REPEATED BOUT EFFECT EXERCISE VARIATIONS
IN
MUSCLE-SPECIFIC
MICHAEL C. ZOURDOS,1 PAUL C. HENNING,2 EDWARD JO,3 ANDY V. KHAMOUI,4 SANG-ROK LEE,5 YOUNG-MIN PARK,6 MARSHALL NAIMO,4 LYNN B. PANTON,4 KAZUNORI NOSAKA,7 AND JEONG-SU KIM4 1
Department of Exercise Science and Health Promotion, Florida Atlantic University, Boca Raton, Florida; 2Military Performance Division, U.S. Army Research Institute of Environmental Medicine, Natick, Massachusetts; 3Department of Kinesiology and Health Promotion, California State Polytechnic University, Pomona, California; 4Department of Nutrition, Food, and Exercise Sciences, The Florida State University, Tallahassee, Florida; 5Department of Health and Sport Sciences, The University and Memphis, Memphis, Tennessee; 6Department of Nutrition and Exercise Physiology, University of Missouri, Columbia, Missouri; and 7School of Exercise and Health Sciences, Centre for Exercise and Sports Science Research, Edith Cowan University, Joondalup, WA, Australia ABSTRACT
Zourdos, MC, Henning, PC, Jo, E, Khamoui, AV, Lee, S-R, Park, Y-M, Naimo, M, Panton, LB, Nosaka, K, and Kim, J-S. Repeated bout effect in muscle-specific exercise variations. J Strength Cond Res 29(8): 2270–2276, 2015—A single bout of unaccustomed exercise confers protective effect against muscle damage from a subsequent bout of similar activity, that is, repeated bout effect (RBE). It remains unknown whether varying muscle-specific exercise between sessions alters the magnitude of the RBE. This study examined the effects of musclespecific exercise variation between consecutive sessions on the RBE. Twenty untrained males (21 6 2 years) were assigned to one of 2 groups (n = 10 per group): (a) 2 sessions of incline curls, Fixed Exercise or (b) 1 session of incline curls followed by 1 session of preacher curls, Varied Exercise, with 7 days between sessions. Subjects performed 5 sets of 6 repetitions at ;50% of maximal isometric elbow flexor strength during each session. Changes in maximal voluntary isometric and isokinetic torque, range of motion, muscle soreness, and serum creatine kinase were measured before, immediately after, and 24, 48, 72, and 96 hours after each exercise session, and the changes were compared between bouts and between groups. There were significant time effects (p , 0.05) for isometric maximal voluntary contraction, concentric maximal voluntary contraction, range of motion, and muscle soreness during sessions 1 and 2 with no between-group differences. Both groups demonstrated a significantly faster recovery of range of motion and soreness to baseline levels after session 2 compared with session 1. Overall, our findings suggest that incline curls conferred a protective effect during subsequent
Address correspondence to Jeong-Su Kim, [email protected]. 29(8)/2270–2276 Journal of Strength and Conditioning Research Ó 2015 National Strength and Conditioning Association
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preacher curls in a similar way to repeating incline curls; therefore, the RBE was not exercise specific.
