Resistance training and endurance training are often performed concurrently in most exercise programs in wellness, fitness, and rehabilitative settings in an attempt to obtain gains in more than 1 physiologic system to achieve total conditioning, to meet functional demands, or to improve several health-related components simultaneously. This is done even though it is believed that concurrent resistance and endurance training may impede the strength gains achieved by resistance training alone (4,8). However, this belief is not supported by unequivocal evidence, and little is known about the effect of this mode of training on initial strength development or strength development for health (12,17).
Research has alluded to a possible optimal resistance training dose to optimize muscular strength development in certain populations, and it appears that training status influences the requisite dose and potential magnitude of strength development (5,18). Individuals seeking muscular strength development for general health need a resistance training dosage of a mean intensity of 60% of 1 repetition maximum (1RM) performed for 3 days per week, whereas recreationally trained nonathletes and athlete populations require mean training intensities of 80% of 1RM and 85% of 1RM, respectively (18). The discrepancies between studies examining the effect of concurrent resistance and endurance training on strength development may be related to differences in design factors, including the mode, frequency, duration and intensity of training, training history of participants, scheduling and length of training sessions and dependent variable selection, age, the upper limit of the subject's genetic potential, and the way in which the 2 modalities of exercise are integrated (4,10,13,14,17,19). In this regard, studies investigating the effect of concurrent training on strength in sedentary or untrained populations have prescribed atypical training programs (i.e., single-leg training design) (19) that either prescribed impractically high training frequencies that repeatedly stressed the same muscle group (3,4,8,13), prescribed unusually high numbers of repetitions per set not conducive to strength development (4), used programs with short durations that do not allow for adequate strength development (4,8), determined strength development at velocity specific measurements (4,17), or compared males and females (4,8) (Table 1). These design factors could impede the comparison between the programs and even compromise strength development in the concurrent programs because of residual fatigue, glycogen depletion resulting from the endurance component, a reduced ability to develop tension, or overtraining (4,8,14,16).
In considering the paucity of data and results from problematic experimental designs, this study aimed to compare the effects of 16 weeks of resistance training with concurrent resistance and endurance training on muscular strength development in previously sedentary, healthy males. It was hypothesized that moderate-intensity resistance and concurrent resistance and endurance training would lead to similar improvements in muscular strength.
Experimental Approach to the Problem
Because of the abundant controversy on the interference of concurrent resistance training and endurance training on strength and its role in initial muscular strength development and for general health, this study was undertaken to determine how sedentary, apparently healthy males adapt to a combination of strength and endurance training as compared with the adaptations produced by strength training alone. The strength training program used in this study was designed to optimize efficiency, safety, and effectiveness in sedentary, apparently healthy males and is similar to that reported previously (3,5,16,18). The study made use of a quantitative pretest, post-test research design and 16 weeks of training under carefully monitored conditions to examine the training effects on initial or general health-related strength development. Strength was measured at baseline, every 4 weeks during the experimental period, and at post-test. All other identical measures were taken before and after the 16-week experimental period.
The study was approved by the Department of Sport and Movement Studies' Research Committee, and written informed consent was obtained from each subject. Thirty-eight sedentary, apparently healthy males were age-matched and were then randomly assigned to either a nonexercising control (Con) group (n = 12), resistance training (Res) group (n = 13), or concurrent resistance and endurance training (Com) group (n = 13). All subjects were required to be sedentary for 6 months before the study, on no pharmacologic agents known to affect muscular strength, had to be weight stable for at least 6 months before the study, and free of medical conditions prohibiting exercise, and all subjects had to be male. Males were selected because men and women display significant differences in strength and strength development. The subjects in the Con group had to remain sedentary throughout the 16-week experimental period. The baseline data of all the subjects is reflected in Table 2.
Body Composition Measurements
Subjects were weighed in kilograms to the nearest 0.1 kg on a calibrated medical scale (Mettler DT Digitol, Mettler-Toledo AG, Ch-8606 GreiFensee, Switzerland) wearing only running shorts. The body fat percentage was obtained by using the 7-skinfold method (triceps, subscapular, supra-iliac, abdominal, frontal thigh, mid-axilla, and pectoral skinfolds) of Jackson and Pollock (11). Skinfolds were measured before any exercise using a manual skinfold caliper (Harpenden John Bull, British Indicators, Ltd., Burgess Hill, UK). Fat mass was calculated by multiplying body mass with body fat percentage divided by 100. Lean mass was calculated as total body mass in kilograms subtracted by fat mass in kilograms.
