Introduction
The energy expenditure (EE) during physical activities is an important aspect of exercise prescription. It has been suggested that health benefits induced by regular exercise are to a great extent related to the total amount of energy spent. In other words, the volume of exercise would be more important for health than its mode, intensity, or duration (25 1
The effectiveness of resistance training within exercise programs aiming at health promotion has been recognized (3 31 32 6,40 4,12 22,26,33
Considering that acute metabolic responses to resistance training have important ramifications to induce changes in body composition and muscular endurance, it is necessary to quantify these responses during resistance training. In this context, some previous studies have investigated the isolated influence of specific training variables on the EE, as the muscle mass involved (15,30 8,11,23,40 10,26 13,21 13,16,23,37 16 9
The length of recovery intervals in resistance training is acknowledged as a variable influencing the performance and fatigue levels along multiple-set training sessions. If the recovery between sets is shortened, the relative intensity of a sequence of exercises will probably increase (19 2 8-12 2
In a few words, the manipulation of the intervals between sets and exercises would eventually influence the intensity and total work, being related to early fatigue, overall training volume and EE. However, the impact of rest intervals (RIs) on the accumulated EE during resistance exercises is not fully understood. We were able to find only 2 studies that have specifically investigated this issue (11,28 28 11 2 within the exercise session, albeit increasing the O2 after the exercise, at least in the first few minutes of recovery. Therefore, the manipulation of RIs between sets would influence the O2 kinetics in resistance training. Unfortunately, both applied exercise protocols are poorly related to actual training practice. The recruited muscle mass and exercise intensity may have influenced the magnitude of oxygen consumption (O2 ), particularly in the postexercise period. Moreover, the O2 assessment was interrupted before returning to baseline values, albeit the evidences indicating that the contribution of resistance training to increase the EE would rather rely on the excess postexercise oxygen consumption (EPOC) than on the exercise session itself (8-11,17,22,29
The possible influence of RIs on the work volume and EE during resistance training may also depend on the amount of muscle mass involved in the exercise session (18 2,18 2 22,33
A better understanding about the influence of the RIs on the O2 during resistance exercises performed with multiple-set protocols and different muscle mass could provide practical implications for maximizing the EE and avoid early fatigue in resistance training programs, with evident repercussion on the total amount of work and expected adaptations. The length of RIs could affect the accumulated fatigue and exercise efficiency throughout the sessions, influencing either the training volume or total caloric expenditure. This information may help design resistance training routines, especially in the context of weight management programs.
Thus, the purpose of this study was to observe the effect of different RIs (1 vs. 3 minutes) on the O2 during and after resistance exercise protocols performed with multiple sets and engaging large and small-muscle mass (leg press [LP] vs. chest fly [CF]). We hypothesized that the influence of RIs on the respiratory responses and accumulated fatigue would be different in the LP and CF exercise protocols. It has been also hypothesized that the RIs would significantly affect the relationship between the O2 assessed within the exercises and postexercise recovery but not the total O2 and EE.
Methods
Experimental Approach to the Problem
To investigate the effect of different RIs between sets on the EE estimated from oxygen uptake in 2 resistance exercises, data were assessed on 6 nonconsecutive days. A within-group study design was used. On the first day, the body surface area (m2 ) and body composition were assessed, and a 24-hour dietary recall was applied to control for the possible influence on the RMR. The workload corresponding to 15 repetition maximum (RM) in the selected exercises was also determined. All these measurements were repeated in a second visit, after 48–72 hours, to test their reliability.
From the third to sixth days, each subject accomplished 4 exercise sessions, separated by 48 hours in a counterbalanced design. Half of the subjects first performed the horizontal LP (larger muscle mass), whereas the other half initiated the experimental protocol by the CF (smaller muscle mass). Within each exercise, the RIs were also randomized through the counterbalanced design (either 1 or 3 minutes).
