Interset rest intervals (RI) refer to the amount of time taken in between sets and exercises. The amount of rest allotted can significantly affect performance of subsequent sets, the metabolic, hormonal, and cardiovascular responses to an acute bout of resistance exercise, and training-induced muscular adaptations. General guidelines recommend at least 2- to 3-minute RI for multiple-joint exercises and 1- to 2-minute RI for single-joint exercises during strength and power training (i.e., lower repetitions), whereas shorter RI (i.e., <2 minutes) are often recommended to enhance muscle endurance in conjunction with higher repetition sets (2). However, RI length often depends on several other factors including training intensity, training goals, and fitness level. Therefore, it is difficult to precisely define an optimal RI, and it could be argued that on a global basis, it may not exist. This brief review will examine the effect of RI on resistance exercise performance, the acute hormonal and metabolic response, and training-induced muscular adaptation in an effort to aid strength and conditioning professionals in selecting the most appropriate RI for individual clients.
EFFECT OF INTERSET REST INTERVAL LENGTH ON PERFORMANCE
Several studies have shown that the number of repetitions performed during a resistance training bout may be compromised with shorter RI. That is, a 3- to 5-minute RI has been shown to produce less performance decrements than a <2-minute RI during compound exercises, such as bench press or back squat (16–18,23,26,27). Specifically, under conditions where the load is held constant (50–90% 1-repetition maximum [1RM]) and all sets are performed to the point of muscular failure (i.e., inability to complete an additional concentric repetition with proper form), resting 3–5 minutes between sets allows for significantly greater repetitions versus resting 1–2 minutes. For example, Ratamess et al. (17) found that a minimum of 3-minute RI are necessary to maintain bench press performance over 3–4 sets at 75–85% 1RM in trained men. Although many causative factors exist that contribute to fatigue, neural fatigue resulting in changes to the voluntary activation of motor units can be impacted by RI. For example, shorter RI (i.e., <2 minutes) may accelerate the onset of neural fatigue, whereas longer RI (e.g., 3–5 minutes) may allow for maximal voluntary activation of motor units and the maintenance of training intensity and volume, which seem to be of primary importance for augmenting strength and stimulating muscle hypertrophy (14,21). Therefore, when training for maximal strength and hypertrophy, it is generally advisable to afford 2- to 5-minute RI in between sets and exercises to limit the negative effects on performance and to prevent the need to lower the exercise intensity during subsequent sets to sustain repetitions within the range conducive to the training goal.
Two key factors, training history and strength level, can influence the RI required during resistance exercise and should be taken into account when prescribing training parameters. Consistent training with shorter RI may result in adaptations that allow for training intensity to be sustained (12). It has also been shown that stronger individuals (based on peak power output) were able to sustain power output over 5 sets of 8 repetitions with 40% 1RM when using a 2-minute RI, whereas weaker individuals needed a minimum of 3 minutes to maintain power output performance (10). Therefore, athletes attempting to achieve specific training volume goals may initially need longer RI but may later adapt so that shorter RI can be utilized without excessive fatigue.
EFFECT OF INTERSET REST INTERVAL LENGTH ON THE ACUTE HORMONAL AND METABOLIC RESPONSE
It is well-established that resistance exercise protocols that activate a large amount of muscle mass and are of sufficient intensity elicit a transient hormonal and metabolic response (13). Several studies have demonstrated that resistance exercise performed with shorter RI (<1.5 minute) provokes a greater transient increase in circulating hormones, including growth hormone, testosterone, and cortisol compared with longer-duration RI (4,9,11,25). Alternatively, Ahtiainen et al. (3) demonstrated that both 2-minute and 5-minute RI between leg press and squat exercises using 10RM elicited significant increases in growth hormone, testosterone, and cortisol and found no significant difference between protocols. Although it has previously been suggested that transient elevations of these circulating hormones (i.e., growth hormone and testosterone) are critical for muscle anabolism, others have shown that transient increases in the hormonal milieu after resistance exercise are not related to exercise-induced muscle hypertrophy (8). Given the equivocal nature of exercise-induced systemic hormonal alterations on muscle anabolism, the notion that a shorter RI provides a superior stimulus for muscle hypertrophy remains questionable.
Shorter RI are also related to increased metabolic stress, which manifests in the accumulation of lactate and hydrogen ions with a subsequent reduction in pH (8,11). Ratamess et al. (17) observed an inverse relationship between RI length and the acute metabolic response. Shorter RI do not provide sufficient recovery time between sets to replenish intramuscular phosphocreatine stores; thus, the continuation of exercise prompts an increased reliance on glycolytic energy production. Although this glycolytically induced metabolic stress may play a role in hypertrophic adaptation (20), metabolic buildup may compromise the ability to sustain repeated high-force muscular contraction (1).
