An important goal of clinical exercise and athletic training programs is to increase strength and muscle mass while improving cardiovascular fitness and optimizing body composition. Such programs typically involve the concurrent use of strength and endurance training sessions; however, this may be problematic when access to training facilities or supervisors is limited or when individuals perceive they have limited time to train. Thus, the identification of a single form of physical training that promotes broad physical fitness adaptations would be of great benefit to physical training specialists.
Clinical and athletic training programs aim to improve a variety of fitness parameters including muscle strength, muscle mass, cardiovascular fitness, and body composition (4,7,15,40). Indeed, strength training has been shown to increase both health (3,22,34) and physical fitness (19,23,33). Traditional strength (TS) training, which requires an individual to lift heavy loads with moderate interset recovery periods using free weights or resistive machines, is commonly performed for these purposes (24) and is particularly associated with substantial increases in bone (8,28,31) and muscle mass and strength (6,16,30,37). These increases in mass and strength are important both for improving many aspects of athletic performance (17,32,41) and movement function and skeletal health in aged and clinical populations (11,36,37,39). Nonetheless, there are several disadvantages of TS training, including that (a) the time required to complete a training session can be long when many exercise sets are performed with reasonable interset rest, (b) health-influencing cardiovascular benefits may be moderate when compared to other forms of training (e.g., aerobic training), and (c) there is often minimal body fat loss after a period of training. These limitations are typically overcome by implementing a concurrent aerobic conditioning program and dietary control.
Alternatively, circuit training, where lighter loads are lifted with minimal rest, has proven very effective for increasing maximum oxygen consumption, maximum pulmonary ventilation, functional capacity, and strength while reducing body fat and improving body composition (9,15,18,29). Thus, circuit training could be used to improve health and physical fitness, without the additional requirement for specific aerobic conditioning. To this end, less time needs to be devoted to physical training to elicit demonstrable improvements in health and physical fitness. A significant drawback of normal circuit training programs, however, is that the loads lifted are typically low, so the stimulus for strength and muscle (18) and bone mass (21) adaptations is minimal. This is a notable problem for aged and clinical populations, for whom increasing muscle and bone mass and functional strength is essential for improving functional capacity, or for those exercisers whose goals include increasing mass and strength for other reasons.
It was recently shown that healthy subjects were able to produce the same muscle performance under heavy loading conditions during a heavy strength training circuit session lasting ∼12 minutes as they could during a TS training session, yet the cardiovascular response to the circuit was significantly greater (1). These data are suggestive of the novel possibility that circuit training programs using heavy loads might elicit substantial strength and bone and muscle mass gains while improving cardiovascular fitness and body composition. To our knowledge, the response to chronic heavy circuit training has not been determined previously despite it being the best possibility of promoting broad physical fitness adaptations. Thus, to determine the efficacy of such a program, this study compared the effects of high-resistance traditional and circuit strength training on muscle size and strength, body composition, and measures of cardiovascular fitness in healthy young adults.
Experimental Approach to the Problem
A longitudinal, randomized design with a 1-week familiarization phase was used. The independent variable was ‘training program.’ Dependent variables included muscle size and strength, power, body composition, and measures of cardiovascular fitness. After the 1-week familiarization phase, the subjects were divided into 3 groups: high-resistance circuit (HRC) training, TS training, and a control group (C). After a week of familiarization, an 8-week training period was implemented for the training group.
Forty healthy (HRC = 15; TS = 15; and C = 10 participants), trained men volunteered for the study (age: 22.7 ± 3.3 years). By the end of the study, 33 participants (body mass: 75.2 ± 8.1 kg; height: 1.76 ± 0.6 m) completed the study with the largest portion of withdrawals resulting from injury sustained outside the research (HRC, TS, and control groups contained 15, 11, and 7 subjects, respectively). Attendance at training sessions was monitored. Two subjects were omitted from the study, because <85% of scheduled sessions were completed. All subjects had been regularly performing resistance training (RT) in a gymnasium (e.g., ca. 6-12 repetitions per set, 3 sets per exercise, 2-4 d·wk−1; no “Olympic” or “power” lifting training) for at least the previous 12 months, could produce a force equal to twice their body mass during an isometric squat lift and had no recent injuries or medical conditions that would prevent maximal exertion. All subjects performed upper- and lower-body RT twice per week. The participants agreed not to take ergogenic aids, supplements, or medications that might influence performance. The subjects read and signed statements of informed consent before participation in the study. Approval for the study was given by the Human Subjects Ethics Committee of the San Antonio Catholic University of Murcia, Spain.