The warm-up consisted of 12 repetitions of the selected exercise with 30% of 15RM. Five minutes after the warm-up period, all subjects performed the LP and CF protocols in 2 blocks of 2 sessions (5 sets of 10 repetitions with the 15RM workloads, concentric and eccentric phases lasting nearly 1 second). The 15RM workload was adopted to allow the subjects to perform 5 sets of 10 repetitions in each exercise. A pilot study revealed that with a higher workload many subjects did not succeed in performing the established repetitions throughout all sets. Moreover, the load corresponding to 15RM seems to meet the American College of Sports Medicine (ACSM) recommendations (3
Subjects
Ten healthy men with at least 1 year of experience with the proposed exercises were selected for the present investigation (26 ± 3 years; 179 ± 6 cm; 78 ± 7 kg; resistance training experience = 1.6 ± 0.4 years; 15RM LP = 69.8 ± 9.0 kg; 15RM CF machine = 30.2 ± 4.3 kg). The experimental protocol was approved by Institutional Ethical Committee and each subject signed a written informed consent before participation in the study. The following additional exclusion criteria were adopted: (a) use of drugs that could affect the cardiorespiratory responses and (b) bone, joint, or muscle problems that could limit the performance in the selected exercises.
Procedures
Fifteen Repetition Maximum Testing
The 15RM load was determined for the horizontal LP and CF machine. A warm-up set of 12 repetitions was performed using 30% of the perceived 15RM. After a 5-minute RI, 3–4 maximal trials were performed to determine the 15RM with 5-minute intervals between trials and 30-minute between exercises. A complete range of motion and proper technique were required for each successful 15RM trial. After an interval of 48–72 hours, the 15RM tests were repeated to assess the reliability of the workload values. The intraclass correlation coefficient and standard error of estimation were calculated for the LP and CF.
Exercise Selection
The choice of a protocol with 5 sets of 10 repetitions performed with the load corresponding to 15RM in the LP and CF must be justified. First of all, in most cases, resistance training sessions are composed of exercises for both lower and upper limbs. The LP and CF are exercises frequently included in training programs for novice or intermediate individuals and do not significantly depend on previous experience to be correctly performed. Therefore, the comparison between the exercises could be done with little influence of potentially confounding factors such as the mechanical efficiency during their performance.
Secondly, real weight training programs hardly include exercises performed with maximum repetitions in the context of weight management. Instead, this kind of program is usually designed with submaximal workload and repetitions, because the subjects typically lack of experience in resistance training (2
Anthropometric Measures, 24-hour Diet Recall, and Resting Metabolic Rate Assessment
The body surface area (m2 ) was determined using a standard protocol (7 14 36 −1 (24 41
The RMR was assessed according to the detailed recommendations proposed by Compher (5 2 was assessed during 40 minutes with a low flow pneumotachometer (2–30 L·min−1 ). The average of data obtained in the last 10 minutes was recorded as corresponding to the RMR.
Breath-by-breath oxygen uptake (O2 ) and ventilation (VE) were measured using a O2000 analyzer (Medical Graphics™, Saint Louis, MO, USA). Gas analyzers were calibrated with a certified standard mixture of oxygen at 17.01% and carbon dioxide at 5.00%, balanced with nitrogen. The flows and volumes of the pneumotachograph were calibrated with a syringe graduated for a 3-L capacity (Hans Rudolph™, Kansas, MO, USA). All measurements were made at the same time of the day, between 9:00 and 11:00 AM, in a temperature-controlled environment (20–22°C), relative humidity between 60 and 70%, and barometric pressure around 760 mm Hg. After an interval of 48–72 hours, all the procedures were repeated to determine the RMR reliability. The intraclass correlation coefficient and standard error of estimation were calculated (Table 1 ).
Table 1: Subjects' characteristics and reliability of 15RM and resting respiratory assessments (N = 10).*
O2 Assessment during and after the Exercise Sequences
Upon arrival at the laboratory, the O2 mask and equipment were put on the subject after they were positioned to perform the selected exercise and before performing the standardized warm-up. The laboratory conditions and criteria for the RMR assessment were reproduced for the O2 assessment during the exercise sequences, including calibration of the equipment and standardized diet before each protocol. Before the exercise protocols, the subjects remained in the supine position for at least 15 minutes or until the respiratory exchange ratio (RER) was ≤0.82.
