Volume Load Rather Than Resting Interval Influences Muscle Hypertrophy During High-Intensity Resistance Training

Abstract

Longo, AR, Silva-Batista, C, Pedroso, K, de Salles Painelli, V, Lasevicius, T, Schoenfeld, BJ, Aihara, AY, de Almeida Peres, B, Tricoli, V, and Teixeira, EL. Volume load rather than resting interval influences muscle hypertrophy during high-intensity resistance training. J Strength Cond Res 36(6): 1554–1559, 2022—Interset rest interval has been proposed as an important variable for inducing muscle mass and strength increases during resistance training. However, its influence remains unclear, especially when protocols with differing intervals have equalized volume. We aimed to compare the effects of long (LI) vs. short rest interval (SI) on muscle strength (one repetition maximum [1RM]) and quadriceps cross-sectional area (QCSA), with or without equalized volume load (VL). Twenty-eight subjects trained twice a week for 10 weeks. Each subject's leg was allocated to 1 of 4 unilateral knee extension protocols: LI, SI, SI with VL -matched by LI (VLI-SI), and LI with VL-matched by SI (VSI-LI). A 3-minute rest interval was afforded in LI and VSI-LI protocols, while SI and VLI-SI employed a 1-minute interval. All subjects trained with a load corresponding to 80% 1RM. One repetition maximum and QCSA were measured before and after training. All protocols significantly increased 1RM values in post-training (p < 0.0001; LI: 27.6%, effect size [ES] = 0.90; VLI-SI: 31.1%, ES = 1.00; SI: 26.5%, ES = 1.11; and VSI-LI: 31.2%, ES = 1.28), with no significant differences between protocols. Quadriceps cross-sectional area increased significantly for all protocols in post-training (p < 0.0001). However, absolute changes in QCSA were significantly greater in LI and VLI-SI (13.1%, ES: 0.66 and 12.9%, ES: 0.63) than SI and VSI-LI (6.8%, ES: 0.38 and 6.6%, ES: 0.37) (both comparisons, p < 0.05). These data suggest that maintenance of high loads is more important for strength increases, while a greater VL plays a primary role for hypertrophy, regardless of interset rest interval.

Introduction

Resistance training (RT) has been widely applied as a primary strategy for increases in muscle strength and mass. These muscular adaptations are influenced by the correct manipulation of main variables such as training frequency, intensity, volume, and interset rest interval (¹). Regarding the rest interval, long rest intervals (LI, 2–3 minutes) for strength increases and short rest intervals (SI, <1 minute) for muscle hypertrophy have been commonly recommended (^{1,11,12,24,25}). These recommendations are based on the premise that LI would allow for an optimal recovery to maintain high intensity and volume load (VL), while SI would result in a higher systemic elevation of anabolic hormones concentration (mainly growth hormone) related to muscle hypertrophy (^1,10,24).

However, results of longitudinal studies on the topic are conflicting, with some studies showing advantage for LI on strength increases (^13,23) and hypertrophy (^4,8,23), while others failing to demonstrate any difference for strength (^2,4,8,16) and hypertrophy (^2,13,18) between LI and SI. Such discrepancies may be attributed to heterogeneity in the experimental designs. For example, Hill-Hass et al. (¹³) compared rest intervals of 20 and 80 seconds; Fink et al. (⁸) compared 30 seconds vs. 3 minutes; Schoenfeld et al. (²³) compared 60 seconds vs. 3 minutes, and Ahtiainen et al. (²) compared 2 minutes vs. 5 minutes. Moreover, various issues may have confounded some of these studies. Specifically, Fink et al. (⁸) manipulated the intensity of the load in addition to the rest intervals, whereby the shorter rest group trained with lighter loads while the longer rest interval group trained with heavier loads. In addition, the study by Ahtiainen et al. (²) used a crossover design whereby subjects trained with short rest for 3 months and long rest for 3 months without a washout period. Other factors such as age and study duration also differed between many of the trials (^2,8,13,23).

