Resistance training is an excellent method for stimulating increases in muscular size, strength, power, and local muscular endurance (3,17). However, gains resulting from the training are dependent on a requisite volume of training being performed at a relatively high intensity; these parameters generally need to increase as training progresses for gains to continue (3). This requirement makes it difficult for many exercisers to train for long enough in each session to achieve significant strength gains because the increased time devoted to resistance training would normally preclude their participation in endurance-type activities that improve cardiovascular function, reduce body fat levels, and ultimately improve general health. In order to reduce the time devoted to strength training but allow a high enough volume of training to be performed, many exercisers use superset (i.e., the alternating performance of 2 exercises with a short rest between each) and circuit training (i.e., the performance of a series of exercises with no rest between them) practices. Of particular interest to many is the practice of circuit training because it allows the development of local muscular endurance and aerobic fitness (1,9-13,18) while also increasing strength and power (5) in a time-efficient manner.
Nonetheless, circuit training has traditionally been performed using relatively low loads for a relatively high number of repetitions in each set in order to improve local muscular and aerobic endurance. This is at odds with the requirement for high loads (>85% of maximum) to be used for maximum muscle size and strength gains (8). Since many studies examining the benefits of the circuit training have used higher volumes with lighter loads (1,9-13,18), it is unclear whether exercisers attempting to use high loads during a circuit program would be able to develop and maintain a high cardiovascular output. Furthermore, the rest period allowed between successive exercise sets significantly affects metabolic (16), hormonal (14,15), and cardiovascular (7) responses, so it also must be determined whether exercisers would be able to use loads similar to those that they would have had they had a significant period of passive rest between sets.
Given that little is known about the physical responses to high-load circuits, we compared physical performance parameters (number of repetitions completed, speed of the concentric phase of the movement, and power developed during the concentric phase of the movement) and cardiovascular load (heart rate) during a heavy-resistance circuit (HRC) to the responses when a standard passive rest was allowed between sets. The study is important to determine whether HRC training could be a viable alternative to more traditional low-resistance circuits.
Experimental Approach to the Problem
A randomized, counterbalanced, crossover design with familiarization was used. The independent variable was intervention, being either passive rest between resistance training sets or active exercise between sets (i.e., a circuit). Dependent variables included set volume, total volume, average and peak concentric movement speed, average and peak concentric power, average set heart rate, average rest heart rate, and total average heart rate.
Ten men (Table 1) who had performed dynamic, free-weight strength training for at least 1 year (intermediate status) volunteered for the research. The subjects did not take ergogenic aids or medications that might influence performance. Written informed consent was obtained from each subject; approval for the study was given by the Human Subjects Ethics Committee of the Catholic University San Antonio of Murcia.
Testing was performed once weekly for 3 weeks, with all testing being conducted at the same time of day. On day 1, subjects were familiarized with the test and training exercises, and their 6 repetition maximum (6RM) loads were determined for bench press, leg extension, and ankle extension exercises according to standard procedures (12). On the subsequent 2 test days, subjects performed 1 of 2 strength training programs; the order was chosen randomly, although the order was counterbalanced (Figure 1).
Traditional Strength Set Training Session
To warm up, the subjects performed three sets of the bench press exercise using the following sequence: 10 repetitions at 50% of 6RM, 1-minute rest, 8 repetitions at 75% of 6RM, 2-minute rest, and 1 set of bench presses to volitional fatigue at 100% of 6RM; subjects performed active stretching between rests. The 6RM load was adjusted for the subsequent test by ∼2% if a subject performed ±1 repetition and was adjusted by ∼5% if a subject performed ± 2 repetitions (6). The main phase of testing commenced after a 5-minute rest. Before the testing, a heart rate monitor (Polar S625X, Finland; 0.2 pulses·s−1) was strapped to the subject's chest; heart rate was continuously recorded for the duration of the test. The bench press test was performed on a modified Smith machine that consisted of a bar that moved freely on rollers in the vertical plane. A rotary encoder attached to the barbell and interfaced with a computer allowed the recording of bar position with an accuracy of 0.002 second; the system was calibrated prior to each testing session and bar velocity and power (using the measured load) were subsequently calculated. The validity and reliability of the device have been reported elsewhere (6).
For testing, the subjects were asked to perform 5 sets of bench presses to volitional fatigue (determined as the point at which full elbow extension could not be gained without assistance) with 3-minute passive rest between sets. The subjects were spotted by an experienced lifter to ensure that volitional fatigue was achieved safely and with the confidence of the subject. Loud verbal encouragement was given throughout. The eccentric phase of the lift was performed for 3 seconds and was timed by a digital metronome, whereas the concentric phase was performed for maximum velocity; the session lasted ∼13.2 minutes. Because of the load lifted, the subjects were able to push maximally throughout the movement range without the bar escaping the subject's grip at the top of the movement. Bar velocity and power during the concentric phase of the movement were measured for each repetition.
Heavy Resistance Circuit Session
After warm-up on all exercises (performed as in the traditional strength training (TS) condition), the subjects again performed 5 sets of the bench press exercise to volitional fatigue. However, in the HRC condition, the subjects also performed 1 set each of 6RM leg extension and ankle extension exercises between bench press sets; ∼35 seconds separated each exercise. Heart rate and bar movement measures were taken as in the TS condition.