KEY WORDS muscle damage, resistance training, eccentric exercise, incline curls, preacher curls
INTRODUCTION
A
single bout of unaccustomed exercise generates a significant degree of skeletal muscle damage, especially when eccentric actions are predominantly stressed (1,3,7,9,16). Exercise-induced muscle damage is directly indicated by morphologic disruptions to normal myofiber ultrastructure (11,27). However, muscle damage can also be evaluated by indirect measures that include localized soreness, swelling, temporary strength, and range of motion (ROM) decrements, as well as elevated muscle-specific proteins in the blood, such as creatine kinase (CK) (5–7,17,19). Previous studies have reported that a single bout of unfamiliar eccentrically biased exercise confers protection against muscle damage from a subsequent session of similar activity (4,6,18,20,21). This phenomenon, known as the repeated bout effect (RBE), has shown, primarily through indirect damage markers, to manifest up to 6 months after the initial bout and is specific only to the exercised muscles (21). The current mechanistic underpinnings for the RBE remain equivocal; however, past research supports the role of myofiber regeneration, changes to connective tissue, or acute increases in motor unit recruitment as contributors to the RBE (12,15,16,25). Several studies have demonstrated that the level of protection afforded by an initial exercise bout was dependent on volume and intensity. For instance, Chen et al reported that eccentric elbow flexion exercise performed within a range of 40–100% of maximal voluntary contraction elicited the RBE, which was evidenced by reduced muscle damage after a similar secondary bout at maximal intensity. Furthermore, the magnitude of the RBE increased in parallel with the intensity of the
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Journal of Strength and Conditioning Research initial exercise session, such that the greater the intensity the larger the degree of protection conferred (6). As for volumedependent effects, previous research investigated the extent to which an initial bout consisting of 2, 6, or 24 maximal eccentric actions protected against muscle damage from a subsequent bout of 24 maximal repetitions and found that regardless of the volume prescribed in the first bout, participants demonstrated the RBE as outcomes from the second session revealed a significantly blunted damage response (22). An important finding in this study was that the magnitude of the RBE was diminished when the initial bout consisted of less volume (i.e., 2 or 6 eccentric actions) than the second (i.e., 24 eccentric actions) (22). Thus, the RBE and thereby the level of protection rendered is maximized when a high-volume exercise bout is preceded by a session of equivalent volume. Nevertheless, these findings from Nosaka et al. (2001b) and other comparable results (5) suggest that the RBE manifests in a volume dependent fashion. Given that training regimens usually incorporate repeated bouts of exercise for a selected muscle group, systematically minimizing muscle damage from session to session through intensity and volume manipulation of the RBE may be beneficial in the context of performance optimization. However, the potential applicability of previous acute findings to practical settings is limited, partly because previous research has only used the same exercise between consecutive bouts. It can be conjectured that by varying muscle-specific exercise from 1 session to the next, the RBE might be diminished or even completely abolished, thereby compromising exercise performance and adaptations because of the adverse effects that sustained muscle damage over the course of time may have on training volume. Therefore, this study was designed to determine whether muscle-specific exercise variation between consecutive exercise sessions would alter the magnitude of the RBE while intensity and volume were controlled. We hypothesized that the magnitude of the RBE would be attenuated in response to an exercise variation for a given muscle group.
METHODS Experimental Approach to the Problem
This study was designed to examine the magnitude of the RBE if muscle-specific exercise was varied between consecutive sessions. Subjects were randomly assigned to one of the 2 groups: fixed exercise (FE) or varied exercise (VE). Each group initially underwent a familiarization session followed by 2 experimental sessions, which were separated by 7 days of rest. During the familiarization session, maximal isometric elbow flexor strength (MIEFS) was determined and used to establish the exercise load to be used for the experimental sessions. Additionally, this familiarization was solely isometric to avoid significant muscle damage and subsequent occurrence of the RBE before the initial experimental session. One week later, subjects returned to the laboratory for the first experimental session. During session 1, both FE and VE performed a damaging exercise bout consisting of 5 sets of 6 repetitions
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(load = 45–55% MIEFS) of the incline dumbbell curl exercise using the nondominant arm with 3 minutes rest between sets. For session 2, FE repeated the exercise protocol used in session 1 (i.e., incline to incline), whereas VE performed the dumbbell preacher curl exercise with the nondominant arm using the same set, repetition, and load configuration as the first session (i.e., incline to preacher). Indirect markers of muscle damage, which have been used frequently in previous research (5,6,10,13,18,20,22) included maximal voluntary isometric contraction (MVC-ISO), maximal voluntary concentric contraction (MVC-CON), ROM, muscle soreness (SOR), and serum CK activity. Furthermore, subjects were also familiarized with the MVC-ISO and isokinetic concentric strength testing during the familiarization session to avoid a learning effect. All indirect muscle damage markers were assessed before and immediately, 24, 48, 72, and 96 hours after exercise for both experimental sessions. Thereby, VE performed varied muscle-specific exercise in the subsequent bout to examine the occurrence and magnitude of the RBE when implementing a variation of exercise selection. Finally, all subjects came to the laboratory at the same time of day for each laboratory visit and were instructed to fast for 2 hours before any blood collection. Subjects