Importantly, studies that observed better results in muscle hypertrophy for LI tended to be performed with repetitions until muscle failure, which is likely to result in a higher VL (^4,8,23). Indeed, increases in muscle hypertrophy resulting from RT have been shown to be closely related to VL (^1,14,20,21). Thus, RT interventions that use similar intensities performed with higher VL with LI could conceivably promote greater increases in hypertrophy. On the other hand, not all studies using LI, leading to a higher VL, have shown additional effects on muscle strength compared with SI (^4,8). This could be related to a training-related specificity, whereby higher RT intensities are known to promote greater muscle strength increases, regardless of the VL or interset rest interval (^{15–17,22,25}).

Considering the influence of RT volume on muscle adaptations, especially muscle hypertrophy, it is essential that different interset rest intervals are compared with both equalized and nonequalized RT volumes to adequately assess the role of the interset rest interval on muscle adaptations. Therefore, the aim of this study was to compare the effects of a LI (3 minutes) vs. a SI (1 minute) interset rest interval, with and without equalized VL, on muscle strength and hypertrophy in response to a 10-week RT program. We hypothesized that (a) greater changes in muscle hypertrophy would be observed with LI due to the associated higher VL, and (b) muscle strength would not be influenced by the different interset rest intervals.

Methods

Experimental Approach to the Problem

This was longitudinal design that compared the effects of 10 weeks of RT with LI (3 minutes) vs. a SI (1 minute) with or without equalized VL, on strength and hypertrophy. After a period of familiarization, the unilateral inclined (45°) leg press maximum dynamic strength test (one repetition maximum [1RM]) was evaluated for both lower extremities. Seventy-two hours after this initial assessment, subjects repeated the 1RM test. One repetition maximum testing was repeated every 72 hours until 1RM outcomes were within 5% of the previous session for each lower limb. All subjects were able to complete 1RM testing within 4 attempts. At least 72 hours after the final 1RM test, the quadriceps cross-sectional area (QCSA) was obtained by magnetic resonance imaging (MRI). Each subject's lower limb was then allocated to 1 of the 4 unilateral training protocols in a randomized, counterbalanced fashion according to 1RM and QCSA values as follows: (a) long rest interval (LI, using a 3-minute interval between sets; N = 14); (b) short rest interval (SI, using a 1-minute interval between sets; N = 14); (c) LI, but performing the same VL of the SI protocol (VSI-LI; N = 14); and (d) SI, but performing the same VL of the LI protocol (VLI-SI; N = 14). Figure 1 describes the randomization process; Table 1 provides baseline data for strength and hypertrophy. As documented with the use of the unilateral model (¹⁶), each limb of a subject is randomized to 1 of 2 treatments that can be applied concurrently, which increases the study's statistical power by reducing the degree of between-subject variability. The average number of sets and repetitions performed per set throughout the training period was controlled, and the VL was calculated as the sum of the training volume (number of sets × number of repetitions × external load) performed throughout the training period.

Figure 1.: Illustration of randomization process in the experimental protocols.

TABLE 1 - Baseline data for strength and hypertrophy.^*†

Groups	1RM (kg)	p	QCSA (cm2)	p
LI (n = 14)	168.6 ± 44.7	>0.05	76.7 ± 14.2	>0.05
VLI-SI (n = 14)	168.2 ± 45.2	>0.05	76.5 ± 14.7	>0.05
SI (n = 14)	171.1 ± 35.9	>0.05	77.2 ± 13.3	>0.05
VSI-LI (n = 14)	170.4 ± 35.6	>0.05	75.8 ± 12.9	>0.05

^*1RM = one repetition maximum; QCSA = quadriceps cross-sectional area.

^†Long rest interval (LI, using a 3-minute interval between sets); short rest interval (SI, using a 1-minute interval between sets); LI, but performing the same VL of the SI protocol (VSI-LI); SI, but performing the same VL of the LI protocol (VLI-SI). Corresponding p-values between limbs.