Subsequently, each subject warmed up as previously described, and after another 5 minutes, the exercise session began. The O2 was assessed using a medium flow pneumotachometer (10–120 L·min−1 ). In a previous study, the reproducibility of the O2 assessed by a similar protocol has been tested in a group of 6 subjects performing the bench press (at the end of 3 sets performed with 10RM and after 20 minutes of postexercise recovery). The intraclass correlation ranged from 0.78 (within sets values) to 0.90 (20 minutes EPOC) (p < 0.05) (9
After the end of the exercise, the O2 was continuously measured, including the transition period between the exercise and recovery. Upon completion of the exercise sequences, subjects remained in the supine position, and the EPOC was recorded for 90 minutes. The same procedures adopted for RMR assessment were applied to measure the O2 during the postexercise recovery. However, it was critical to change the pneumotachometer (medium to low flow) to accurately measure the O2 . The change was made between the fifth and tenth minutes of recovery. A period of approximately 3 minutes was required to reprogram the software, recalibrate the system, and restart the measure, discarding the first minute of the restart (that is, the ninth minute). During the first 5 minutes of recovery, the O2 was assessed every minute to describe the kinetics of the EPOC fast component. From the 10th minute on, the O2 was assessed every 10 minutes. Therefore, the postexercise O2 data were obtained immediately after the last set of a given exercise and at 1, 2, 3, 4, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90 minutes postexercise.
Individual breath-by-breath data points for the O2 (ml·kg−1 ·min−1 ) were averaged for the entire set and for each minute of the RIs. The time corresponding to the initiation of each set, the time to complete each set, and the RI between sets were recorded. The total O2 (L) for a given exercise was calculated by summing up the values obtained during the sets and RIs.
Calculations (O2 net and Energy Expenditure)
The absolute O2 (L·min−1 ) and RER during each exercise, set, recovery intervals, and during the 90-minute postexercise recovery were measured and used to calculate the total O2 in all exercise protocols. The total O2 for a given protocol was defined as the overall oxygen consumption during the exercise performance, recovery intervals, and 90-minute EPOC. Within session (for each exercise and RI) and EPOC, EE were estimated based on the net O2 defined as the resting O2 (RMR) subtracted from the total O2 . The energy cost in kilocalorie estimation was based on a caloric equivalent of 5.05 kcal·L−1 , according to the method proposed by Wilmore et al. (40
Statistical Analyses
The Levene (0.82, p = 0.60) and Shapiro–Wilk (0.96, p = 0.78) tests were applied to confirm normal data distribution. Standard statistical methods were used to calculate means and SD s. Differences in total O2 , EPOC, and EE were tested by 2-way analysis of variance (ANOVA) with repeated measures (exercises × RI). The O2 during the sessions was compared by 3-way ANOVA with repeated measures (exercises, RI, and sets). The duration of EPOC during recovery was also determined by an ANOVA for repeated measures. In all cases, Fisher's post hoc tests were used to determine pairwise differences when significant F ratios were obtained. Cohen's d effect sizes for the differences between means were calculated. A probability level of p ≤ 0.05 was adopted for statistical significance. The same statistical software was used for all calculations (Statistica™ 6.0, Statsoft, OK, USA).
Results
Table 1 presents the characteristics of the subjects in relation to anthropometric variables, 24-hour dietary recall, 15RM workloads, and resting respiratory data. The reliability results for the 15RM and respiratory assessments are also exhibited.
As expected, all subjects performed 10 repetitions in every set of each exercise protocol, which confirmed that the same load volume was maintained in each set of the experimental situations (CF = 302 ± 43 kg × reps and LP = 698 ± 90 kg × reps). Additionally, the duration of the concentric and eccentric phases was also controlled (nearly 1 second each) to avoid the possible influence of the speed of movement on muscle tension and EE. No significant differences (p = 0.83) were found for the time to perform the 10 repetitions in each set of both exercises (range from 19 ± 3 to 21 ± 3 seconds).
