Increasing muscle strength is generally deemed as an important consideration for training programs with the purpose to improve performance, general health, and fitness (2,9). The paramount activity to promote increases in muscle strength is resistance training. Optimally designed resistance programs are based on scientific principles that control training variables; this is normally achieved by manipulating volume, intensity, exercise type, between-set rest intervals, and other variables (6,9,12).
According to Ratamess et al. (17), the between-set rest interval is one of the most important variables to be controlled by the researcher, athlete, coach, fitness trainer, or practitioner. An adequate between-set rest period is necessary to offset the detrimental effects of fatigue and facilitate muscle recovery, allowing for the performance of more work during training sessions (15,16,18,25-27). However, it has also been proposed that the development of fatigue and metabolite accumulation may actually be important factors for promoting muscle adaptation in response to resistance training (14,20,21,23), which favors the use of short between-set rest intervals (4,22).
The acute effects of different between-set rest intervals have been extensively studied. However, studies on chronic effects are scarce. Previous studies that have examined the chronic effects of resistance training suggested that varying the rest interval between sets may bring about different muscle adaptations, but the results are inconsistent. Robinson et al. (19) examined the effects of rest interval manipulation (180, 90, and 30 s) on measures of maximum strength over a 5-week resistance training period in trained young men and reported that the increase in maximum strength was significantly different only between 180 seconds and 30 seconds. Later, Pincivero et al. (15) used either a short rest interval (40 s) or a long rest interval (160 s) during unilateral isokinetic training for the knee extensors and flexors of 15 college-age nontrained men and suggested better isokinetic adaptations for the longer rest intervals.
Recently, Hill-Haas et al. (7) compared 20-second to 80-second rest intervals between sets in 18 active women during 5 weeks of resistance training. According to the results, there was a greater improvement in muscle strength with 80-second rests. In contrast, Ahtiainen et al. (3) reported no differences between 2- and 5-minute rest periods on muscle strength gains in previously strength trained men after 24 weeks of training. Also, using a similar training protocol to the present study, Willardson and Burkett (27) reported no difference on strength gains between 2-and 4-minute rest intervals in trained subjects. Thus, because of the controversies, differences in training protocols, training duration, and population training level, as well as limited literature regarding the chronic effects of different rest intervals in muscle strength gains on untrained subjects, the main purpose of this study was to examine the effect of between-set rest interval manipulation, with the same work output, on muscle strength in young, nonresistance trained men after 12 weeks of resistance training.
Experimental Approach to the Problem
Forty participants were initially required to attend 3 to 4 sessions to get familiarized with the resistance training program. Participants were then required to attend 2 additional sessions to test for 1 repetition maximum (1RM) in the leg press and bench press exercises, using standardized procedures for strength testing (10). The tests were repeated at the end of the training period.
The subjects were then matched for upper- and lower-body strength and randomly assigned to 2 resistance training groups. However, after drop-outs, matching was hindered for leg press strength. The 2 groups undertook 12 weeks of whole-body resistance training. Training was conducted 2 days a week, with a minimum of 48 hours between sessions. Both groups performed the same exercises and were instructed to perform 8 to 12 repetitions until volitional fatigue at a speed of 4 seconds per repetition (2 s for the concentric phase and 2 s for the eccentric phase). This range of repetitions was chosen based on the recommendations of the American College of Sports and Medicine (9). One group used short-rest intervals (SR, work rest ratio approximately 1:3) and the other used long-rest intervals (LR, work rest ratio approximately 1:6).
Forty college-aged men volunteered to participate in the study. However, because of time contraints, 6 subjects (4 in the LR and 2 in the SR) dropped out of the study or did not complete the final 1RM tests. Therefore, 16 subjects in the LR group (22.4 ± 2.6 yr; 73.1 ± 13.6 kg; 171.9 ± 8.2 cm) and 18 subjects in the SR group (21.4 ± 3.2 yr; 73.8 ± 14.0 kg; 175.9 ± 7.8 cm) successfully completed the study. The men were selected at random from the respondents to fliers distributed over the university and by word of mouth.
The inclusion criteria for participation in the study included being at least 18 years of age, no resistance training experience, and being free of clinical problems that could be aggravated by the protocol. To be included in the analyses, subjects had to attend more than 85% of the training sessions. Although they were untrained in a resistance training sense, all were physically active, with involvement in other activities such as walking, jogging, martial arts, and team sports. All participants were notified of the research procedures, requirements, benefits, and risks before providing informed consent. The institutional research ethics committee granted approval for the study.
