Muscle power is an important factor during athletic performance and daily tasks performed by youth and seniors (ACSM, 2009; Nogueira et al., 2009). Studies reported that muscle power increases after strength training (ST) programs performed with high-velocity (ACSM, 2009). ST is commonly performed with multiple sets of repetitions protocols (ACSM, 2009; Peterson et al., 2005), and the magnitude of strength and maximal power output gains rely on the manipulation of several variables, such as rest interval (RI) between sets (ACSM, 2009; Willardson, 2006), muscle action velocity, training load, and volume (ACSM, 2009; Peterson et al., 2005).
The American College of Sports Medicine (ACSM) position stand recommends that power training should comprise 3 to 6 repetitions per set with a load ranging from 0 to 60% of one repetition maximum (1RM) using high muscle action velocity (ACSM, 2009). The ACSM also recommends rest periods of at least 2 to 3 minutes between sets for core exercises, and 1 to 2 minutes for assistance exercises (ACSM, 2009). However, according to the ACSM there is no strong current scientific evidence to support these RI recommendations. Moreover, this recommendation was based on a consensus of panel members using clinical experience (i.e. evidence D) (ACSM, 2009). In addition, the RI between sets recommendation from the ACSM was also based only on the study of Abdessemed et al. (1999). They investigated the effects of three different RI (1-, 3- or 5 minutes) between sets on power output during bench press exercise. The authors reported a decrease on power output and higher blood lactate only after the 1 minute RI.
Furthermore, Pincivero et al. (1998) compared muscle power output from quadriceps and hamstring during a reciprocal action exercise protocol using short (40 s) and long RI (160 s) during four maximal effort sets on an isokinetic dynamometer. Muscle power production was reduced only with the 40 s RI. The authors reported that shorter RI may not be sufficient to allow power output recovery between sets of resistance exercise. However, these investigations were conducted using single-joint knee extension isokinetic exercise (Pincivero et al., 1997) and bench press exercise (Abdessemed et al., 1999), which may have limited application for lower-body multi-joint isoload exercises. In addition, most athletes that rely on lower-body power (i.e. volleyball, basketball, soccer, American football, and etc.) usually perform squat power training to improve knee extensors power performance.
Therefore, due to the limited amount of scientific evidence on the effect of RI on lower-body power training, the purpose of the present study was to compare maximal power output, muscular activity and blood lactate concentration following 1, 2 or 3 minutes RI between sets during squat training in young resistance-trained men. Our hypotheses was that 2 and 3 minutes RI between sets would allow greater neuromuscular recovery and lower metabolic impact (i.e., lower lactate responses) when compared to 1 minute RI during a multiple-sets squat power training exercise protocol.
Experimental approach to the problem
A repeated-measures design was used on a nonrandomized convenience sample of younger resistance trained men. Subjects were tested for power output (average and peak), blood lactate concentration, and muscle activity of the vastus medialis, vastus lateralis and rectus femoris during three different testing protocols. The exercise protocol was six sets of six repetitions of squat exercise, and the RI between sets was 1, 2, and 3 minutes. The RI between sets was randomly assigned across the three testing days separated by a minimum of 72 hours. A squat exercise was used because it is often used for training in sports (Gullett et al., 2009).
A priori statistical power (1-[beta]) was calculated for each dependent variable using G * Power software (Faul et al., 2007). The following design specifications were taken into account: [alpha] = 0.05; (1-[beta]) = 0.8; effect size f = 0.25; test family = F test and statistical test = ANOVA repeated measures, within-within interaction. The sample size estimated according to these specifications was 12 subjects. Therefore, twelve resistance-trained men (age = 22.7 [+ or -] 3.2 years; height = 1.79 [+ or -] 0.08 m; body mass = 81.8 [+ or -] 11.3 kg; 1-RM smith squat = 131.7 [+ or -] 26.7 kg) volunteered to participate in this study. Participants were considered "resistance-trained" (at least six months of consistent RT experience) based on the criteria set by the American College of Sports Medicine (Kraemer et al., 2002). In general, the subjects in their habitual training routine practiced noncompetitive resistance exercise for the purpose of muscle hypertrophy. Their training routine included four to six sessions per week, performing six to 12 sets per muscle group, and six to 12 maximum repetitions per set with 60 to 120 s of RI between sets. Subjects were excluded if they had a history of musculoskeletal injuries or any disease that could compromise their health during the study. The investigation was approved by the local institutional review board for use of human subjects, which is in accordance with the Declaration of Helsinki, and all the subjects read and signed an informed consent form before participation. Subjects were asked to maintain their normal hydration status and dietary intake, to avoid any strenuous exercise in the 48 h before the experimental sessions, and avoid smoking, alcohol and caffeine consumption for 24 h before all tests.
The maximum strength of the lower limbs (1RM) was assessed during the squat exercise using a Smith machine and according to the recommendations of Brown and Weir (2001). A pre-determined range of motion and proper technique were required for each successful 1RM trial. In addition, an elastic band was placed behind the subject to keep the bar displacement and knee angle (~80[degrees]) constant on each squat repetition. The position of each subject were recorded and reproduced throughout the study. The testing procedures were performed by the same investigator, and the determination of 1RM load was conducted on two non-consecutive days (test-retest) with a minimum of 72 h between tests. The intraclass correlation coefficient (ICC) was used to determine the test-retest reliability (r = 0.99) (Stratford, 2008). The determination of 1RM enabled the calculation of training loads (60% of 1RM) used during the experimental sessions.
Figure 1. Experimental design. 1RM test Squat exercise 1RM test Squat exercise Experimental Sessions (Random order) 6 x 6 reps using 1 min of rest interval 6 x 6 reps using 2 min of rest interval 6 x 6 reps using 3 min of rest interval Experimental sessions
An overview of the experimental protocol is presented in Figure 1. The three experimental sessions were conducted after three to seven days following the 1RM determination. Each session consisted of performing the squat exercise with 60% of their pre-determined 1RM (Loturco et al., 2013...