Twenty healthy college aged men participated in this study. Subjects had previous recreational resistance training experience, however, had not engaged in resistance training for at least 6 months before the start of the study. Mean age, body mass, and height were not significantly different (p . 0.05) between FE (n = 10; age = 19.7 6 1.6 years; body mass = 79.5 6 10.5 kg; height = 182 6 7 cm) and VE (n = 10; age = 21.4 6 2.4 years; body mass = 83.2 6 17.8 kg; height = 179 6 6 cm). Subjects were informed of the experimental risks and signed an informed consent form before the investigation. The investigation was approved by the Florida State University Institutional Review Board. Subjects were also instructed to refrain from any nutritional supplementation, additional exercise, and therapy for muscle recovery during the entire experimental period. Procedures
Maximal Isometric Elbow Flexor Strength Testing. Subjects were tested for MIEFS to determine the exercise load administered during subsequent experimental sessions. This testing procedure was administered as an alternative to strength testing for the experimental exercises themselves (i.e., incline and preacher curl) to ensure that the exercises were indeed novel to the subjects. To assess MIEFS, subjects were seated on a standard weightlifting bench with their trunk vertically positioned against an upright back support set at 908. For a warm-up trial, subjects performed a 5-second isometric contraction with the nondominant arm against a dumbbell representing 50% of estimated MIEFS at 908 of elbow flexion. After resting 3 minutes, subjects performed an additional warm-up set with 70% of estimated MIEFS. Subjects rested 3 minutes and then attempted 90% of estimated MIEFS as their first trial. If the load was maintained at 908 of elbow flexion for 5 seconds VOLUME 29 | NUMBER 8 | AUGUST 2015 |
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RBE, Muscle Damage, Resistance Training while upholding an upright seated position, the trial was deemed successful. If the trial was successful, investigators increased the load based on subject perception of the previous attempt; if unsuccessful, the last successful load defined the subject’s MIEFS. Investigators determined MIEFS within 5 trials, which were each separated by 3 minutes of rest. Incline Curl Exercise Protocol. For session 1, both FE and VE performed the incline curl exercise with the nondominant arm. Only FE repeated the protocol during session 2. Subjects were seated on a standard incline weightlifting bench with 508 of trunk flexion and both arms hanging freely. This resting body position permitted 508 of shoulder extension. Subjects were instructed to maintain this shoulder position throughout the entire exercise. Investigators then provided the subjects with a standard dumbbell bar adjusted with the appropriate load. The load administered was between 45 and 55% MIEFS, which was confirmed, through indirect markers, to induce muscle damage during pilot testing. Within this load range, subjects were able to perform 6 maximal repetitions for each of the 5 sets. Subjects were instructed to perform the exercise with the arm supinated. The tempo for each repetition was 1-second concentric action to 2-second eccentric action through the full ROM and was assisted by use of a metronome. Subjects were provided 3 minutes of rest between each set. An illustration of the incline curl exercise can be seen in Figure 1A. Preacher Curl Exercise Protocol. Varied exercise performed the dumbbell preacher curl exercise with the nondominant arm for session 2. Subjects were seated on a commercially standard preacher curl apparatus with their trunk in a vertical position. The subjects’ nondominant arm was rested supinated on the frontal pad, which was angled to allow 508 of shoulder flexion. Each subject performed the exercise using the same set, repetition, and load configuration as the first session. Rest period between each set and tempo for each repetition was used in similar fashion as session 1. An illustration of the preacher curl exercise can be seen in Figure 1B. Criterion Measures
Maximal Voluntary Isometric Contraction. Subjects were positioned on an isokinetic dynamometer (Biodex Medical Systems; Biodex Multijoint System 3, New York, NY, USA) and secured properly to restrict movement around nontested joints to measure isometric torque. First, 2 isometric contractions at 50% of MIEFS were performed for warm-up using a dumbbell. To assess MVC-ISO, the nondominant arm was positioned and fixed at 908 of elbow flexion and 08 of shoulder abduction and flexion. The center of rotation for both the tested elbow joint and the dynamometer were aligned. Subjects then performed two 5-second maximal isometric contractions with 2 minutes of rest between contractions. The greatest peak torque value achieved during the 2 isometric contractions was recorded as the subject’s MVC-ISO. The position setting for each subject was recorded to minimize variability among subsequent assessments.