Because of the sedentary nature of the subjects and in an attempt to avoid injuries, each subject underwent a 10 repetition maximum (10RM) evaluation (7) on each of the prescribed exercises used in the Res and Com sessions. The prescribed exercises included shoulder press, latissimus dorsi pull-downs, seated chest press, low pulley row, crunches (modified sit-ups), unilateral leg press, unilateral knee extensions, and unilateral prone hamstring curls. The 10RM evaluations took place at the start and completion of the experimental period and every 4 weeks during the experimental period. Each subject performed a 5-minute warm-up, 8 static stretching exercises, followed by the completion of 5 to 10 repetitions of each of the prescribed exercises at 40% to 60% of their estimated 1RM. After this warm-up, each subject had to stretch the muscle/muscle group concerned and then completed 10 repetitions at approximately 70% of estimated 1RM. If the subject was successful at performing 10 repetitions, the weight was increased conservatively to the next weight increment allowed by the test exercise equipment. The subject rested 3 to 5 minutes before attempting to complete another 10 repetitions at the new weight increment. This procedure was followed until each subject completed no more than 10 repetitions. This value was then recorded as the weight lifted for the amount of repetitions, and each subject's 1RM value was then calculated using the following formula: 1RM = weight lifted/[1.0278 - (repetitions to fatigue × 0.0278)] (7). To determine the number of repetitions to be used during subsequent training sessions for crunches, each subject had to perform a maximum number of repetitions during 1 minute (15).
Periodized Training Programs
All subjects in the Res and Com groups were under direct supervision during the training sessions and were familiarized with the equipment before start of the experimental program. Table 3 provides a summary of the design of the main study. To control for the potential overtraining effects of stressing the same muscle groups too frequently and to equate the exercise programs in number of training days, a 3-day weekly regime was implemented (13,16). All exercise sessions were preceded by 5 minutes of easy cycling (heart rate [HR] < 100 beats per minute) and concluded with 5 minutes of cycling and 8 stretches for 2 sets of 30 seconds (1,2).
Subjects in the Res group performed 3 sets of 15 repetitions for each of the tested exercises at 60% of the estimated 1RM (18) during each training session using a combination of Polaris weight machines and York free weights. The subjects were required to perform 3 sets of crunches at 60% of the maximum number of repetitions performed in 1 minute during the baseline testing (15). In an attempt to equalize for time across the 3 exercising groups and to avoid overtraining, the present investigation made use of sessions that used both endurance training and resistance training in equal proportions. In this regard, each subject in the Com group was required to perform 2 sets of 15 repetitions at a workload of 60% 1RM and 2 sets at 60% of the maximum number of repetitions performed in 1 minute during the baseline testing. The Com group subjects were also required to exercise using a combination of treadmills, rowers, steppers, and cycle ergometers for 22 minutes at an intensity of 60% of their individual age-predicted HR maximum. Age-predicted maximum HRs were determined by subtracting age from 220. The endurance intensity was readjusted every 4 weeks with a 5% increase in HR and the exercise program adjusted accordingly. The estimated 1RM for each subject was re-evaluated every 4 weeks and his exercise program adjusted accordingly (15). A 60- to 90-second rest period was allowed between each set and each of the various exercises (20).
Data were analyzed using commercial software (Statistical Package for Social Sciences, Version 14, Chicago, IL, USA). Standard statistical methods were used for the calculation of the means and SD, whereas a dependent samples t-test was used for statistical comparison. Cronbach's alpha and the intraclass correlation coefficient (ICC) average measure was used to calculate and quantify test-retest reliability using the Con group's dependant variables, whereas Spearman's correlation coefficient was used to establish relationships between the variables. Alpha levels were set at p ≤ 0.05 for establishing statistical significance.
The Res training resulted in a significant decrease in body mass (p = 0.041), whereas Com training had no significant impact on body mass (p = 0.507). The lean mass of the Res and Com subjects both significantly increased after their respective programs (p = 0.005; p = 0.001, respectively).
Sixteen weeks of Res training significantly increased muscular strength during shoulder press (p = 0.001), latissimus dorsi pull-downs (p = 0.001), seated chest press (p = 0.001), low pulley row (p = 0.001), crunches (p = 0.001), leg press (p = 0.001), knee extensions (p = 0.001), and hamstring curls (p = 0.001) (Table 4). Similarly, 16 weeks of Com training resulted in significant increases in the weight lifted during shoulder press (p = 0.001), latissimus dorsi pull-downs (p = 0.001), seated chest press (p = 0.001), low pulley row (p = 0.001), crunches (p = 0.001), leg press (p = 0.001), knee extensions (p = 0.001), and hamstring curls (p = 0.001).