One Repetition Maximum Test
In the week before the experiment and 5 to 7 days after the last training session, the load for 1RM was determined for each subject in the bench press and the leg press exercises using the protocol suggested by Kraemer and Fry (10). The initial tests were repeated in all subjects, and data were analyzed by Pearson product moment correlations to estimate day-to-day 1RM reliability (r = 0.93 for bench press and 0.94 for leg press). Also, no significant differences were found between day-to-day upper- and lower-body 1RM loads.
Resistance Training Intervention
All training sessions were closely supervised to ensure safety and compliance with the procedures. For both groups, a whole-body multiple-set resistance training program was implemented using a combination of free weights and machines. The sessions consisted of 5 exercises, 2 for the upper body, 2 for the lower body, and 1 for the midsection. To maintain the same total work output, improve external validity, and follow the ACSM (9) recommendations, subjects performed 2 sets of 8 to 12 repetitions. Training loads were adjusted carefully; if a subject could not perform 8 repetitions or could lift the load more than 12 times, he was instructed to adjust the load to ensure the completion of the required number of repetitions. Each subject maintained a training log in which the number of repetitions performed and the weight used in each set were recorded.
Training was conducted 2 days a week, with a minimum of 48 hours between sessions, for 12 weeks. The time under tension for each set was approximately 30 to 40 seconds. One group used a short (SR) work rest ratio (approximately 1:3) interval (sets beginning every 2 min), and the other used a long (LR) work rest ratio (approximately 1:6) interval (sets beginning every 4 min). During the training sessions, music tracks with 120 bpm were played to facilitate control of movement speed.
The 2 groups trained separately. The LR group trained at 4 pm and the SR group trained at 5 pm. Each group was divided into 3 subgroups. There were 3 different resistance training programs, as shown in Table 1. Each subgroup performed 1 of the 3 programs during a given class, and the subsequent program was performed in the next class, following the order A→B→C. Therefore, each program was repeated every 3 sessions.
All values are reported as mean ± SD. Paired t-tests were used to compare pre- and postvalues of the 1RM tests within groups, and an independent t-test was used to compare training volume between groups. To compare differences in strength gains between groups, final values of 1RM load for LR and SR were compared with analysis of covariance (ANCOVA) using the baseline values as covariates. Statistical significance was set at p ≤ 0.05. The version 14.0 of SPSS (SPSSS, Chicago, IL, USA) was used in the statistical analysis.
Training volume did not differ between groups for any exercise (p > 0.05). The results of the 1RM tests before and after 12 weeks are presented in Figures 1 and 2 for bench press and leg press, respectively. The 1RM load for the bench press and leg press exercises significantly increased for both groups (p < 0.05). The increases in bench press 1RM load were 10.5 ± 6.4% for the LR and 14.4 ± 8.1% for the SR group. For the leg press exercise, increases were 17.8 ± 12.3% for the LR and 17.5 ± 9.2% for the SR group. The results of ANCOVA did not reveal significant differences between SR and LR for the bench press or leg press 1RM (p > 0.05).
The purpose of this study was to investigate the chronic effects of training with different between-set rest duration on muscle strength in nonresistance trained young men. After 12 weeks of training, there were similar and significant increases in upper- and lower-body muscle strength for both groups. The main limitation of the study was the lack of a control group; however, the purpose was to compare strength training protocols rather than determine how much strength could be gained over a 12-week period. In addition, control groups were also not used in previous studies examining chronic effects of rest interval on muscle strength (3,7,15,19,28).
Generally, it has been recognized that, to maximize increases in muscle strength through resistance training, one needs sufficient rest between sets (6,9,12). This is probably based on the fact that longer rest intervals allow the performance of higher work volume during the training sessions (4,16,18,25,26,27). However, studies using different total work volumes reported no significant differences between short and long rest periods (3,28). In addition to the well-known importance of mechanical factors (9,13), recent studies have suggested that muscle strength and size may increase in the absence of high force contractions (1,8,23,24). In agreement with these suggestions, some authors proposed that fatigue and metabolite accumulation may have an important role in the adaptation promoted by resistance training (20,21,23). This may be an endorsement for the use of shorter rest intervals (22) because shorter rest intervals have been shown to produce more pronounced force decrements and higher metabolite and growth hormone accumulation (5,11,25,26,27). Recently, our group reported that shorter rest intervals produce more increases in serum growth hormone, even with less work volume (4).