Standard statistical methods were used for the calculation of means, SDs, and Pearson's correlation coefficients (r, R 2). Differences between active and passive conditions were examined using repeated-measures analysis of variance, with intervention (rest) as factor. Intraclass reliability was assessed in the first set of bench presses in each of the 2 conditions. The α level for all tests was set at 0.05.
Bar velocity, bar power, and heart rate data are shown in Table 2. There was no difference in the number of repetitions performed in the 2 conditions (TS = 21.7 ± 3.6 repetitions; HRC = 20.8 ± 3.3 repetitions). The intraclass correlation coefficient (ICC) was high (0.97) for the 6RM bench press exercise.
There were also no differences in the maximum or average bar velocities achieved during the concentric movement phase of the bench press exercise between the 2 conditions (ICC for average velocity = 0.79). Subsequently, there were also no differences with respect to the mean and peak power (ICC for mean power = 0.74), as shown in Figure 2.
Despite there being no detectable performance differences for the bench press exercise, the average heart rate was significantly higher in the HRC condition compared to TS condition (HRC = 129 ± 15.6 beats·min−1, ∼71% maximum heart rate [HRmax]; TS = 113 ± 13.1 beats·min−1 ∼62% HRmax). As shown in Figure 3, this difference is attributable to a significantly lower resting heart rate in the TS condition (set 5: TS = 111 ± 16.3 beats·min−1; HRC = 131 ± 20.0 beats·min−1); there were no differences in the heart rates achieved during the bench press exercise, although differences (HRC > TS) tended to be greater as the exercise session progressed.
Circuit training has been commonly used as a tool to simultaneously stimulate muscle size/strength increases alongside improvements in local muscle endurance and aerobic capacity. However, the loads used during circuits have traditionally been low in order to allow a greater amount of work to be performed, whereas the loads that stimulate maximum strength adaptations and muscle mass gains are necessarily high (8). In order to better understand the physical performance and cardiovascular effects of using heavy loading in a circuit session, we examined bar velocity and power, total workload, and heart rate responses during a bench press training session in which alternative exercises (leg extensions and ankle extensions) were/were not performed during the 3-minute rest period. We found that the performance of alternating exercises during rest did not significantly affect the ability for the subjects to perform the concentric phase of the bench press lift (as indicated by a lack of difference in both bar velocity and, logically, power), nor was the total volume for the bench press exercise different between the 2 groups. Thus, the total loading imposed on the subjects was the same regardless of whether the rest period was active or passive. Although it remains to be tested, we predict that the stimulus for hypertrophy would have been similar in the 2 conditions; future research examining both endocrine and myocellular responses to the training is warranted.
Importantly, we also found that the heart rate responses differed markedly during the rest period between the 2 conditions (e.g., set 5: HRC = 131 ± 20.0 beats·min−1; TS condition = 111 ± 16.3 beats·min−1), which resulted in the average heart rate throughout the 13.2-minute exercise program also being higher in the HRC condition. Interestingly, heart rates measured during the bench press exercise were not different between the groups. Regardless, the heart rate data indicate that a greater cardiovascular load was imposed in the HRC condition. This would likely be important for stimulating adaptations that improve cardiovascular function and, ultimately, exercise endurance; interset recovery during resistance training has been previously shown to have an impact on cardiovascular responses (7). Indeed, when using age-predicted HRmax as a surrogate for the measured maximum (2), we found that the HRmax achieved in the HRC were approximately 71% of maximum. This is well within the 60-90% range that is suggested by the American College of Sports Medicine (ACSM) (4) for the development of cardiorespiratory fitness and promotion of body composition changes. By contrast, heart rates during the TS condition were only ∼62% of maximum, which falls at the lower end of this range. Extension of the current circuit program to the use of other exercises could also easily increase the exercise time above the minimum limit (20 minutes) set by the ACSM. Our results strongly suggest that longer term studies be conducted in order to more completely assess the impact of higher resistance circuit programs compared to more traditional circuits on both muscle strength and cardiovascular function.
Circuit training is an excellent strategy for reducing the time devoted to strength training, while allowing a sufficient volume of training to be performed. Nonetheless, circuit training has traditionally been performed using relatively low loads for a relatively high number of repetitions in order to improve both local muscular and aerobic endurance. The present data indicate that heavy-resistance (6RM; 3-minute active rest between sets) circuit training allows the same loading to be imposed as achieved with traditional strength training (3-minute passive rest between sets). We therefore predict that the stimulus for hypertrophy would have been similar in the 2 conditions. We also found that heavy resistance circuit training imposed a greater cardiovascular load (∼71% of HRmax) than traditional strength training (∼62% of HRmax). Consequently, this training form would likely be important for stimulating adaptations that improve cardiovascular function and exercise endurance. Given these results, longitudinal studies investigating both the endocrine-myocellular adaptations and muscle strength-cardiovascular adaptations are warranted.
The researchers express their gratitude to the subjects in this investigation who made it possible. In addition, authors acknowledge the personal trainer Jesús V. Gómez (Corpore Sport Gym, Torres de Cotillas, Murcia, Spain), Dr. Luis Carrasco (Universidad de Sevilla, Sevilla, Spain), and Professor Carlos Pérez (Ergotech, Zarandona, Murcia, Spain) for their assistance throughout this project. The results from this study do not constitute endorsement of the products by the authors or by the National Strength and Conditioning Association.
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