Subjects completed 1 week of familiarization RT before the 8-week training period. During the 1-week familiarization phase, the subjects performed 3 RT sessions. The purpose of such training was to ensure the subjects were performing similar training and that their training compliance was high before the specific training phase. In each RT session, loads were adjusted so that subjects could perform only 6 repetition maxima (6RM) of leg curl, bench press, standing calf raise, lat pull-down, half squat, and preacher (biceps) curl exercises, which were typical of the RT exercises used by the subjects in their previous training and were used in this study.
Traditional Strength Training
Subjects in the TS group performed RT 3 times a week for 8 weeks with at least 1 day of rest between each training day. The volume progressed from 3 sets for each exercise in the first week to 6 sets in the eighth week (i.e., 1 set increase every 2 weeks). To warm up, the subjects performed 2 sets of 3 exercises (leg curl, bench press, standing calf raise) using the following sequence: 10 repetitions at 50% of 6RM, 1-minute rest, 8 repetitions at 75% of 6RM, 2-minute rest, and then the first main training set (i.e., 100% of 6RM). The 6RM load was adjusted for the subsequent test by approximately 2% if a subject performed ±1 repetition, and was adjusted by approximately 5% if a subject performed ±2 repetitions (22). In every session of each week, the subjects lifted weights that allowed only 6 repetitions to be performed (6RM, ca. 85-90% of 1RM). The eccentric phase of each exercise was performed for approximately 3 seconds, whereas the concentric phase was performed at maximum velocity. This sequence was standardized in the first training week and eccentric phase duration was regularly timed as feedback for the subjects. Three minutes of passive rest separated sets (Figure 1). After completion of the first 3 exercises, the subjects completed the other 3 exercises (lat pull-down, half squat, and preacher curl) with the same warm-up sequence (∼5 minutes). The subjects were supervised by an experienced lifter to ensure that volitional fatigue was achieved safely, and the control of the rest was strict. The total training time in the TS group ranged between 105 minutes (3 sets) and 125 minutes (6 sets).
High-Resistance Circuit Training
Training performed by the HRC group differed from TS only in the rest interval between the exercises. Although subjects in the TS group performed the exercises one after the other with a rest between each set, HRC subjects executed the training in 2 short circuits. The first circuit consisted of leg curl, bench press, and standing calf raise exercises. The second circuit consisted of lat pull-down, half squat, and preacher curl exercises (Figure 1). These 2 short circuits were performed for 3-6 series (progressing biweekly). A ∼5-minute period between the circuits allowed for a warm-up using the exercises used in the second series. Approximately 35 seconds separated each exercise, which was enough time only to move safely between exercises. The warm-up, intensity, and the volume of exercise were the same as that completed by TS. Again, the subjects were supervised by an experienced lifter to ensure that volitional fatigue was achieved safely, and the rest periods were strictly adhered to. The total training time in the HRC group ranged between 55 minutes (3 sets) and 78 minutes (6 sets).
Before and after the 8-week specific training phase, and over successive days, testing included (a) body composition examination using dual x-ray absorptiometry (DEXA), (b) maximum dynamic strength (1RM) testing on the bench press and half squat exercises, and peak power output in the bench press exercise using resistances of 30, 45, 60, 70, and 80% of 1RM, and (c) maximum lactate (Lactmax) and maximum power testing during a 30-second Wingate test. Two days later, a shuttle-run test was completed. Test order was repeated in the same order at the same time of day at posttraining. Three days of rest separated the last training session of each phase from the start of testing. Subjects were required to attend testing in a fully hydrated state.
Maximum Dynamic Strength
The subjects were carefully familiarized with the testing procedures of voluntary force production of the muscle groups tested. In all tests of physical performance, external verbal encouragement was given to each subject.
A modified Smith machine that consisted of a bar that moved freely on rollers in the vertical plane was used to measure maximal bilateral concentric force production of the lower and upper limbs (ULs). The 1RM half squat strength was recorded as the maximum weight that subjects were able to lift in a half squat (90° knee angle); an assistant gave a “go” signal when the lifter attained the proper joint angle (this was practiced extensively in familiarization). For safety, we limited the range of motion to a maximum of 80° by placing an adjustable chair under the subjects, and each repetition was inspected to ensure the knee angle was ∼90°. After the general warm-up, subjects performed a specific warm-up using 50% (10 reps), 75% (6 reps), and 85% (3 reps) of their estimated 1RM. After this warm-up, the subjects' resistances were fixed at a critical value of 5% below the expected 1RM and were gradually increased after each successful performance until failure. The interval between each trial was 2-3 minutes, and 1RM was achieved within 3-5 attempts. The same procedure was used for the 1RM bench press strength. In this test, the bar could not be bounced off the chest, the feet had to remain in contact with the floor, and the buttocks had to remain in contact with the bench. In both tests, the last acceptable extension with the highest possible load was taken as 1RM.
