scale PL 200, Filizola S.A., Sao Paulo, Brazil, an accuracy of 0.1 kg) and height (professional stadiometer Sanny, Sao Paulo, Brazil, an accuracy of 0.1 cm) were measured. Corporal density was measured using the technique of body scanning by the Dual X-ray Absorptiometry (DXA) (Hologic, Waltham, MA, USA). Participants were recommended to not perform any physical exercise for at least forty-eight hours and to add libitum hydration the day before. The participants remained in the supine position with arms beside the body and hands in neutral position. Feet and knees 10 cm away tied with a Velcro band for avoiding any movement that might interfere with the image visualization during the procedure. The analyzed variables were: free fat mass, fat mass, and body mass. DXA calibration followed the manufacturer recommendations as well as the measurements were performed for an experienced and blind for the experiment evaluator.

2.5 Data analysis

The researchers conducted the Shapiro-Wilk test to analyze the data distribution. Levene’s test was used to assess the homoscedascity of the groups. All the data are described as the mean and standard deviation. Factorial repeated measures ANOVA 2 $\times$ 3 was used for analyzing the interaction between time (pre vs. post) and intervention (1 set vs. 3 sets vs. 5 sets) for the HRV index (RMSSD, SDNN, and pNN50) and maximum muscular strength. Bonferroni’s Post hoc was used when necessary to identify statistical differences. The RMSSD and SDNN were converted into logarithm (InRMSSD and InSDNN) to avoid outliers and to simplify the analyses. Moreover, effect size (ES) was calculated to reveal practical differences. According to Rhea [20], the following criteria were adopted: ES $<$ 0.50 $=$ trivial, 0.50 $⩽$ ES $<$ 1.25 $=$ small effect size, 1.25 $⩽$ ES $<$ 1.9 $=$ moderate effect size, and ES $⩾$ 2.0 $=$ large effect size. All data were analyzed using the software SPSS 21.0, alpha level adopted was 5%.

3. Results

In our study, 27 out of 31 participants finished the forty-week experiment (Fig. 2). Four participants dropped out of the study because personal reasons before the start of the investigation. The descriptive data of age, HRV indexes (RMSSD, SDNN, pNN50), maximum muscular strength, and body mass are in Table 2.

Table 2

Descriptive values (mean and standard deviation) of research variables in baseline

Variables	Baseline
	Mean	SD
Age (years)	23.5	4.7
RMSSD (ms)	32.7	8.1
SDNN (ms)	49.6	18.2
pNN50 (%)	17.2	4.9
1RM bench press (kg)	94.7	19.1
1RM leg press 45 ${}^{\circ }$ (kg)	203.0	33.7
Fat free mass (kg)	62.8	8.0
Fat mass (kg)	15.7	7.3
Body mass (kg)	78.4	9.3

Note. RM $=$ repetition maximum; SD $=$ standard deviation.

Table 3

Mean and standard deviation of HRV (RMSSD, SDNN and pNN50) and maximum muscular strength according to intervention (1 vs. 3 vs. 5 sets) and time (pre-vs. post)

Variables	1 set ( $n=$ 27)		3 sets ( $n=$ 27)		5 sets ( $n=$ 27)		Effects	$F$	$p$
RMSSD (ms)
Pre	32.8	$\pm$ 8.1	32.5	$\pm$ 8.7	33.7	$\pm$ 10.7	Group	28.64	0.01
Post	31.3	$\pm$ 9.9	34.1	$\pm$ 10.5	37.2	$\pm$ 9.8 ${}^{*,\text{a},\text{b}}$	Time	30.10	0.01
$\mathrm{\Delta }$ %	$-$ 4.7	$\pm$ 1.5	4.9	$\pm$ 3.7	10.4	$\pm$ 4.2	GxT	37.02	0.01
SDNN (ms)
Pre	50.1	$\pm$ 18.2	49.0	$\pm$ 10.2	48.9	$\pm$ 11.5	Group	26.38	0.02
Post	48.3	$\pm$ 19.5	51.1	$\pm$ 20.4	54.8	$\pm$ 17.0 ${}^{*,\text{a},\text{b}}$	Time	29.55	0.01
$\mathrm{\Delta }$ %	$-$ 3.7	$\pm$ 1.4	4.2	$\pm$ 1.1	12.1	$\pm$ 4.5	GxT	32.80	0.01
pNN50 (%)
Pre	18.5	$\pm$ 5.8	17.9	$\pm$ 5.5	18.3	$\pm$ 5.6	Group	23.93	0.03
Post	16.4	$\pm$ 5.2	17.8	$\pm$ 4.6	19.1	$\pm$ 4.2 ${}^{*,\text{a},\text{b}}$	Time	25.41	0.02
$\mathrm{\Delta }$ %	$-$ 12.8	$\pm$ 1.3	$-$ 40.6	$\pm$ 0.2	4.3	$\pm$ 1.9	GxT	29.92	0.02
1RM BP (kg)
Pre	95.1	$\pm$ 20.4	94.1	$\pm$ 21.2	94.1	$\pm$ 19.4	Group	61.7	0.01
Post	93.1	$\pm$ 18.7	98.2	$\pm$ 20.0 ${}^{*,\text{a}}$	101.3	$\pm$ 17.4 ${}^{*,\text{a},\text{b}}$	Time	59.3	0.01
$\mathrm{\Delta }$ %	$-$ 2.1	$\pm$ 0.9	4.4	$\pm$ 2.2	7.7	$\pm$ 4.0	GxT	68.4	0.01
1RM LP (kg)
Pre	203.3	$\pm$ 41,3	204.1	$\pm$ 42.1	201.5	$\pm$ 42.0	Group	50.9	0.02
Post	205.9	$\pm$ 37.8	210.0	$\pm$ 35.1 ${}^{*,\text{a}}$	218.5	$\pm$ 28.4 ${}^{*,\text{a},\text{b}}$	Time	54.2	0.01
$\mathrm{\Delta }$ %	1.3	$\pm$ 0.4	2.9	$\pm$ 1.7	8.4	$\pm$ 2.8	GxT	60.3	0.01