Test-retest reliability using the Con group's dependant variables indicated that when all dependant variables were considered, the ICC was significant at a 95% confidence level at pretest (ICC = 0.868), post-test (ICC = 0.837), and when considering both pre- and post-test (ICC = 0.927). When considering the individual strength measures, the ICC was significant (p ≤ 0.05) for shoulder press (ICC = 0.967), latissimus dorsi pull-downs (ICC = 0.966), seated chest press (ICC = 0.922), low pulley row (ICC = 0.911), crunches (ICC = 0.955), leg press (ICC = 0.867), knee extensions (ICC = 0.762), and hamstring curls (ICC = 0.762). As such, under ICC average measures, the average (or sum) of the scores of the 8 exercises (retest) are highly reliable, with an interval of 0.713 to 0.956 with 95% confidence.
Even though the Res training significantly increased body mass by a mean of 0.55 kg, it does not appear as if this could have resulted in the significant muscular strength increases. This is because significant (p ≤ 0.05) relationships were only found between body mass and lattisimus dorsi pull-down (R = 0.298), seated chest press (R = 0.348), low pulley row (R = 0.330), crunches (R = 0.377), and leg extension strength (R = 0.369). No significant (p ≤ 0.05) relationships were observed between body mass and shoulder press (R = 0.241), leg press (R = 0.235), and leg curl strength (R = 0.181). In turn, significant relationships were found between lean mass and shoulder press (R = 0.425), lattisimus dorsi pull-down (R = 0.494), seated chest press (R = 0.574), low pulley row (R = 0.560), crunches (R = 0.510), leg press (R = 0.503), leg extension (R = 0.538), and leg curl strength (R = 0.386). As such, the increases in lean mass after Res and Com training could have resulted in the significant increases in all the measured strength parameters.
The primary results of this study demonstrated that resistance training and concurrent resistance and endurance training both improved strength in all of the 8 prescribed exercises and that concurrent resistance training and endurance training was as effective in developing muscular strength initially or for general health as resistance training alone in previously sedentary or untrained, healthy males. This finding is in agreement with similar previous concurrent training studies (3,5,16,17). However, this is in contrast with studies that have found that strength development was inhibited when concurrent programs were compared with resistance-only programs (4,6,8,9,10,19).
The results of the present study make it difficult to support the general concept of the interference effect because this appears to hold true in specific situations only and only when optimal strength development is required. This may be so because those individuals seeking muscular strength development for general health may need a much lower resistance training dosage than recreationally trained nonathletes and athletes, and thus these lower training doses may limit or negate the interference effect. Furthermore, the addition of sets to a resistance training program does not necessarily appear to result in enhanced gains in strength, regardless of whether endurance training is included in the training program. This is evidenced by the additional set of resistance training that was prescribed to the Res group not resulting in a larger increase in strength.
Future studies should examine the interference effects arising from the order of resistance and endurance training exercises on strength (e.g., endurance training before resistance training or vice versa). This may be needed because resistance training before endurance training may lead to an increased strength-training effort, whereas performing endurance training before resistance training may lead to glucose and glycogen depletion before the resistance training component of the program, leading to peripheral fatigue and even overtraining. In this regard, further systematic research is necessary to quantify the inhibitory effects of concurrent training on strength development and to identify different training approaches that may overcome any negative effects of concurrent training. The results of this study indicate that concurrent training, in which the training occurs in the same session, is an effective, well-rounded exercise program that can be prescribed as an means to improve initial or general strength in sedentary, healthy males.
Recent research indicated that distinct muscular adaptations and dose-response relationships may exist for certain populations (5,18). This finding and that of supporting studies combined with the fact that the incidence of hypokinetic diseases continue to grow increases the importance of training programs that are able to elicit gains in more than one physiologic system to achieve total conditioning, to meet functional demands, or to improve several health-related components simultaneously. The present study's finding that performing simultaneous resistance and endurance training in the same workout does not impair strength development in sedentary, healthy males has important practical relevance for the construction of strength training programs, especially for disease prevention and in rehabilitative settings. This is because by concurrently performing resistance and endurance training, individuals may not only develop their strength, which improves quality of life and allows for an independent lifestyle, but also allows an individual to elicit the unique benefits each mode of exercise has to offer. Furthermore, clinicians must appreciate that although concurrent resistance and endurance training may not be compatible for optimal or peak strength development, this mode of training is effective for developing initial or health-related strength in individuals seen in wellness and fitness settings.
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