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Figure 1. A) Incline curl exercise protocol: during rest, the subjects will be positioned with 508 trunk flexion and 508 shoulder flexion (nondominant arms handing freely). When performing the exercise, subjects will maintain shoulder position while flexing the elbow through the full ROM. B) Preacher curl exercise protocol: Subjects will be positioned on a standard preacher curl bench with 508 shoulder flexion while resting the nondominant arms on the angled pad. When performing the exercise, subjects will maintain shoulder position while flexing the elbow through the full ROM. ROM = range of motion.
Maximal Voluntary Concentric Contraction. Subjects were positioned on the dynamometer similar to the MVC-ISO assessment aforementioned to measure isokinetic torque. To assess MVC-CON, the nondominant arm was positioned and fixed at 458 of shoulder flexion and 08 of shoulder abduction. As a warm-up, subjects performed 2 sets of 5 submaximal repetitions with 2-minute rest intervals. Then, subjects performed 2 sets of 5 maximal isokinetic contractions at 908$s21 with 2-minute rest between sets. The MVC-CON was determined by the greatest peak torque value obtained among all repetitions performed during the test.
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Journal of Strength and Conditioning Research Range of Motion. Elbow joint ROM was assessed through goniometry and determined by the difference in the subject’s relaxed joint angle and flexed joint angle in the nondominant arm. Reference points for goniometer placement (proximal apex of deltoid, axis of rotation of the elbow, styloid process, and dorsal tubercle of the radius) were determined by palpating the arm and marked with semipermanent ink to ensure consistency among subsequent assessments. The relaxed joint angle was assessed with the tested arm rested along the side of the body, whereas the flexed joint angle was measured by fully flexing the subject’s elbow. Range of motion was calculated by subtracting the flexed angle from the relaxed angle. Muscle Soreness. To determine SOR, the investigator applied palpations at 4 different locations on the subject’s nondominant arm using the index and middle fingers. One palpation was applied at 3 sites on the upper arm: 25, 50, and 75% of humerus length (i.e., distance from the distal lateral epicondyle of the humerus to the lateral border of the acromium). The final palpation site was on the lower arm, 3 cm below the elbow crease. To maintain homogeneity among assessments, the same investigator conducted all measurements throughout the experiment. Also, each site was marked by semipermanent ink so that palpations could be administered on the same anatomical location on each subject for all assessments. After each palpation, subjects were instructed to indicate a level of perceived soreness on a 100-mm visual analog scale (VAS). On the VAS, 0 mm indicated “no pain” and 100 mm represented “extreme pain.” After administering the palpations, the investigator flexed and extended the corresponding elbow joint, and subjects specified the perceived soreness level on the VAS. Muscle soreness was determined as the mean of all VAS values for each assessment. Serum Creatine Kinase Activity. A 10 ml sample of venous blood was collected from the antecubital vein through sterile venipuncture techniques. For each draw, blood was collected in a serum separating tube. The blood was allowed to clot for 30 minutes at room temperature and then centrifuged for 10 minutes to obtain serum. After separation, all serum samples were aliquoted and stored at 2208 C until analysis. Creatine kinase activity was measured by an automated analyzer (Sirrus Clinical Chemistry Analyzer; StanBio Laboratory, Boerne, TX, USA) using a commercial enzymatic assay kit (CK Liqui-UV; StanBio Laboratory, Boerne, TX, USA). Samples were analyzed in duplicate of which the means were used for subsequent analysis. Our coefficient of variation between duplicates was less than 10%. Statistical Analyses
A 1-way analysis of variance (ANOVA) was used to determine whether there were baseline differences between groups. A 2 (group) 3 2 (session) 3 6 (time) multifactorial ANOVA was used to analyze all dependent variables. In the event of a significant main or interaction effect, Tukey’s post hoc test was used to detect time points showing significant
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mean differences between sessions or between groups. Level of significance was set at p # 0.05.