Because both relative and absolute O2 changed identically, only relative O2 data are presented. Data for O2 during the exercise performance and recovery (EPOC) are presented respectively in Figure 1 and Table 2 . The peak O2 associated with a given set always occurred in the first few seconds of the subsequent RI. In general, the O2 increased throughout the successive sets in all exercise protocols, which was mainly detected during the RIs (p < 0.001). There were significant differences between the exercises (LP1 vs. CF1, p < 0.001, effect sizes ranging from 0.18 to 0.33; LP3 vs. CF3, p < 0.001, effect sizes ranging from 0.21 to 0.34). There were also differences associated with the RIs in the LP protocols (LP1 vs. LP3, p < 0.003, effect sizes ranging from 0.32 to 0.60), but not in the CF protocols (CF1 vs. CF3, p > 0.79, effect sizes ranging from 1.06 to 1.69). Hence, the O2 within the exercise sessions was influenced by both muscle mass (greater O2 for the LP) and RIs at least for the LP (greater O2 for the 1-minute RI).
Table 2: Relative
O
2 throughout 90 minutes of postexercise recovery (EPOC) (
N = 10).*†
Figure 1: Mean values and analysis of variance results for the oxygen uptake (
O
2 ) at rest and during exercise sequences. S1–S5: first to fifth sets; I1–I4: first to fourth rest intervals; Int 1 min: 1-minute rest interval; Int 3 min: 3-minute rest interval. *Significant difference between exercises within a given rest interval (1–3 minutes); †Significant difference between rest intervals within a given exercise (leg press or chest fly); ‡Significant difference between RMR and all sets and intervals. The bars represent the confidence limits for 95%.
The O2 during the first minutes of recovery was influenced by the peak O2 attained in the exercise sessions. The EPOC was higher after the LP compared to CF regardless of the RIs until the fourth minute of recovery (p < 0.001 and effect sizes <0.21). The comparison between RIs within a given exercise showed significant difference just for the LP at the first minute of recovery (p = 0.002 and effect size = 0.24). The O2 remained significantly higher compared to the baseline value up to 20 minutes for CF3 (p < 0.03, effect sizes from 0.22 to 0.41) and up to 40 minutes for CF1 (effect sizes from 0.34 to 0.49), LP1 (effect sizes from 0.39 to 0.52), and LP3 (effect sizes from 0.29 to 0.50; p < 0.0001) during postexercise recovery. It is noteworthy to mention that in all cases the relative O2 remained higher than RMR for approximately 60 minutes, though such differences were not statistically significant.
The EPOC magnitude (or EPOC net) was calculated by subtracting the O2 during recovery from RMR, considering as end point the moment in which there was no significant difference between the O2 and RMR (LP1 = 7.36 ± 1.10 L at 40-minute recovery; LP3 = 5.80 ± 1.00 L at 40-minute recovery; CF1 = 4.73 ± 0.99 L at 40-minute recovery; CF3 = 3.77 ± 0.93 L at 20-minute recovery). A significant effect of the exercise on the EPOC net was found (p = 0.003, effect sizes from 0.31 to 0.46), but the same was not observed for the RIs (p = 0.06, effect sizes from 0.61 to 0.83) (Table 2 ). As mentioned, with the exception of CF3, the EPOC duration was quite similar across all exercise protocols. In every situation, approximately 45% of the EPOC net was obtained until the fifth minute of recovery phase, whereas another 55% corresponded to the measurements made between the 10th and 90th minutes.
The results for the accumulated O2 within the exercise sessions, RIs and total, are presented in Table 3 . The O2 during the exercise sequences and EPOC were generally higher for the LP compared to CF (except CF3 vs. LP3 during EPOC), regardless of the RIs (p < 0.001, effect sizes from 0.16 to 0.31). The total O2 net (within exercise sessions + total EPOC − RMR) was not affected by the RIs (p = 0.52, effect sizes from 0.44 to 0.78), but it was always higher in the LP compared to CF (p < 0.001, effect sizes from 0.19 to 0.37).
