Individual and team sports, such as wrestling and football, require the expression of several components of physical fitness. Strength and endurance, among other fitness components, are demanded in varying degrees in all forms of athletic competition. The modern competitive athlete, regardless of discipline, frequently participates in various forms of resistance and endurance exercise simultaneously to develop strength and endurance to optimize physical performance. Published books, fitness guides, and scientific articles advise strength and conditioning professionals to prescribe programs that include both strength and endurance training concurrently (7,15). Additionally, the American College of Sports Medicine position stand on “The Recommended Quantity and Quality of Exercise for Developing and Maintaining Cardiorespiratory and Muscular Fitness, and Flexibility in Healthy Adults” (2) promotes the inclusion of both resistance and endurance training components in the exercise prescription for health-related fitness. Thus, it is of considerable interest from a practical point of view to determine whether concurrent resistance and endurance training influences the nature and magnitude of physiological adaptations compared with resistance or endurance-only training.
The adaptive responses of skeletal muscle to exercise training are considered to be primarily dependent on the intensity and mode of training performed (16,26,50). Resistance training involves the performance of low-repetition near maximal muscular contractions and has been shown to increase maximal contractile force (6). Such resistance training is primarily anaerobic. Endurance training, which represents a type of aerobic exercise, involves the performance of dynamic submaximal muscular contractions using large muscle groups and has been shown to increase maximal oxygen uptake (V̇o2max)(29,48). Muscular strength increases little or not at all as a result of endurance training (5,19,25) and V̇o2max changes are minimal or nonexistent after resistance training (25,28,33). Thus, it seems inconsistent to prescribe resistance training to individuals who seek to improve endurance or endurance training to individuals who wish to improve strength, because such prescription may violate the principle of training specificity (i.e., systemic vs. localized stimulus and corresponding adaptation). Despite this inconsistency, many competitive athletes and fitness enthusiasts participate in simultaneous resistance and endurance exercise training under the theory that they are improving their athletic performance and health-related fitness.
Skeletal muscle adaptations to either resistance or endurance exercise programs may be different and possibly antagonistic when both types of training are imposed concurrently. For example, resistance training induces primarily skeletal muscle hypertrophy (39) principally caused by an increase in myofibrillar protein content (12,38,40,59), resulting in an increase in maximum voluntary strength. Furthermore, resistance training may also induce a decrease in mitochondrial volume density (41) and capillary density (40,54,58). All of these changes may be detrimental to endurance performance. In contrast, endurance training has been shown to increase capillary density (3,14,35,53), mitochondrial volume density (30), and oxidative enzyme activity (22,53). Such changes favor improvements in endurance performance. Furthermore, endurance training may also induce a decrease in strength (43) and muscle fiber size (35,57). Thus, it seems that skeletal muscle may not be able to adapt optimally to the two antagonistic stimuli when they are concurrently imposed.
Despite the antagonism during concurrent training previously described, it may be possible to integrate select resistance and endurance training regimens to derive an additive or dual effect. Evidence that concurrent training may have an additive effect is supported by studies where resistance training increased short-term (4-6 minutes)(25,28) and long-term (60-90 minutes) endurance (27) and strength (7). Furthermore, some endurance training programs have resulted in muscle hypertrophy (22) and increased strength (49).
The question of whether concurrent training has an antagonistic or additive effect on strength development has been the topic of great interest over the past 30 years (11,18,56). Despite extensive research efforts, this area is not well understood and remains controversial. Several investigations used diverse experimental protocols comparing concurrent resistance and endurance training with resistance-only and endurance-only training and reported a plethora of conflicting results. Specifically, some studies report no compromise in strength or endurance development when integrating training regimens (4,8,42,52,55,61). Nelson et al. (45,46) concluded that endurance performance is impaired without a notable change in strength, whereas Hennessey and Watson (24) provided evidence that both strength and endurance performance are compromised. However, it should be noted that there is a great deal of evidence supporting the notion that the development of strength is significantly reduced when combining resistance and endurance training without concomitant reduction in endurance development (13,17,25,31,32,36,37,51).
