Total aggregate load volume over the 8 weeks was greater on an absolute basis for LONG compared to SHORT (51,385 ± 9420 vs 44,755 ± 12,166 kg, respectively): these results were not significantly different between the groups (p = 0.18), but the observed power for this analysis was only 0.26. There were no significant correlations between total load volume or changes in load volume and changes in the various measurements, but the observed power for these analyses was only 0.05–0.07.
There was no significant time by treatment interaction for changes in elbow flexor thickness (p = 0.16; CI for difference in change between groups = −0.06, 0.31). There was a significant main effect of time (P = 0.001). LONG significantly increased elbow flexor MT from baseline to poststudy by 5.4% (p < 0.01). The increase for SHORT of 2.8% showed a trend (p = 0.08) for statistical significance.
A group-time interaction trend was noted for greater increases in triceps brachii thickness for LONG compared to SHORT (p = 0.06; CI = −0.01, 0.56). There was a significant main effect of time (P = 0.009). LONG significantly increased triceps brachii MT from baseline to poststudy by 7.0% (p < 0.01). The increase for SHORT of 0.5% was not statistically significant (p = 0.83).
There was a significant time by treatment interaction for changes in anterior quadriceps femoris thickness, with significantly greater increases in favor of LONG compared to SHORT (p = 0.04; CI = 0.00, 0.69). There was a significant main effect of time (P < 0.0001), with both LONG and SHORT increased MT from baseline to poststudy of 13.3% (p < 0.001) and 6.9% (p < 0.01), respectively.
There was no significant group by time interaction for changes in vastus lateralis thickness (p = 0.77; CI = −0.27, 0.36). There was a significant main effect of time (P = 0.002), with both LONG and SHORT increasing MT from baseline to poststudy of 11.5% (p < 0.01) and 10.0% (p < 0.01), respectively.
There was a significant time by treatment interaction for 1RMSQUAT, with significantly greater increases in favor of LONG compared to SHORT (p < 0.01; CI = 6.1, 32.9). There was a significant main effect of time (P < 0.0001), with both LONG and SHORT showing a significant increase in 1RMSQUAT from baseline to poststudy of 15.2% (p < 0.001) and 7.6% (p < 0.001), respectively.
There was a significant time by treatment interaction for 1RMBENCH, with significantly greater between-group increases in favor of LONG compared to SHORT (p = 0.02; CI = 2.2, 32.5). There was a significant main effect of time (P = 0.0001). LONG showed a significant increase in 1RMBENCH from baseline to poststudy of 12.7% (p < 0.001). The increase for SHORT of 4.1% (p < 0.09) showed a trend for statistical significance.
The 50%BENCH task was performed with a load that corresponded to 50.4% and 50.1% of the pretesting 1RM strength for SHORT and LONG, respectively. There was no significant time by group for changes in 50%BENCH (p = 0.27; CI = −2.7, 8.0). There was a significant main effect of time (P = 0.001); both the LONG and SHORT showed a significant increase in 50%BENCH from baseline to poststudy by 23.2% (p < 0.01) and 13.0% (p = 0.03), respectively. For the group as a whole, a strong positive relationship (r = 0.75) was seen between % change in 1RMBENCH and % change in 50%BENCH repetitions (Figure 1).
Our study produced several important findings. Consistent with our hypothesis, there was a clear benefit to longer rest intervals from a strength standpoint. Both 1RMSQUAT and 1RMBENCH were significantly greater for LONG compared to SHORT, and effect sizes were at least double that in favor of the longer rest condition for these measures. Contrary to our hypothesis, there was a strong suggestion that longer rest periods had a greater impact on hypertrophic outcomes. Muscle thickness was significantly greater for LONG compared to SHORT in the anterior thigh, and the effect size differences imply that these differences were meaningful. Regarding the triceps brachii, there was a trend for greater increases with LONG compared to SHORT (p = 0.06), and the 95% CI (−0.01, 0.56) suggests a high probability for an effect. Moreover, effect sizes markedly favored LONG compared to SHORT (0.37 versus 0.03, respectively). Although increases in biceps brachii thickness were not different between groups, the effect sizes again favored LONG compared to SHORT (0.39 versus 0.18, respectively). It should also be noted that significant increases in MT for the triceps brachii and biceps brachii were only seen in LONG. Interestingly, increases in thickness of the lateral thigh were similar between conditions. Finally, although both groups saw significant increases in local upper body muscle endurance, rest interval duration did not seem to influence this outcome.