Resistance training was performed for 10 weeks twice per week. The 1RM was reassessed 72 hours after the tenth RT session to adjust training load in the following weeks (6–10 weeks). The QCSA was also assessed 72 hours after completion of the last training session to ensure there were no confounding effects of edema, with subsequent assessment of 1RM performed 48 hours later. Subjects were requested to abstain from alcohol and unaccustomed exercise in 48 hours before the testing sessions, as well as caffeine in the 24 hours preceding the tests. In addition, they were required to maintain their diet pattern. Subjects arrived at the laboratory at least 2 hours after their last meal and immediately began their warm-up.

Subjects

Thirty-four healthy, young, and recreationally active individuals participated in the study (18–34 years old). Subjects were not engaged in any kind of regular RT and/or aerobic training for at least 6 months before the experimental period and were free from cardiovascular and/or neuromuscular disorders. All subjects signed the free and informed consent term before participation. Exclusion criteria included the usage of any dietary supplements for at least 2 months before the study, as well as any previous administration of anabolic steroids. Six subjects withdrew from the study due to personal reasons; therefore, data from 28 (18 men, 10 women) (mean ± SD, 22.5 ± 5.7 years, 169.5 ± 9.5 cm, and 65.6 ± 13 kg) were included in the analysis. Informed consent was received from all individuals who met the inclusion criteria before participation in the study. The study was conducted according to the Declaration of Helsinki and was approved by research ethics committee of the University of São Paulo.

Procedures

Maximum Dynamic Strength Test Procedures

The maximum dynamic strength was determined as the maximum weight lifted in a single and complete repetition of unilateral leg press 45° exercise according to the guidelines of the American Society of Exercise Physiologists (³). The test was performed on an inclined leg press machine (45° leg press, G-001, Gervasport, Cotia, SP, Brazil) for both right and left legs. The knee joint amplitude was set at 90° using a goniometer (the lateral femoral condyle was used as the point of intersection, while the lateral malleolus and the greater trochanter were fixed as the extremities). The subjects performed a general 5-minute warm-up running on a treadmill at 9 km·h⁻¹, followed by 3 minutes of light stretching of the lower limbs. A specific warm-up was performed using 1RM outcomes measured during the initial assessment session. Subjects performed one set of 8 repetitions with approximately 50% 1RM and one set of 3 repetitions with approximately 70% 1RM. Warm-up sets were separated by a 2-minute rest interval. Three minutes after the specific warm-up, subjects began 1RM testing by performing single repetitions of progressively heavier loads until failure. The heaviest load lifted during the trials with proper technique was considered as the 1RM. Subjects rested 3 minutes between attempts, and all subjects reached their 1RM within 5 attempts. All tests were supervised by two-experienced researchers (A.R.L. and K.P.) who provided strong verbal encouragement during all attempts. The coefficient of variation between 1RM values performed 72 hours apart was 3.7%.

Quadriceps Cross-Sectional Area

The QCSA was obtained through MRI (Signa LX 9.1; GE Healthcare, Milwaukee, WI). Subjects were positioned supine with straps used to restrain lower limb movements during image acquisition. The position of the straps were standardized and placed such that they did not influence the QCSA measures. An initial reference image was captured to determine the perpendicular distance from the greater trochanter of the femur to the inferior border of the lateral epicondyle of the femur, which was defined as the segment length. Quadriceps cross-sectional area was measured at 50% of the segment length with 0.8-cm slices for 3 seconds. The pulse sequence was performed with a field of view between 400 and 420 mm, time of repetition of 350 ms, eco time from 9 to 11 ms, 2 signal acquisitions, and a matrix of reconstruction of 256 × 256 mm. The images were then transferred to a computer (Mac OS X, version 10.5.4; Apple, Cupertino, CA) and analyzed using open-source software (OsiriX, version 3.2.1; OsiriX Imaging Software, Geneva, Switzerland). The quadriceps images were traced in triplicates by a specialized independent researcher, and the mean values were used for further analysis. The segment slice was divided into skeletal muscle, subcutaneous fat tissue, bone, and residual tissue. Quadriceps cross-sectional area was calculated by subtracting the bone and subcutaneous fat from the total area. The coefficient of variation between 2 QCSA measures performed 72 hours apart was 0.95%.