The physiological mechanisms that mediate adaptive responses of skeletal muscle to concurrent training remain unclear. Possible explanations for compromised strength development may be related to alterations in neural recruitment patterns and attenuated muscle hypertrophy (11,18,36). Other conceivable explanations for compromised strength development during concurrent training include fiber type transformation (36), endocrine responses (8,36), and overtraining syndrome (10,24,45,21). Furthermore, the adaptive responses of skeletal muscle to concurrent training regimens probably also depend on the initial fitness level of the trainees, the training modes, the frequency, intensity, duration, and volume of training, and the method used to integrate the two forms of training (42,52). Consequently, it may be possible to combine resistance and endurance training without compromises in strength or endurance. In fact, Balabinis et al. (7) recommend concurrent training for early-phase strength improvement objectives.
Nevertheless, no published data are available from well-designed and controlled studies that compare the effect of different modes of endurance exercise on strength performance in concurrent training regimens. Different modes of endurance exercise are thought to impose specific physiological, biochemical, and biomechanical demands upon the skeletal muscle and seem to result in skeletal muscle adaptations specific to the mode of training (47). Similarly, resistance exercise is thought to impose specific physiological, biochemical, and biomechanical demands and seems to result in skeletal muscle adaptations specific to the resistance exercise movement. Hence, it may be possible that the physiological, biochemical, and biomechanical demands of select endurance training modes effect skeletal muscle adaptation differentially when combined with resistance training requiring similar biomechanical movement patterns in the same skeletal musculature, in which case, a concurrent training regimen involving an endurance training mode closely matching the biomechanical movement patterns of prescribed resistance training may result in either compromised or enhanced strength development when compared with a concurrent training regimen involving an endurance training mode with a less specific biomechanical movement pattern to the prescribed resistance training. That is, the closer the endurance exercise is biomechanically to the resistance exercise, the more likely an antagonistic or additive skeletal muscle adaptation will appear.
The present study examined the effect of two different modes of lower-body endurance exercise (i.e., incline treadmill walking and cycle ergometry) on lower-body strength development with concurrent resistance training designed to improve lower-body strength (i.e., leg press 1 repetition maximum [RM]). Although both modes of lower-body endurance training involve lower-body skeletal musculature, the author hypothesized that cycle ergometry would result in the expression of either a more pronounced antagonistic or an additive effect on strength development because of its more specific biomechanical movement pattern to the leg press criterion measure. Additionally, the two concurrent training groups' strength development was compared with a resistance-only group. The identification of the exercise protocol resulting in the greatest amount of lower-body strength development was the central focus of this investigation.
Experimental Approach to the Problem
The present study examined the effect of two different modes of lower-body endurance exercise (i.e., cycle ergometry and incline treadmill walking) on lower-body strength development with concurrent resistance training designed to improve lower-body strength (i.e., leg press 1RM). To determine whether it is possible to adapt to concurrent resistance and endurance training, the following considerations were taken into account in the study design: (a) both resistance and endurance training would involve the same muscle groups; (b) the two concurrent training groups would perform endurance exercise modes that were biomechanically different; and (c) the frequency, intensity, duration, and volume of endurance exercise were reduced compared with previous concurrent designs in an effort to avoid overtraining, injury, or both. The identification of the exercise protocol resulting in the greatest strength development was the central focus of this investigation.
The study participants (N = 30) were nonsmoking sedentary apparently healthy young men and women volunteers (18-25 years) from the university community without previous experience in intense resistance or endurance training. Both men and women were included in the sample because there is evidence that women respond to concurrent training similar to men despite endocrine differences (23,37,60) and for greater statistical power. Before screening and pretesting, the volunteers were provided with both a complete written and oral explanation of the study, and each volunteer signed an informed consent document approved by the University Institutional Review Board. Volunteers were also asked to complete a self-administered medical history and physical activity readiness questionnaire to ensure that all the subjects were free of cardiovascular, musculoskeletal, or metabolic diseases that could preclude them from participating in the study.
Before pretraining measurements, 30 volunteers were randomly assigned to 1 of 3 individual training groups. The individual groups were separated into the following categories: lower-body resistance only (R; N = 10), combined lower-body resistance and cycle ergometer endurance (RC; N = 10), and combined lower-body resistance and incline treadmill endurance (RT; N = 10). After pretest measurements, several subjects were reassigned to a different group to match or equalize the training groups.