On a modified Smith machine, a rotary encoder (Real Power, Globus, Codogne, Italy) 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 before 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 (13).
Fifteen minutes after testing for the maximum strength of the ULs, the subjects were asked to perform 5 sets of 3 repetitions of bench presses using resistances of 30, 45, 60, 70, and 80% of 1RM with a 3-minute passive rest between sets. The subjects were spotted by an experienced lifter to ensure that maximum velocity was achieved safely and the subject was confident under the weight. Loud verbal encouragement was given throughout. The eccentric phase of the lift was performed over 3 seconds and was timed by a digital metronome, whereas the concentric phase was performed at maximum velocity. Because of the significant 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; experienced spotters ensured that the bar was stopped at the top of its trajectory. Bar velocity and power during the concentric phase of the movement were measured for the exercise and for each repetition.
Total and regional bone, fat and lean (body mass - [fat mass + bone mass]) masses were assessed by DEXA (XR-46, Norland Corp., Fort Atkinson, WI, USA). The DEXA scanner was calibrated using a lumbar spine phantom as recommended by the manufacturer. Subjects were scanned in the supine position. Lean mass (g), fat mass (g), total area (cm2), and bone mineral content (BMC) (g) were calculated from total and regional analysis of the whole body scan. Areal bone mineral density (BMD; g·cm−2) was calculated using the formula BMD = BMC × area−1. Lean mass of the limbs was assumed to be equivalent to the muscle mass. The test-retest reliability (intraclass correlation coefficient [ICC]) for this device was very high (R2 = 0.999; p = 0.001) in both cases.
Maximum lactate and maximum power were determined using the Wingate Anaerobic Test. A cycle ergometer (828E, Monark, Vansbro, Sweden) fitted with a photoelectric cell to count the number of revolutions of the pedals was used. Seat height was adjusted to suit the subject, and toe clips were used to prevent the feet from slipping off the pedals. Subjects warmed up by pedaling for 3 minutes against a 2-kp load at 60 rpm. At the end of each minute, they were required to pedal as fast as possible against the actual relative resistance that they would be working against for 5 seconds. After the warm -p, and after 2 minutes of rest, the test started. The subjects were instructed to pedal as fast as possible from the beginning of the test and to try to maintain maximum pedaling speed throughout the 30-second period. The resistance applied was adjusted relative to body mass (0.075 × body mass, in kg) (27). The pedal revolutions were recorded mechanically for 30 seconds by a cycle monitor. Maximum power was calculated as the highest power output reached over a 5-second interval (38), and maximum lactate was determined as the maximum value registered after the 30-second test. Blood lactate was recorded using a finger prick test every 2 minutes after the Wingate test until the lactate concentration started to decrease. Blood was collected in a capillary tube and then placed in the portable blood lactate analyzer (Accusport, Sports Resource Group, Boehringer Mannheim, Hawthorne, NY, USA). This portable lactate analyzer has been found to be valid and reliable (5).
Aerobic endurance was assessed using the 20-m multiple-shuttle-run test (25). The first 2 stages of the 20-m test were used as familiarization and for a light warm-up before starting the test. Each subject ran between markers placed 20 m apart, starting at a speed of 8.5 km·h−1 (2.36 m·s−1). The running speed was increased by 0.5 km·h−1 (0.14 m·s−1) every minute. The running pace was regulated by a prerecorded CD of the instructions being played loudly during the testing, which signaled when the subjects needed to be at one or the other end of the 20-m course. Subjects tried to complete as many stages of the shuttle-run test as possible, and the test was terminated when they were unable to maintain the prescribed pace. The subjects were given a warning the first time they were behind the sound signal, and the test was stopped on the third warning. Results of the 20-m shuttle-run test are presented as time devoted to complete the test.
A multivariate analysis of variance (MANOVA) was used to examine pretraining differences in test performances; no significant differences were found so the groups were equivalent before the intervention. Main and interaction effects resulting from the intervention were analyzed using single or MANOVAs with repeated measures. Significant time effects were further analyzed using paired samples t-tests. Interaction (time × group) effects were examined using independent samples t-tests on the change scores for each group (i.e., pre-post changes). In some instances, the assumption of equality of variance was violated, according to Levene's test of equality of variance. In these cases, unequal variances were assumed. Effect sizes were calculated using Cohen's d and were reported when appropriate (small effect size; d = 0.2-0.3). An alpha level of 0.05 was set as the criterion for statistical significance for all analyses.