Resistance Training Program

All subjects completed 2 training sessions per week for 10 weeks. Each training session began with a general (treadmill running at 9 km·h⁻¹ for 5 minutes) and specific (1 set of 5 repetitions of unilateral inclined leg press with 50% 1RM) warm-up. After a 1-minute rest, subjects in the LI and SI groups performed 3 sets of unilateral inclined leg press to 90° knee flexion until concentric failure (not able to perform another concentric repetition using correct technique) with 80% 1RM. Subjects in the LI group rested 3 minutes between sets and subjects in the SI group rested 1 minute between sets. Depending on protocol allocation, subjects then performed the unilateral incline leg press with their other lower extremity using the VLI-SI or VSI-LI protocols to concentric failure with 80% 1RM until the same LI or SI VL was achieved. Repetition tempo was standardized to 2 seconds (1 second for the concentric and 1 second for the eccentric phase of the repetition) controlled by a metronome.

Statistical Analyses

Data are presented as mean ± SD, relative changes, effect sizes (ES), and 95% confidence intervals (CI). Data normality was tested by the Shapiro-Wilk test and visual inspection to observe the presence of outliers; however, no outliers were observed. After data normality had been confirmed, a mixed model for repeated measures was applied for 1RM and QCSA, with “Protocol” (LI, SI, VSI-LI, and VLI-SI) and “Time” (pre-training and post-training) used as fixed factors, and ‘Subjects' as random factors. The absolute changes (i.e., POST-PRE) in 1RM and QCSA were compared between the protocols using a one-way analysis of variance (ANOVA). The total number of sets, repetitions, and VL were also compared using a one-way ANOVA. In all analyses when a significant F value was found, the Tukey post hoc was used for multiple comparisons. In addition, ESs were calculated using Cohen's d; qualitative descriptors for ES interpretation were assigned as follows: <0.2, negligible effect; 0.2–0.39, small effect; 0.40–0.75, moderate effect; and >0.75, large effect (⁶). The significance level set was p ≤ 0.05. Analyses were conducted using the SAS software v. 9.3. (SAS Institute, Inc., Cary, NC).

Results

Maximum Dynamic Strength

No significant differences between protocols were observed for leg press 1RM before training (LI: 168.6 ± 44.7 kg, VLI-SI: 168.2 ± 45.2 kg, SI: 171.1 ± 35.0 kg, VSI-LI: 170.4 ± 35.6 kg; p > 0.05). After training, leg press 1RM significantly increased (Figure 2A) for LI (within-protocol effect: p < 0.0001; 27.6%; ES = 0.90; 95% CI = 0.09–1.71), VLI-SI (within-protocol effect: p < 0.0001; 31.1%; ES = 1.00; 95% CI = 0.19–1.82), SI (within-protocol effect: p < 0.0001; 26.5%; ES = 1.11; 95% CI = 0.28–1.93), and VSI-LI (within-protocol effect: p < 0.0001; 31.2%; ES = 1.28; 95% CI = 0.44–2.13). The absolute change analysis (Figure 2B) did not reveal any significant differences between protocols for the 1RM increases (all comparisons, p > 0.05).

Figure 2.: A) Maximum dynamic strength (1RM, in kg) before (PRE) and after (POST) a 10-week unilateral resistance training program in the long resting interval (LI), volume-matched long resting interval with a short interval (VLI-SI), short resting interval (SI), and volume-matched short resting interval with a long interval (VSI-LI) protocols. B) Absolute change in 1RM in the LI, VLI-SI, SI, and VSI-LI protocols. *Significant within-protocol effect (p < 0.05). 1RM = one repetition maximum.