Our finding that LONG produced greater strength increases compared with SHORT is in line with general RT guidelines, which recommend rest periods of 3 minutes or more between sets to maximize absolute strength (9,28). Longer rest periods can allow for the completion of a higher number of repetitions (29) and the maintenance of a higher training intensity and volume (30), and thus may allow for greater muscle activation per set. However, two previous studies showed that varying the rest intervals between sets had no impact on strength outcomes (3,6), whereas another study showed a benefit to shorter rest intervals (23). Of these studies however, two used volume-equated and/or repetition-equated methodological approaches (3,23), which may nullify the aforementioned benefits of longer rest intervals on training capacity. In addition, the remaining study assessed muscular strength using 5RM testing on a Smith Machine rather than a 1RM with free weights (6), which is often considered the “gold standard” for assessing strength in nonlaboratory settings (14).
Regarding increases in muscle mass, our findings were consistent with those of Buresh et al. (6), who reported significantly greater increases in arm CSA and a trend for greater increases in leg CSA with rest durations of 2.5 minutes versus 1 minute. The veracity of the results of Buresh et al. (6) can be questioned because CSA was estimated from anthropometric measurements. In this study, we provide direct site-specific measures showing that longer rest intervals produce significantly greater increases in thickness of the anterior thigh and strong indication of greater growth in the upper arm. These findings are at odds with those of Ahtiainen et al. (3) who found no differences in CSA of the quadriceps femoris with rest intervals of 2 versus 5 minutes and of Villanueva et al. (23) who found greater increases in lean body mass with rest intervals of 1 versus 4 minutes. These studies, in conjunction with this study, reveal an important consideration in interpreting the results from studies examining rest intervals; that is, rest intervals should be considered as an absolute (e.g., 1 minute vs 5 minutes), rather than an arbitrary, relative value (e.g., short vs long). For example, both Ahtiainen et al. (3) and this study sought to directly compare adaptations after training with short vs long rest intervals. However, we used a 1-minute vs 3-minute protocol, whereas Ahtiainen et al. (3) used 2-minute vs 5-minute protocol. Accordingly, it can be inferred that a rest interval of 1 minute is likely too short in duration to promote maximal hypertrophic gains, whereas a 2-minute rest period provides sufficient recuperation in this regard.
The divergent findings from Ahtiainen et al. (3) and Villanueva et al. (23) for strength development and muscular hypertrophy may be due to differences in research design. Both of these studies equated volume between groups, which is in contrast to this study and Buresh et al. (6). In the study of Ahtiainen et al. (3), this resulted in the shorter rest interval group performing on average 1 more set per exercise. Given the dose-response curve of training volume on strength development and muscle hypertrophy (20), the extra sets may have counteracted the negative effect of the shorter rest period on training adaptations, causing equal adaptations in both groups. Moreover, as previously noted, Ahtiainen et al. (3) afforded 2-minute rest between sets, which may have allowed for sufficient recovery and thus negated any detrimental effects associated with shorter rest periods.
Villanueva et al. (23) equated not only total training volume but also repetitions per set and the number of sets. This inherently resulted in the shorter rest interval group training closer to muscular repetition failure per set, which has been found to increase strength development and muscle hypertrophy (17). Training closer to repetition failure may facilitate training adaptations by increasing motor unit recruitment and intramuscular metabolic stress in the form of phosphate metabolites, lactate and H+ accumulation, hypoxia, and lowered pH. In addition, the population examined by Villanueva et al. had a mean age of 68 (±4.1) years. Increasing age is accompanied by well-known functional declines attributed to changes in both the morphology of skeletal muscle tissue and neurological networks that control them (21). Regardless of the differences in methodological approaches used between our study and those of Villanueva et al., the presumption that these differing populations are equally responsive to a training variable such as rest interval duration requires further support.