In the present study, the rest intervals ranged between 3 and 3.5 minutes in the LR protocol and 1 and 1.5 minutes in the SR protocol. This range is similar to that recommended for strength and hypertrophy gains, respectively (9,12). In a previous study, Pincivero et al. (15) randomly assigned 15 college-aged individuals to either a short (40 s) or a long rest interval group (160 s). One leg of each subject was assigned to a 4-week, 3-days-a-week isokinetic training of concentric extension and flexion at 90°/s. The results indicated that the longer between-set rest period resulted in greater improvement in hamstring muscle strength. However, the study used isokinetic training, which involves maximum contractions in every repetition, and the rest-interval lengths were more extreme than those used in the present study.
Hill-Haas et al. (7) divided 18 active women in 2 groups: 1 group exercised with 20-second rest intervals between sets, whereas the other group had 80-second rest intervals between sets, both performing 30 repetitions per set and equivalent training volumes during 5 weeks. The authors found greater improvement in repeat sprint ability with the 20-second rest protocol, whereas the use of 80-second rest intervals lead to greater improvements in the 3RM leg press. Comparisons between the study of Hill-Hass et al. (7) and the present findings are limited because of the large difference in the length of the rest intervals (their “long” rest interval lasted approximately the same as our “short” rest interval) and the large difference in the number of repetitions performed.
Ahtiainen et al. (3) studied the adaptations to strength training protocols involving different rest intervals in 13 recreationally trained men. The study involved 2 separate 3-month training periods in a cross-over design. One training protocol involved between-set rest intervals of 2 minutes, whereas the other involved 5 minutes of rest, and both had the same work volume. According to the results, both protocols resulted in similar gains in muscle strength. Except for the differences in rest duration and characteristics of the subjects, our results are similar to the results obtained by Ahtiainen et al. (3).
Similarly, Willardson and Burkett (28) reported that large strength gains (18.2%) can be achieved with a minimum of 2-minute rest intervals, and little additional gains (21.4%) are derived from resting 4 minutes between sets. In their study, 15 trained men were placed into either a 2-minute (n = 7) or 4-minute (n = 8) rest interval group. Subjects performed 2 squat workouts per week. No differences were observed between groups. The present study also found similar strength gains in untrained subjects for the leg press exercise for the LR (17.8%) and for the SR (17.5%) group. Ahtiainen et al. (3) and Willardson and Burkett (27) concluded that, after a certain threshold volume, the length of the rest period does not make a systematic contribution to the neuromuscular response. Therefore, we can also hypothesize that the untrained subjects from both groups in the present study may have reached this threshold volume necessary to gain a certain amount of strength, which also reduced the significance of increasing the rest intervals between sets for nontrained subjects.
Using a resistance training protocol comparable with the present study, Robinson et al. (19) also examined the effects of different rest interval manipulation (180, 90, and 30 s) on measures of maximum strength over a 5-week period in trained young men. Similar to the present study, their results did not show any significant difference in the increases in 1RM strength between 3- and 1.5-minute rest intervals. However, Robinson et al. (19) reported an approximately 7% increase in lower-body muscle strength, whereas the present study found approximately 17%. This may be a result of the fact that Robinson et al. used resistance trained subjects and the present study used nonresistance trained college age men. Moreover, the present study had a longer duration (12 wk) than the study of Robinson et al. (19).
In summary, it has been previously suggested that longer interval duration would enhance strength gains because of the performance of a higher work volume (4,16,18,25,26,27). However, in the present study, the total work volume performed by the 2 groups was not different, which may explain the lack of difference in strength gains. The present results suggest that, in nontrained young men using 8 to 12 repetitions, the use of LR and SR produced similar gains in muscle strength. This implies that the length of the rest interval does not influence muscle strength adaptations in this group. However, it is important to note that the results are limited to muscle strength, and the manipulation of rest intervals may interfere with other adaptations such as muscle hypertrophy, power, and endurance. Thus, one can assume that a program could be designed with either long or short rest intervals if the purpose is to increase muscle strength, depending on personal preferences and time availability.
An optimal gain of muscle strength depends on the type of exercise, training load, number of repetitions per set, contraction velocity, rest interval between sets, and others factors. Also, the manipulation of these factors is dependent of the subject's training status. The present study of novice lifters shows that a work rest ratio of 1:3 or 1:6 between 2 consecutive sets to failure results in similar gains in upper- and lower-body muscular strength after 12 weeks of resistance training. The time spent in the gym for the SR group was 18 minutes compared with 36 minutes for the LR group. Thus, from our results, personal trainers and strength coaches should feel confident that their beginning lifters are able to achieve the same strength gains with less rest between sets, thereby spending less total time in the weight room.
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