Note. HRV $=$ heart rate variability; 1RM $=$ one repetition maximum; BP $=$ bench press; LP $=$ leg press; $\mathrm{\Delta }$ % $=$ percentage change; GxT $=$ group vs. time interaction; ${}^{*}p<$ 0.05 vs. pre-intervention; ${}^{\text{a}}p<$ 0.05 different to “1 set”; ${}^{\text{b}}p<$ 0.05 different to “3 sets”.

Figure 2.

Flowchart of evaluated participants.

3.1 Maximum muscular strength

According to maximum muscular strength, results did not show differences for bench press ( $F_{(4,23)}=$ 1.72, $p=$ 0.49) and leg press 45 ${}^{\circ }$ ( $F_{(4,23)}=$ 2.05, $p=$ 0.37) when comparing baseline with pre-condition measurements (1, 3, and 5 sets). This indicate that the washout period (8-week) was enough for return the maximum muscular strength to baseline value.

The findings indicated interaction between time and intervention for bench press ( $F_{(5,22)}=$ 68.39, $p<$ 0.01) (Table 3). The 5 sets condition presented better performance when compared to 3 ( $p=$ 0.03; ES $=$ 0.6) and 1 set ( $p=$ 0.01; ES $=$ 1.6). Also, 3 sets condition showed better results when compared to 1 set ( $p=$ 0.01; ES $=$ 1.1). Likewise, results demonstrated an interaction between time and intervention for leg press 45 ${}^{\circ }$ ( $F_{(5,22)}=$ 60.3, $p<$ 0.01) (Table 3). The 5 sets condition showed superior values when compared to 3 sets ( $p=$ 0.02; ES $=$ 0.7) and 1 set ( $p=$ 0.01; ES $=$ 1.8), as well as 3 sets when compared to 1 set ( $p=$ 0.01; ES $=$ 1.3).

3.2 HRV

The results did not show differences for RMSSD ( $F_{(4,23)}=$ 2.51, $p=$ 0.32), SDNN ( $F_{(4,23)}=$ 2.19, $p=$ 0.39) and pNN50 ( $F_{(4,23)}=$ 1.87, $p=$ 0.46) when comparing baseline with pre-condition measurements (1, 3, and 5 sets). This indicate that the washout period (8-week) was enough for return the HRV indicators to baseline values.

The analyses showed an interaction between time and intervention for RMSSD ( $F_{(5,22)}=$ 37.02, $p<$ 0.01). There was an observable improvement for the 5 sets condition ( $p=$ 0.01; ES $=$ 0.9) when compared to 1 and 3 sets (Table 3). Also, it revealed an interaction between time and intervention for SDNN ( $F_{(5,22)}=$ 32.80, $p<$ 0.01). The 5 sets condition ( $p=$ 0.01; ES $=$ 0.8) showed superior values when compared to 1 and 3 sets (Table 3). Similarly, the study revealed an interaction between time and intervention for pNN50 ( $F_{(5,22)}=$ 29.92, $p<$ 0.02). The 5 sets condition ( $p=$ 0.01; ES $=$ 0.5) presented better values when contrasted with 1 and 3 sets (Table 3).

4. Discussion

From a practical point of view, the present study points out the effect of RT volume on HRV in young untrained adults. Once the findings of this study reveal a positive effect of the higher RT volume on HRV indicators, strength and conditioning professionals could prescribe greater RT volume for healthy untrained adults increasing the HRV, consequently, improving performance as well as reducing the risk of cardiovascular disease. The main finding of this study was that the 5 sets condition yielded greater improvement in HRV indicators when compared to 3 and 1 sets conditions in healthy untrained male adults. This seems to be the first study showing the effect of different RT volumes on HRV.