Table 3: O
2 and energy expenditure during exercise, postexercise recovery, and total (
n = 10).*†
Discussion
The present investigation aimed to describe the O2 during and after resistance exercises performed with large and small-muscle mass and different RIs between sets. Evidently, any generalization of our results must consider the specific load range and training volume used in the exercise protocols. The major findings were (a) the O2 increased along the consecutive sets in both exercises. The LP induced about twice the peak O2 compared to CF, regardless of the RIs; (b) the exercises performed with 3-minute RIs were related to higher overall O2 , but in general the 1-minute RIs induced higher EPOC; (c) the total EE estimated from the net O2 was mainly influenced by the type of exercise, being higher for the LP regardless of the RIs (>1.5 times the total EE compared to CF); (d) the EPOC net was influenced by the exercise type but not by the RIs. In fact, differences because of the intervals occurred just for the LP at the first minute of recovery; (e) the EPOC duration was similar for most exercise protocols (about 40 minutes). The exception was the EPOC after the CF performed with 3-minute RIs, which lasted approximately 20 minutes.
There are several difficulties in comparing the respiratory responses associated with resistance training. Many potential confounding factors may influence the results, as the subjects' physical activity level or dietary behavior (20 Table 1 ) were coherent with current recommendations (24,41
The O2 assessment is another issue to consider, including the calibration procedures of the metabolic gas analyzer and type of pneumotachometer used during exercise and recovery. This study seems to be the first to adjust the pneumotachometer to the actual air flow during the exercise and recovery. In fact, no previous studies considered this aspect into account when assessing the O2 during resistance exercise protocols. The EPOC assessment period should also be standardized to allow the comparison across studies. Prior studies varied considerably in regard to the time reported for EPOC, with duration ranging from 5 minutes (27 11 26 9,11,13,26,28 2 was measured during 90 minutes after the exercise and the end of the EPOC was defined as the moment in which the O2 returned to the RMR values.
It has been shown that the O2 increase within multiple sets of resistance exercises would be better assessed during the RIs than during the exercise itself. Indeed previous studies have shown that the O2 is expected to rise during the first minute after the end of a set (9,28,33 Table 3 ). The fact that O2 differences between exercises and sets were detected only when the RIs were accounted for is a critical finding in a practical research perspective.
The factors that most contribute to the EE and fatigue during aerobic activity are duration and intensity. Unfortunately, it is not possible to quantify the effect of duration alone in a multiple-set resistance exercise session. As previously demonstrated (9,28 28 2 during exercise and 30 minutes of postexercise recovery in a group of 8 trained men who performed 10 randomized protocols (5 sets of 10 and 5 repetitions of the bench press performed with 75 or 85% of 1RM) using several different RIs (30 seconds, 1, 2, 3, 5 minutes). It has been shown that the O2 was progressively higher throughout the sets as the RI was shortened, mainly because of accumulated fatigue. A continuum of responses was reported such that acute O2 was highest with short RIs.
In other words, shortening the RIs restrains the recovery and therefore enhances the fatigue in the subsequent sets. Previous research indicated that both high training intensity and short recovery intervals (<1 minute) result in early muscle fatigue during resistance training (17,39 2 would increase as a result of hypoxemia or ventilatory compensation in the presence of metabolic acidosis (38 2 was obtained after the last set of the LP performed with 1-minute RIs (19.3 ± 7.6 ml·kg−1 ·min−1 ). The O2 range within the exercise protocols was 7.3–19.2 ml·kg−1 ·min−1 , slightly higher than the range reported by Ratamess et al. (28 −1 ·min−1 ) and somewhat lower than the results reported by Mazzetti et al. (21 −1 ·min−1 ). However, in the CF (smaller muscle mass), the effect of the RIs on the O2 was less evident. No significant difference was found in the comparison between protocols performed with different RIs. Therefore, at least within the range of the number of repetitions and sets performed in our study, it is feasible to assume that the interval between sets is more important to prevent early fatigue when exercising larger than smaller muscle groups. Abbreviating the RIs had a significant impact in the accumulated O2 along the sets performed in the LP but not along the CF. To some extent, these results contradict the largely accepted premise that large-muscle group exercises would be less susceptible to accumulated fatigue than small-muscle exercises (35 2 was significantly affected in the LP when the interval was shortened, whereas no impact was detected in the CF. Future studies should better address this question.