Bench Press and Squat Strength
Repeated measures ANOVA revealed a significant effect of time (p = 0.001) and an interaction effect (bench press, p = 0.001; squat, p = 0.002); there was no group effect (d < 0.2 in both variables). Bench press strength improved for the HRC group (19.5 ± 10.6 kg; p = 0.001) and traditional training (TS; 17.7 ± 9.0 kg; p = 0.001) but not the control group (Control; 1.94 ± 5.2 kg; p = 0.328) (Figure 2). Squat strength increased for HRC (44.2 ± 24.1 kg; p = 0.001) and TS (45.0 ± 21.3 kg; p = 0.001) but not for Control (11.3 ± 14.6 kg; p = 0.065). Independent t-tests showed that the increase in HRC was greater than in Control for bench press (p = 0.017) and for squat (p = 0.049) strength; the assumption of equality of error variances (Levene's test) was not satisfied, so unequal variances were assumed. The increase in TS was also greater than for Control in bench press (p = 0.001) and squat (p = 0.001). There were no differences in the changes in HRC vs. TS.
Bench Press Power
For the bench press (Table 1), there was an increase in power in HRC at 30% (p = 0.003), 45% (p = 0.001), 60% (p = 0.002), and 80% (p = 0.015), and increases in TS at 30% (p = 0.005), 45% (p = 0.001), 60% (p = 0.002), 70% (p = 0.002), and 80% (p = 0.007). There were no between-group differences in the bench power changes (d ≤ 0.2 for all variables) and no statistical change in the control group.
Shuttle Run Test
The repeated measures ANOVA revealed a significant interaction effect (p = 0.018). Paired t-tests revealed a significant decrease in the running time of Control subjects (−24.6 ± 18.5 seconds; p = 0.012) (Table 2) but a significant increase in the running time of HRC (29.6 ± 44.3 seconds; p = 0.027; d = 0.2) and TS (36.7 ± 48.8 seconds; p = 0.032) subjects. These improvements were not related to the subjects running harder in posttraining tests because there was no difference in maximum heart rate at the volitional end of the test. Independent t-tests showed that the changes in HRC (p = 0.014) and TS (p = 0.006) were significantly different from the changes in Control (d = 0.2). There were no differences in the changes between HRC and TS (p = 0.535).
There was no change in the peak lactate values recorded after the Wingate test for any group; however, there was both a time and time × group interaction effect for maximum power (d ≤ 0.2 for both tests). For maximum power, paired t-tests showed no change in Control (36.6 ± 68.6 W; p = 0.175) or HRC (25.1 ± 57.8 W; p = 0.128) but a significant increase in TS (63.3 ± 61.6 W; p = 0.007).
The repeated measures ANOVA revealed a significant main effect of time for the change in body fat (%) (p = 0.005; d = 0.2), total lean mass (p = 0.009, d = 0.2) and total fat mass (p = 0.017, d = 0.2). There were no changes in total BMD and BMC in any of the training groups. Paired t-tests revealed no changes in the Control group. The HRC decreased their body fat percentage by 1.5 ± 1.6% (p = 0.002), and TS showed a trend toward a decrease of 1.1 ± 1.9% (p = 0.082); HRC also increased lean mass by 1.5 ± 1.9 kg (p = 0.002) and TS increased lean mass by 1.2 ± 1.6 kg (p = 0.036) (Table 3).
The major finding of the present research was that 8 weeks of HRC training, which was similar to that which has been previously shown to elicit a considerable cardiovascular response (1), resulted in strength and muscle mass improvements that were similar to those obtained with TS training. This is an important finding because circuit training using lower loads has previously been shown not to promote comparable strength and muscle mass increases as traditional weights training (10). These results are also important because the strength increases were obtained in healthy men who had a consistent history of strength training (albeit using loads that were lighter than those used in this study) where further increases in strength are considered difficult to obtain (2). The ability for HRC training to improve strength and muscle mass in already-trained individuals is suggestive that it might be of substantial benefit in athletic populations, and perhaps be of even greater benefit in elderly and clinical populations who begin training programs with a limited physical capacity.