The 3 training programs (R, RC, and RT) were designed specifically to stress the lower-body skeletal musculature. The R group participated in a 2-day per week lower-body resistance training program. The RC group participated in the same lower-body resistance training program as R and a 2-day per week cycle ergometer training program. The RT group participated in the same lower-body resistance training program as R and a 2-day per week incline treadmill endurance training program. RC and RT endurance training programs used an identical exercise intensity based on an age-predicted maximum heart rate so that both concurrent treatment groups were metabolically comparable.
Each resistance and endurance training program performed by the individual groups was designed to produce improvements in either strength or aerobic capacity, respectively, when performed independently. Both the resistance and endurance training protocols provided progressive overload in accordance with the ACSM's 2002 position stand on exercise progression. However, the volume and intensity of the endurance training programs were moderate and used a lower frequency when compared with other concurrent designs in an effort to avoid overtraining and injury (42). RC and RT alternated the order of their endurance and resistance training each training session to balance the quality of training between endurance and resistance exercise similar to other concurrent designs (51). All training sessions were performed on alternate days, allowing a minimum recovery of 48 hours between training sessions.
Subjects were tested for determination of height, body mass, body composition, and maximum dynamic strength (leg press 1 RM). The 1 RM leg press was selected to measure maximum dynamic lower-body strength because it is a multijoint movement but lacks the skill and experience required by the back squat exercise. All measurements in the study, except 1RM, were made before and after 9 weeks of training. 1 RM was measured before training, after 3 weeks of training, after 6 weeks of training, and after training was completed.
Height was measured to the nearest centimeter using a stadiometer. Body mass was assessed using a beam-balanced scale to the nearest 0.1 kg. Body composition was determined from skinfold measurements using a Lange caliper (Cambridge Scientific Industries Inc, Cambridge, Md). A 3-site regression equation developed by Jackson and Pollock (34) was used to calculate body density from skinfold measurements, and the Brozek (9) equation was used to convert measurements to percent body fat. All skinfold measurements were performed by one technician to ensure consistency between pre- and post-test data.
Lower-body maximum dynamic strength was determined by using a 1RM test on the bilateral leg press movement (Hammer Strength, Cincinnati, Ohio). The starting position for each trial required that the volunteer be seated with the knees flexed slightly greater than 90 degrees, as measured by a goniometer. One repetition maximum was defined as the maximal weight that can be pressed once during the regular execution of the criterion exercise. To familiarize the volunteers with the equipment and to minimize the incidence of muscle strain, all volunteers performed two progressive warm-up sets of 10 repetitions at submaximal loads set by the investigator's discretion. Thereafter, volunteers performed single repetitions with 2 to 3 minutes of recovery between trials, and resistance was increased by 10-20 kg until the 1RM was reached.
The testing order was established in accordance with recommended single-session measurements (1). The pre- and post-testing order was as follows: height, body mass, body composition, and maximum dynamic strength. Additionally, maximum dynamic strength assessments were made after week 3 and week 6 to document changes in strength over time. Specifically, these measurements were made at the beginning of week 4 and week 7 using the protocol previously described.
Lower-Body Resistance Training
The lower-body resistance program that R, RC, and RT participated in was performed 2 days per week on alternate days and was designed to produce marked improvement in strength of the lower-body musculature. The program consists of the 3 following isotonic exercises in order of execution: leg extension, leg flexion, and leg press, and these were performed on Hammer Strength variable resistance exercise equipment. It is important to note that the term isotonic in this study does not imply that a constant internal torque is being applied but rather a constant external torque from the resistance exercise equipment. The resistance exercise program followed a modified periodization model that began with higher-volume, lower-resistance training sessions and progressed toward lower-volume, higher-resistance training sessions (37). The manipulation of these variables for the three resistance exercises is presented in Table 1. All strength training sessions were supervised by the investigator to ensure that the subjects completed the workouts to the specifications of the design and that levels of motivation and coaching were comparable.