According to maximum muscular strength, the results revealed a dose-response effect for RT volume. 5 sets condition yielded greater improvement in maximum muscular strength when compared to 3 and 1 sets. Similarly, 3 sets condition produced an enhancement in maximum muscular strength when compared to 1 set. Those results corroborate a systematic review with a meta-analysis conducted by Grgic et al. [16], that showed an increase in RT volume is associated with an enhancement of muscular strength.

The results of the present study revealed improvements on HRV (increase on HRV indicators in the time domain) for 5 sets condition when compared to 1 and 3 sets. RT volume positively affects the autonomic nervous system, although there appears to be a volume threshold for HRV improvement in untrained adults, since 3 and 1 sets conditions did not change HRV indicators. An increase in plasma volume and in baroreflex sensitivity, as well as lower metaboreflex activation [11], may help to explain the present results in the 5 sets experimental condition; however, it should be further investigated. Although the mechanisms underlying these effects were not assessed, previous studies showed that higher RT volume reduces catabolic hormones (cortisol) and increases anti-inflammatory cytokines (interleucyine-6 and interleucybe-10) [21], which are associated with HRV enhancement. The reduction in cortisol’s concentration shows a relation to the decreased adrenaline and norepinephrine, which reduce the activation of the sympathetic nervous system, causing improvement of resting HRV [22]. The increase in the concentration of the anti-inflammatory cytokines seems to increase the vagus nerve activity, which also improves resting HRV [8]. Unfortunately, the study did not measure blood sample and therefore could not confirm this hypothesis. At the level of the cardiac structure, RT induces left ventricle hypertrophy. Thereby, the ventricle eccentric hypertrophy, which seems common in participants of the RT program, is characterized by an increase in ventricle wall thickness that does not reduce the size of the chamber. Moreover, the increase in the left ventricle thickness relates to the reduction of systolic blood pressure [23]. Thus, once the blood pressure is controlled, it might indicate a better autonomic nervous system functioning and an improvement in HRV which could explain the findings of the 5 sets experimental condition.

The findings of this study on HRV contradict other scientific investigations [10, 11]. Collectively, these studies suggest that RT might not improve resting HRV in healthy young adults because they already have a normal cardiac autonomic function. However, it is important to highlight that these studies [10, 11] adopted RT volume equal to or lower than 3 sets per exercise. Iglesias-Soler et al. [24] compared an RT protocol that led to muscular failure with another one with rests between repetitions. However, they did not find differences in the HRV. This similarity to the findings of the present investigation for 1 and 3 sets conditions might be due to the reduced RT volume (10 to 30 repetitions to each exercise).

The results of this study for a higher volume training session (i.e., 5 sets) do not support previous findings that indicated that RT induces a reduction in cardiac vagal modulation [11, 25]. However, differences in protocols deserve to be noted. The respective studies [11, 25] analyzed the effect of RT on HRV after a single session. Additionally, a common characteristic of those investigations [10, 11, 25] was to adopt rest between sets below three minutes. The previous studies analyzed the effect of intensity on HRV, whereas the present study focused on the effect of volume on HRV. Thereby, the differences in protocols might explain dissimilarity in the findings. On the other hand, Fortes et al. [9], despite adopting a lower RT volume than the present study (3 sets per exercise), revealed an HRV improvement after eight weeks of RT in male healthy adults.

Although the present study reveals interesting results that might elucidate some questions in the literature, it presents some limitations that ought to be mentioned. In fact, an echocardiogram was not used for assessing images of the heart which could explain improvements of the heart in the 5 sets condition. Also, inflammatory and hormone markers were not analyzed [26]. Therefore, the present study’s findings must be handled with caution. However, this study used the repeated measures crossover design with washout intervals, that might suppress some cited limitations.

5. Conclusion

The results showed that higher RT volume (5 sets condition) improvement on HRV in young untrained adults. Performing 600RM weekly (200RM 3 times a week, with 5 sets of 10RM to each exercise), adopting large muscle groups (bench press, leg press 45 ${}^{\circ }$ , seated row, and leg curl), was enough to increase 6 to 12% of the HRV after 8 weeks of RT.

Nevertheless, it is noteworthy that 3 (360RM weekly) and 1 sets (120RM weekly) produced similar findings (without changes to HRV). Thereby, practically, if the aim is to improve HRV in young healthy untrained adults, the best strategy seems to perform 5 sets per exercise in each RT session within an eight-week period.

In summary, RT volume improves cardiac autonomic modulation and strength in healthy untrained adults. Therefore, a higher RT volume should be considered to enhance the above-mentioned parameters in this population.