Henselmans and Schoenfeld (12) hypothesized that the effect of the interset rest interval is primarily mediated by its effect on total training volume and not different between strength development and muscular hypertrophy. This study could not significantly correlate the change in training load to the magnitude of training adaptations; however, our data were statistically underpowered for these analyses. We therefore cannot rule out the possibility that the greater training load achieved by the longer rest period group was responsible for the greater training adaptations. In the study of Buresh et al. (5), the significantly greater upper body muscle hypertrophy co-occurred with significantly greater training loads in the upper body, whereas the lower body muscle hypertrophy difference did not reach statistical significance and co-occurred with a nonsignificantly different training load in the lower body. Moreover, there is compelling evidence for a dose-response effect of RT volume on training adaptations (15,16,20,27). The higher workloads might have a particular impact on the development of type I fibers which, because of their endurance-oriented nature, would benefit from longer times under load. As such, the hypothesis from Henselmans and Schoenfeld (12) requires further research, ideally in the form of a study with a volume-equated group and a nonvolume-equated group.
To the authors' knowledge, no previous study has evaluated the effects of varying rest interval duration on muscular endurance. Somewhat surprisingly, we found no significant differences between resting 1 versus 3 minutes on 50%BENCH. We did, however, observe a strong positive correlation (r = 0.75) between % change in 1RMBENCH and % change in 50%BENCH for the group as a whole. Reducing the amount of rest between sets decreases the ability for clearance of metabolic substrates (1). Theoretically, consistently training in this manner over time should result in adaptations for enhanced buffering capacity that would translate into a greater ability to perform repetitions with submaximal loads. Alternatively, increases in maximal muscular strength may be associated with a reduced cost when performing tasks with the same absolute submaximal load. Although each group increased 1RMBENCH, only LONG reached statistical significance, thereby suggesting that longer rest periods may have greater impact on improving muscular endurance. This hypothesis runs counter to generally accepted RT guidelines (28) and thus warrants further investigation as correlation is not necessarily indicative of causality. It should be noted that results are specific to upper body muscular endurance and cannot necessarily be generalized to those of the lower extremities. Further research is needed to clarify whether differences in this outcome exist between body segments.
The study had several limitations. First, the duration of the training protocol was relatively short. Although the 8-week study period produced significant increases in muscle strength and hypertrophy in most of the outcomes assessed, it remains possible that between-group differences would have diverged over a longer time frame. Second, although subjects were advised to maintain their usual and customary diets, we cannot rule out the possibility that differences in either energy or macronutrient consumption influenced results. Third, volume load could not be adequately determined for the machine-based exercises, as renovation of the university gym forced the use of alternative machines. Although the movement patterns of these machines were identical, they differed in mode of action (cable pulley versus pivot) and thus had different load schemes (load corresponded to a number rather than a true load) that precluded accurate volume load assessment. Thus, it is possible that the volume load data obtained from the 3 barbell exercises did not adequately reflect the actual total volume load performed by each group. Finally, MT measurements were taken only at the midportion of each muscle. Although it is common to use these measures as a proxy of whole-muscle growth, there is evidence that hypertrophy often manifests in a regional-specific manner, with greater protein accretion occurring at the proximal and/or distal aspects of a given muscle (24,25). Thus, it remains possible that subjects may have experienced differential changes in proximal or distal muscle growth in one condition versus the other that would not have been observed with the testing methods employed.
The present study provides evidence that longer rest periods promote greater increases in muscle strength and hypertrophy. Our findings are consistent with current recommendations for maximal strength gains but run counter to general hypertrophy training guidelines (9,28). When the results are taken together with those of Ahtiainen et al. (3) and Buresh et al. (6), it would seem that a minimum rest interval of ∼2 minutes should be recommended for maximizing gains in muscle size. Beneficial effects of longer rest intervals may be mediated by a higher volume loads, but our study was underpowered to make this determination.
Although our results suggest that longer rest periods be employed for enhancing muscular adaptations, we cannot infer that these findings will necessarily hold true when other training variables are manipulated. It is also noteworthy that there was considerable variability within groups and even between muscle groups in the same participants. This may imply that, when manipulating training variables, susceptibility for adaptations may be specific to the individual and/or muscle group. Moreover, integrating phases of short rest in combination with longer rest periods may evoke responses that could translate into greater muscular gains over time. This possibility warrants further study. Finally, time constraints must also be considered with respect to rest interval duration. Sessions for the LONG lasted more than twice long as those for the SHORT. The cost-benefit tradeoff must therefore be taken into account if training time is an important factor.
The authors thank Dymatize Nutrition for supplying the whey protein used in this study. The authors also thank Ryan Thiele MA, ATC, LAT for lending his time to best ensure the safety of participants throughout study.
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Keywords:Copyright © 2016 by the National Strength & Conditioning Association.
rest period; rest interval; muscle hypertrophy; muscular adaptations; rest between sets