Lower-Body Endurance Training
The RC and RT groups performed 2-day per week endurance training programs. Both groups exercised at a constant exercise intensity corresponding to 65% of the subject's age-predicted maximum heart rate. Cyclists were instructed to maintain between 60 and 80 revolutions per minute and increase the pedal resistance as required to achieve the target heart rate response. Walkers were instructed to identify a comfortable yet brisk walking speed preventing running and increase the grade from this speed as required to achieve the target heart rate response. The manipulation of exercise intensity and duration for both RC and RT are presented in Table 2. All endurance training sessions were supervised by the investigator to ensure that the subjects completed the workouts to the specifications of the design and that levels of exercise intensity and duration were achieved.
Analysis of variance (ANOVA) data were used to detect significant differences between pre- and post-test criterion measurements. All pretest criterion measures were compared using a one-way ANOVA, and if significant, the Bonferroni post hoc test was used. Additional ANOVA analyses were used at 3- and 6-week midpoints to detect possible neurological or morphological explanations for observed performance changes. Statistical significance was set at p ≤ 0.05 for all comparisons.
The purpose of this investigation was to determine the mode of lower-body endurance exercise when combined with lower-body resistance training that would allow maximum lower-body strength improvement or maintenance without overtraining. This study is unique in that it is the first to report the simultaneous comparison of concurrent resistance and endurance training on lower-body strength development using different modes of endurance exercise. The subjects in this study were tested for height, body mass, body composition, and maximal dynamic lower-body strength pre- and post-training. Intragroup differences at pretraining were not significant (Table 3). Results of the ANOVA comparisons revealed significant changes in body composition variables (Table 4) and lower-body strength over time (Table 5). All assumptions of linear statistics were met.
The effect of training on body mass revealed several significant findings. First, when men and women were combined, body mass of R was significantly greater than RC and RT post-training. Second, the body mass of men only was significantly greater than RC and RT post-training. Body composition of men only was significantly lower for RC and RT compared with R. These findings are consistent with the findings of previous concurrent designs.
In terms of strength development over time, several interesting relationships emerged from the data analyses. First, when men and women were combined, percent change in strength revealed significantly greater gains in R compared with RT at week 6 and significantly greater gains in strength in R compared with RC and RT post-training. When considering men only, percent change in strength was significantly greater for R compared with RT at week 3, percent changes in strength were significantly greater for R compared with RC and RT at week 6, RC was significantly stronger than RT at week 6, R was significantly stronger than RC and RT post-training, and RC was significantly stronger that RT post-training. Percent change in strength in women was significantly greater in R compared with RT post-training. These findings provide evidence and confirm previous investigations that reported participation in concurrent resistance and endurance exercise regimens compromises the ability to develop strength when compared with resistance training alone. Additionally, and perhaps the most important finding, the data provided evidence that endurance training biomechanically specific to the concurrent resistance training may minimize adaptation interference when concurrently training compared with resistance only training. Figure 1 represents strength development over time for the 3 training groups when men and women were combined.
The issue of the proper dose of exercise to bring about a desired effect is crucial in the prescription of exercise. Over the last several decades exercise professionals have learned that the proper dose of exercise differs greatly, depending on the desired outcome. The primary findings of this study suggest that strength development may be compromised with concurrent training, and when compared with resistance-only training, the mode of endurance training combined with concurrent resistance training may play a role in strength development. The mechanisms responsible for the attenuation of strength development remain speculative. Compromised strength developments previously reported may be related to alterations in neural recruitment patterns and attenuated muscle hypertrophy (11,18,36). Other conceivable explanations for compromised strength development during concurrent training include fiber type transformation (36), endocrine responses (8,36), and overtraining syndrome (10,24,45). Furthermore, the adaptive responses of skeletal muscle to concurrent training regimens probably also depend on the initial fitness level of the trainees, the training modes, the frequency, intensity, duration, and volume of training, and the method used to integrate the two forms of training (42,52).
In the present design, an effort was made to avoid overtraining and injury in as much as the endurance training component of the two concurrent groups was reduced in terms of frequency, intensity, and volume of training when compared with other concurrent designs. Nakao et al. (44) reported no attenuation of strength with a single bout of concurrent endurance exercise of moderate intensity per week. Thus, it was hypothesized that the reduction of endurance exercise in the present design would allow strength development in the concurrent groups to be of a similar magnitude as the resistance-only training group. Nevertheless, the present data suggest that strength development is attenuated in concurrent training regimens, even when frequency, intensity, and volume of endurance training are reduced. Moreover, it is recommended that individuals with the singular objective of developing strength in a specific muscle group avoid endurance training in that muscle group.