Quadriceps Cross-Sectional Area

No significant differences between protocols were observed for QCSA before training (LI: 76.7 ± 14.2 cm², VLI-SI: 76.5 ± 14.7 cm², SI: 77.2 ± 13.3 cm², VSI-LI: 75.8 ± 12.9 cm²; p > 0.05). After training, QCSA significantly increased (Figure 3A) for LI (within-protocol effect: p < 0.0001; 13.1%; ES = 0.66; 95% CI = −0.13 to 1.45), VLI-SI (within-protocol effect: p < 0.0001; 12.9%; ES = 0.63; 95% CI = −0.16 to 1.42), SI (within-protocol effect: p < 0.0001; 6.8%; ES = 0.38; 95% CI = −0.39 to 1.16), and VSI-LI (within-protocol effect: p < 0.0001; 6.6%; ES = 0.37; 95% CI = −0.41 to 1.15). On the other hand, absolute change analysis (Figure 3B) revealed that the LI and VLI-SI produced greater QCSA increases (9.8 ± 3.7 and 9.5 ± 4.0 cm², respectively) than SI or VSI-LI (5.1 ± 2.6 and 4.8 ± 3.2 cm², respectively) (p < 0.05).

Figure 3.: A) Quadriceps cross-sectional area (QCSA, in cm²) before (PRE) and after (POST) a 10-week unilateral resistance training program in the long resting interval (LI), volume-matched long resting interval with a short interval (VLI-SI), short resting interval (SI), and volume-matched short resting interval with a long interval (VSI-LI) protocols. B) Absolute change in CSA in the LI, VLI-SI, SI, and VSI-LI protocols. *Significant within-protocol effect (at p < 0.05); #significant between-protocol effect compared with SI and VSI-LI (p < 0.05).

Number of Sets, Repetitions, and Volume Load

The average total number of sets and repetitions performed by LI, VLI-SI, SI, and VSI-LI were 3.0 ± 0 sets and 16.1 ± 5.2 reps, 4.5 ± 1.5 sets and 11.6 ± 5.1 reps, 3.0 ± 0 sets and 9.8 ± 2.9 reps, and 2.3 ± 0.6 sets and 13.4 ± 5.5 reps, respectively. The LI and VLI-SI performed a greater VL than SI and VSI-LI (133,614 ± 45,683 kg and 133,648 ± 45,675 kg vs. 96,392 ± 25,608 kg, 96,369 ± 25,603 kg, respectively; p = 0.049).

Discussion

The present study investigated the effect of different interset rest intervals (1 minute vs. 3 minutes), with and without equalized VL, on muscle strength and hypertrophy after 10 weeks of RT. Consistent with our hypothesis, our findings demonstrate that both muscle strength and hypertrophy are not directly influenced by the interset rest interval. Thus, we confirmed that greater increases in hypertrophy were observed when higher VL was performed, regardless of the interset rest interval used.

The findings of this study partially call into question some recommendations from the literature on the use of longer intervals for greater increases in muscle strength (^1,11,12,24). It has been suggested that a ≥3-minute interset rest interval is needed to maximize increases in muscle strength (^1,24). Longer intervals would allow for the individual to maintain a higher intensity, perform more repetitions per sets, and consequently obtain higher VL, which in turn would result in greater strength increases (^25,26). In fact, some authors have shown that greater strength increases were obtained by use of longer intervals (^13,23). However, these results are not unanimous because other studies failed to replicate this finding, even when the longer intervals allowed a higher VL (^4,8,18). Recent evidence demonstrates that regardless of VL, the highest intensity of the RT seems to determine the greatest increases in muscle strength (^5,15–17,22). Specifically, higher intensities may promote greater recruitment of motor units, higher firing rate of motor units, and greater changes in agonist-antagonist coactivation rate compared with lower intensities (^9,19). Our findings are consistent with recent literature, demonstrating that under intensity-equated conditions (80% 1RM), the higher VL attained in LI and VLI-SI did not translate into greater strength increases compared with SI and VSI-LI (Figure 2A, B). Collectively, these findings indicate that the length of the rest interval does not significantly influence strength increases, at least for durations as low as 1 minute, when training is performed at a high intensity of load. Moreover, our findings suggest that using shorter rest intervals can make RT sessions more time efficient without compromising increases in strength. It should be noted that these findings are specific to training at a relatively moderate intensity of load (80% 1RM). General guidelines recommend heavier loading (1RM–6RM) to maximize strength increases (¹), and our findings cannot necessarily be extrapolated to the use of these higher loads. Future research is warranted to better determine how manipulations of rest interval length affect long-term strength adaptations with the use of very heavy loads.