A second major finding of the study was that the circuit training elicited similar increases in muscular power as traditional heavy strength training, as measured using a range of loads (30-80% of 1RM) in the bench press test. Given that circuit training is typically associated with greater muscle fatigue and hence positive adaptations in muscular endurance (12), this result might seem somewhat surprising. However, the increased power could be attributable to 2 main factors: (a) the low-repetition, high-load training resulted in comparable strength improvements to the TS training, and (b) the loads were lifted with the intention of moving fast. In this sense, increases in both force-generating capacity and muscle shortening speed (for a given load) or rate of force development could be expected, and thus increases in power are likely to result. In future studies, the importance of lifting the loads quickly in the concentric phase should be explored further; the possibility exists that this movement constraint is important for improvements in muscle power with this type of training.
A secondary, but interesting, result is that the strength and muscle mass changes in the circuit training group were accompanied by significant reductions in body fat (1.5%), as measured by DEXA, and increases in performance in the 20-m shuttle-run test. The decrease in body fat percentage resulted from a simultaneous increase in lean mass (+1.5 kg) and decrease in fat mass (−1.1 kg) with the circuit training, which compared to lean and fat mass changes of +1.2and −0.8 kg, respectively, with traditional heavy strength training. It would be of interest to determine whether these (nonsignificant) between-group differences become more apparent with longer training periods, as substantial changes in already-training individuals are notoriously difficult to obtain. Further research is also required to examine the effects of dietary interventions that may be implemented concurrently with these training practices because these will strongly influence body fat loss (26); subjects in this study were asked to maintain their normal diets but no restrictions were placed on them. The increase in the performance in the 20-m shuttle-run test is also intriguing. One explanation for the improvement is that the circuit training elicited a strong cardiovascular benefit, which resulted in transferable improvements in aerobic performance (20). However, there was also a significant increase in the TS training group, who would not likely have received such benefits. Another explanation is that the subjects performed better on the second occasion because of a learning effect, however, the statistical decrease in the non-training control group, who ran for ∼25 seconds less after 8 weeks, is suggestive that learning effects were small or nonexistent. More than likely, the increased endurance performance in the strength training groups resulted from an increase in functional capacity resulting from their increase in strength and power. Although VO2max was not being directly measured, one might hypothesize a greater oxygen use in maximal exercise when muscle mass increases, and this possibility should be examined in further research. Either way, the present data are suggestive that increases in strength, power and muscle mass might also be associated with improvements in functional performance in unrelated activities, which has implications for athletic and clinical populations alike.
Despite there being similar increases in strength, power and muscle mass, the TS group improved more in their peak cycle power (as measured in the first few seconds of the 30-second Wingate test). This result is difficult to explain because both groups improved their lower limb muscle strength equally (as measured in the squat lift test) and had similar increases in lean mass in the lower limb muscles (data not shown) according to the DEXA scans. Also, the lack of change in peak lactate concentrations after the 30-second test in both groups is suggestive that peak anaerobic metabolism was not altered, although, the possibility exists that any changes were mirrored by increases in the rate of lactate clearance. Although the result is curious, and might be examined in further detail in future studies, the functional significance of the result in this cycle-specific test is probably minor compared to the more important increases in strength and muscle power shown in the other tests.
It is worth mentioning that, although there were significant increases in muscle mass after training, there were no changes in BMD or BMC in either the training or control groups (Table 3). This result is unsurprising given that the subjects were active young men who had a high BMC before the study. Increases in BMC tend not to occur in healthy, skeletally mature men (14), and changes in BMC need long-term training However, given that it has been shown to have positive effects in elderly subjects (35), and the circuit training performed in this study elicited the same neuromuscular adaptations as traditional training, it could be hypothesized that heavy circuit training would have a significant positive effect on BMC in older individuals. Future research should examine further the effects of this type of training on skeletal health in older populations.
The present data show that 8 weeks of high-resistance (6RM) circuit training is both well tolerated and can lead to substantial strength, power and muscle mass gains in young, healthy individuals. Remarkably, these gains are identical to those obtained with traditional, heavy strength training. These results are of particular significance for athletic populations who might have relatively little time to devote to physical conditioning because of the need to perform more specific sports practice. The results also have implications for program design because both groups were able to complete the same work and achieve the same strength increases but the HRC group did so in less time. Thus, the HRC was more efficient and might therefore be useful for individuals who perceive that a lack of time available for training is a substantial exercise deterrent. Nonetheless, given the previously detailed benefits of circuit training on maximum strength, health and aerobic capacity, and the previously documented cardiovascular load induced by heavy resistance circuit training, a more extensive program of research should also examine its effects on strength, muscle and bone mass, and health status, in elderly and clinical populations.
The researchers would like to express gratitude to the subjects in this investigation who made this study possible. In addition, the authors would like to acknowledge “Los Rectores” Sport Gym (Espinardo, Murcia, Spain) for their assistance throughout this project. No funding was received for this article. 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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