A plausible explanation for the attenuation of strength development observed in the concurrent groups when compared with resistance-only training may be related to the duration of endurance training. The endurance component of both concurrent training regimens required that RC and RT exercise at a constant intensity (65% of their age predicted maximum heart rate) for 20 minutes during the first 3 weeks, 30 minutes from weeks 4-6, and 40 minutes during the final 3 weeks of training. Corresponding levels of strength development for each group at these time intervals clearly suggest that as the duration of endurance training increased, the attenuation of strength development also increased. Hence, it may possible to develop strength of a similar magnitude as resistance-only training in concurrent designs if the duration of endurance training is kept at 20 minutes. One could argue that 20 minutes of endurance training twice per week would not produce substantive improvements in aerobic capacity; however, if performed more frequently (i.e., 4-5 times per week), it is possible that such aerobic adaptations would occur and without compromised strength development.
Another conceivable explanation for attenuated strength in the present study observed in the concurrent groups compared with resistance-only training may be related to the order in which resistance and endurance training were applied. In the present study, RC and RT performed resistance training before endurance training in the first week and reversed the order to endurance training first and resistance training second. Of note, performing endurance training before resistance training may have fatigued these subjects to the point where they could not achieve sufficient intensity levels to promote strength development during resistance training. However, because the concurrent groups were warmed up on endurance-first sessions, they may have actually reached higher intensity levels when resistance training. Nevertheless, the author recommends that future concurrent investigations using moderate endurance training be developed which place resistance training before endurance training because intensity in resistance training is paramount.
The present data provided evidence that endurance training biomechanically specific to the resistance training mode may minimize antagonism in concurrent training when compared with resistance-only training. A possible explanation for this conclusion may be related to specificity of type of contraction. For example, in the leg press resistance exercise and cycle ergometry endurance exercise, the quadriceps produce movement primarily concentrically. In contrast, treadmill endurance exercise stresses the quadriceps muscle group eccentrically. It is possible that the different types of contractions played a role in the attenuation of strength development of RT when compared with RC. Furthermore, there is evidence that eccentric contractions result in higher levels of muscle soreness and may require more time for full recovery when compared with concentric muscle contractions (1). Thus, if the goal of an exercise prescription is to maximize strength development in a particular resistance exercise movement combined with concurrent endurance exercise, the present data suggest combining resistance training with a biomechanically specific endurance training mode.
Finally, it may also be possible that the revolutions per minute (RPM) while cycling played a role in the results of the study. The RC group was instructed to cycle between 60 and 80 RPM. It is conceivable that subjects that trained at the lower part of the accepted RPM range may have imposed a form of high repetition resistance training. The extent that cycling RPM effects strength development in concurrent training remains unclear and requires further inquiry.
In conclusion, the findings confirm previous studies that reported attenuated strength development with concurrent resistance and endurance training compared with resistance-only training. More importantly, this study indicates that the biomechanical movement patterns of select modes of endurance exercise in concurrent training regimens may play a role in the development of strength and maintenance of strength. Specifically, it seems that cycling is superior to treadmill endurance training for an individual with the goal of developing strength in a multijoint movement (i.e., leg press or squat) in the lower-body because it more closely mimics the biomechanical movement of these exercises.
The findings of this investigation confirm previous studies that reported reduced strength development with concurrent resistance and endurance training compared with resistance-only training. More importantly, this study indicates that the mode of endurance exercise combined with resistance training in concurrent training regimens may play a role in the development of strength. The author recommends the following guidelines when designing concurrent training regimens: (a) individuals with the primary objective of developing strength in a specific muscle group should avoid endurance training in that muscle group; (b) individuals with the objective of developing strength combined with moderate endurance training should perform resistance training before endurance training because of the intensity requirements of resistance training; and (c) individuals with the objective of developing strength in a specific muscle group while also training for endurance in the same muscle group should select an endurance training mode of a similar biomechanical movement pattern.
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