Regarding muscle hypertrophy, a higher VL was found to positively influence the accretion of muscle mass regardless of the interval used in the RT protocol (1 or 3 minutes) (Figure 3B). Our findings are consistent with results from previous studies showing greater increases in muscle mass for protocols with longer intervals when VL was higher for these protocols compared with short intervals (^4,8,23). Further support for these findings can be found in studies showing similar increases in muscle hypertrophy when VL was equated between protocols of long and short rest intervals (^2,13,18). In addition, throughout the literature, there is strong evidence showing that higher VL favors greater increases in muscle mass (^1,14,20,21). Although the mechanisms are still unclear, it is suggested that the cumulative effect of greater RT volumes elicits higher rates of protein synthesis and intracellular anabolic signaling thereby translating into greater hypertrophic increases over time (²⁰). Taken together with our findings, the evidence suggests that rest intervals of 1 or 3 minutes produce similar hypertrophic increases provided VL is equated between conditions.

This study had several noteworthy limitations. First, we did not strictly control subjects' nutritional intake. However, the within-subject, randomized design should have minimized any potential confounding effects attributed to dietary variation between subjects. Regarding the within-subject design, we cannot rule out the possibility of a cross-education effect confounding strength changes, although this was probably minimized given that both lower limbs were submitted to the same training intensity (¹⁶). Second, we did not control menstrual cycle of the women participating in this study. However, in addition to the within-subject design, women were evenly distributed between the groups (5 women and 9 men in each group), reducing the possible influence of this factor on results. Third, our findings are specific to young, untrained men and women, and as such, these results cannot necessarily be generalized to other populations such as elderly or RT-trained subjects. Finally, although we used a gold-standard measurement for hypertrophy evaluation (MRI), measurements were obtained from the midpoint of the thigh; therefore, it is not known whether differential hypertrophic responses manifested at the proximal and distal regions of the thigh.

In conclusion, our results show that longer and shorter intervals do not affect muscle strength increases when a higher intensity is similarly maintained between the protocols, despite a greater VL with longer rest. This finding indicates that intensity is the primary determinant in muscle strength increases. On the other hand, the interset rest interval seems to have no influence on muscle mass when VL is matched, suggesting that greater increases in hypertrophy may be obtained when both longer and shorter intervals are performed provided a higher VL is attained. Despite the lack of blood data analysis, the aforementioned findings challenge the theory that the acute systemic hormonal elevations promoted by short-rest intervals are a main drive for greater muscle hypertrophy and rather suggest that VL should be prioritized when selecting a rest interval to maximize increases in muscle mass.

Practical Applications

Based on our findings, we propose that if the individual's goal is to maximize muscle strength increases, rest intervals should be “sufficient” to allow for the maintenance of high intensities of load during each set. The “self-selected” rest interval hypothesis proposed by De Salles et al. (⁷) might be a viable strategy because this approach has been shown to be effective in maintaining high levels of training intensity by taking into account the self-reported perception of recovery. Alternatively, when the goal is to achieve muscle hypertrophy, long intervals (3 minutes) allow for a higher VL with fewer sets to be performed, while short intervals (1 minute) require more sets to obtain higher VL, both of which are efficacious for increasing muscle mass. In other words, it could be possible to use both long and short rest intervals as long as a high VL is performed.