Resistance exercises have been shown to elicit an increase in O2 during substantial time following training sessions (10,28,34 2 . Haltom et al. (11 28 2 within the exercise protocol was higher for the longer intervals. The combination of 85% 1RM with 30-second RI produced the highest EPOC (55.9 kcal) in comparison with longer RIs (2 minutes: 41.3 kcal; 5 minutes: 46.7 kcal; p < 0.05). The same pattern was observed for the bench press performed with 75% 1RM (30 seconds: 51.8 kcal; 3 minutes: 45.8 kcal; 5 minutes: 44.5 kcal; p < 0.05).
In this study, there was no difference for the total EPOC and net EE when comparing most of the exercise situations. A significant difference was found only for the EPOC assessed after CF1 and LP1 (p = 0.009). Hence, our results did not confirm the hypothesis that shorter RIs would enhance the EPOC and increase the postexercise EE, because this variable did not have significant influence on the EPOC magnitude. The main determinant factor for the EPOC was undoubtedly the type of exercise. The EPOC assessed after the LP was almost twice the value obtained after the CF.
Given that dieters do not really care if they lose weight during or after the exercise bout, the most important issue for the practitioner would be to identify the determinant factors of the total O2 and EE because of resistance training. There is evidence indicating that exercise volume is a major determinant of the total oxygen uptake and EE during resistance exercise sessions. Collectively, the available studies have shown that the total O2 would be more elevated in protocols with high exercise volume compared to with low exercise volume (intensity × number of repetitions × number of sets). Other variables such as RIs, speed of movement, exercise order, or session format (9,10,13,16,27,28 2 . Concisely, a similar O2 during resistance exercise sessions is expected to occur when the total work is similar (20,37).
In this context, it has been demonstrated that the EE during resistance exercise sessions is proportional to the recruited muscle mass (15,30,33 2 net in the LP protocols was always higher in comparison with the CF. Exercising larger muscle groups increased the total work and the metabolic responses. On the other hand, the RIs were not determinant of the overall energy cost (exercise + intervals + EPOC) in both LP and CF protocols (Table 3 ). Such information may be especially useful in the context of weight management programs.
In conclusion, the O2 during and after resistance training sessions of similar workload and number of repetitions was influenced by the exercised muscle mass and between-set RIs. The O2 in the LP was always higher compared to that in the CF. The RIs influenced the metabolic strain throughout the sets−the shorter RI yielded higher accumulated O2 in the LP, whereas no significant difference was found between the CF protocols. Hence, the idea that smaller muscle mass would be more susceptible to early fatigue when shortening the RIs was not confirmed.
The O2 in the first minute of the EPOC tended to be higher after the protocols being performed with shorter intervals, but the total EPOC was not influenced by this variable. The EPOC magnitude was somewhat higher for the LP, but neither muscle mass nor RIs affected the EPOC duration. No significant effect on the total O2 could be accounted for the RIs. Therefore, the total O2 and EE relied substantially on the type of exercise rather than on the RIs.
Practical Applications
The present results suggest that the RIs between sets have little influence on the accumulated fatigue throughout resistance exercise sessions when small-muscle groups are recruited. On the other hand, there would be a closer relationship between the duration of RIs and accumulated fatigue within multiple sets of exercises engaging larger muscle groups, albeit this is probably not an important issue when the EE enhancing is the main target. Indeed, the RIs do not seem to influence the total EE when resistance training sessions of a similar volume are performed. A given volume of larger muscle group exercises would induce greater EE compared to smaller muscle group exercises because of a higher EPOC magnitude regardless of the RI duration. Hence, in the context of weight management programs, it would be preferable to novice and intermediate individuals to include exercises with larger muscle mass (to increase EE) and longer between-set RIs (to avoid early fatigue and increase training volume). The same concern with the RIs would not be necessary when exercising small-muscle groups.
Additionally, shortening the RIs decreases the O2 within sets and increases the O2 in the initial minutes of the postexercise recovery. Consequently, through an applied research perspective, the shorter the RIs, the higher the need to assess the O2 during recovery to correctly estimate the energy cost associated with resistance training sessions.
Acknowledgments
This study was partially supported by the Carlos Chagas Filho Foundation for the Research Support in Rio de Janeiro (FAPERJ, process E-26/150.751/2007) and by the Brazilian Council for the Research Development (CNPq, process 305729/2006-3). We would like to thank the group of subjects for their participation.
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