The notion that a shorter RI (<1.5 minute) maximizes muscle hypertrophy based on the hormonal and metabolic milieu in the postexercise period has recently been challenged. McKendry et al. (15) observed a blunted muscle protein synthesis (MPS) and anabolic intramuscular signaling response after sets of moderate-intensity, high-volume resistance exercise separated by 1-minute RI compared with 5-minute RI, despite a greater testosterone and lactate response after the 1-minute RI trial. Notably, participants performed a significantly lower total training volume during the 1-minute RI trial compared with the 5-minute RI trial. Because muscle hypertrophy occurs as a result of cumulative increases in MPS with each successive bout of resistance training, it is plausible that longer RI would promote greater muscle protein accretion over time. It is important to highlight that other contributing mechanisms of skeletal muscle hypertrophy exist beyond hormonal and metabolic changes, such as stretch and mechanical load. Although important, no studies to date have directly investigated these mechanisms through manipulation of RI.
EFFECT OF INTERSET REST INTERVAL LENGTH ON MUSCULAR ADAPTATION
It is generally well accepted that manipulating resistance exercise parameters can influence skeletal muscle remodeling. Traditionally, shorter RI have been associated with resistance training programs focusing on hypertrophy, whereas longer RI have been associated with programs focusing on strength. Hypertrophy-style routines typically involve greater volumes, moderate intensities (<85% 1RM), and shorter RI (<90 seconds), whereas strength-style routines typically involve lower volumes, higher intensities (>85% 1RM), and longer RI (3–5 minutes). Among the studies that have compared these 2 styles of training in resistance-trained individuals, both routines appear to elicit similar magnitudes of muscle growth, although strength outcomes favor the strength-style of training (7). However, whether the differences in rest periods per se are responsible for greater improvements in strength remains questionable because the subjects were exposed to a different intensity of training during working sets.
Few studies have explicitly examined the influence of RI manipulation on exercise-induced hypertrophy and strength improvement. Schoenfeld et al. (22) demonstrated that a 3-minute RI elicited greater increases in strength and muscle thickness compared with a 1-minute RI after an 8-week total body resistance training program in young trained men. Similarly, Buresh et al. (5) reported greater increases in arm cross-sectional area after a 10-week resistance training program utilizing a 2.5-minute RI compared with a 1-minute RI in young untrained men; however, no differences between groups were observed for strength outcomes. Robinson et al. (19) also demonstrated greater strength improvements after a 5-week resistance training program with a 3-minute RI compared with a 30-second RI in young trained men. Conversely, Villanueva et al. (24) found that elderly untrained men achieved significantly greater gains in lean body mass and maximal strength after an 8-week resistance training program with a 1-minute RI compared with a 4-minute RI. To further confound matters, others have found no significant difference in strength (6,28) and hypertrophy (3,6) when comparing different RI lengths in young trained men.
In summary, with the exception of 1 study (24), research has not supported the hypothesis that shorter RI provides an advantage for hypertrophy or strength outcomes. Nevertheless, it is difficult to distinguish between the effects of RI per se and the effects of altered training volume. Rest interval manipulation may only have minor effects on muscle hypertrophy compared with other training parameters, such as total training volume, which suffers when RI is insufficient.
Training intensity and volume seem to be reduced proportionally as RI length is reduced during multijoint resistance exercise. It seems that at least 2–3 minutes of RI would provide sufficient recovery so as not to compromise total workout volume and subsequent hypertrophy and strength outcomes. For assistance or single-joint exercises, a shorter RI of 1–2 minutes may suffice. Moreover, the majority of literature does not support the hypothesis that training for muscular hypertrophy requires shorter RI than training for strength development. However, training with shorter RI has shown to promote adaptations that ultimately facilitate the ability to sustain high-intensity performance.
The required RI for full recovery varies depending on several factors, including training intensity, complexity of the given exercise, type of muscle contraction, activated musculature, exercise order, training status, and strength level. Additionally, stopping repetitions before muscular failure would allow for a more rapid recovery between sets compared with training to muscular failure. Therefore, rather than prescribing a fixed RI, it is prudent to recommend that athletes training for muscular strength or hypertrophy perform subsequent sets when psychologically and physiologically ready. Ultimately, several RIs can be implemented within a periodized model to achieve the desired physiological adaptations. Manipulation of training variables including RI is dependent on the specific training goals of the individual.
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