RESULTS Maximal Voluntary Isometric Contraction
Fixed exercise demonstrated a significant (p # 0.05) decrease in MVC-ISO from pre-exercise to immediately (217%), 24 (28%), 48 (210%), 72 (28%), and 96 (28%) hours postexercise in session 1 (Figure 2A). For session 2, FE showed a significant decline (p # 0.05) from pre-exercise to immediately postexercise (210%); however, MVC-ISO fully recovered to baseline by 24 hours postexercise (Figure 2A). In contrast, VE demonstrated significantly decreased MVC-ISO immediately postexercise only (time effect, 210%) without significant differences between sessions 1 and 2 (Figure 2B). When FE was compared with VE within each session, MVC-ISO was not significantly different (p . 0.05). As expected, significant time effects were noted as MVC-ISO significantly (p # 0.05) decreased from pre-exercise to immediately (216%), 24 (27%), 48 (28%), and 72 (27%) hours postexercise in session 1. For session 2, MVC-ISO significantly decreased (217%, p # 0.05) pre-exercise to immediately postexercise but showed full recovery by 24 hours postexercise. Maximal Voluntary Concentric Contraction
Comparison between sessions 1 and 2 for FE (Figure 2A) and VE (Figure 2B) individually are shown in Figure 1. Fixed exercise and VE each failed to show a significant change in MVC-CON from pre-exercise to any postexercise time point regardless of session. However, VE showed a significant time effect as MVC-CON decreased (28%, p # 0.05) from preexercise to immediately postexercise, with values returning to pre-exercise levels by 24 hours postexercise. Additionally, when comparing FE and VE within each session, there were no significant differences in MVC-CON (i.e., no group 3 time interaction). A main time effect, however, was observed in session 2 as MVC-CON significantly decreased (26%, p # 0.05) from pre-exercise to immediately postexercise. Range of Motion
Figure 1 shows the comparison between the first and second session for FE (Figure 2A) and VE (Figure 2B) individually. Fixed exercise showed a significant (p # 0.05) session by time interaction for ROM, which demonstrated a 48-hour earlier recovery of ROM to pre-exercise values in session 2 compared with session 1. In detail, FE demonstrated a significant (p # 0.05) decrease in ROM from pre-exercise to immediately (27.978), 24 (27.678), 48 (26.708), 72 (24.678), and 96 (23.608) hours postexercise in session 1. In session 2, FE showed a significant decline in ROM from pre-exercise to immediately (26.048), 24 (24.008), and 48 (23.938) hours postexercise; however, ROM fully recovered to baseline by 72 hours postexercise. Varied exercise demonstrated a significant (p # 0.05) session by time interaction for ROM, which indicated a 48-hour faster recovery of ROM to pre-exercise values VOLUME 29 | NUMBER 8 | AUGUST 2015 |
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RBE, Muscle Damage, Resistance Training after the second session compared with the first. Specifically, VE showed significant (p # 0.05) reduction in ROM from pre-exercise to immediately (210.078), 24 (27.428), 48 (27.428), and 72 (24.178) hours postexercise in session 1, as ROM fully recovered to baseline by 96 hours postexercise. For session 2, VE exhibited a significant (p # 0.05) decrease in ROM from pre-exercise to immediately (26.268) and 24 hours (22.768) postexercise; ROM returned to baseline by 48 hours postexercise. No significant differences between FE and VE were evident during each individual session (i.e., no group 3 time interaction). Muscle Soreness
Figure 2. Comparison between the first and second session for changes in maximal isometric strength, maximal isokinetic strength, ROM, SOR, and serum CK activity for the fixed exercise (A) and varied exercise (B) groups before (pre), immediately after (post), and 24–96 hours after exercise. Error bars are expressed in SD. ROM = range of motion; CK = creatine kinase; SOR = muscle soreness.
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Comparison between sessions for FE (Figure 2A) and VE (Figure 2B) individually are shown in Figure 1. Fixed exercise exhibited a significant (p # 0.05) session by time interaction, which indicated a 96hour faster recovery of SOR to baseline after the second session compared with the first. In detail, FE showed a significant (p # 0.05) increase in SOR from pre-exercise to 24 (+1.44 mm), 48 (+1.60 mm), 72 (+1.01 mm), and 96 (+0.47 mm) hours postexercise in session 1. For session 2, FE failed to exhibit any significant changes in SOR from preexercise to any postexercise time points. Varied exercise also demonstrated a significant (p # 0.05) session by time interaction demonstrating a more rapid recovery of SOR to baseline after the second session compared with the first by 72 hours. Specifically, VE indicated a significant (p # 0.05) increase in SOR from preexercise to 24 (+0.26 mm), 48
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Journal of Strength and Conditioning Research (+0.16 mm), and 72 (+0.12 mm) hours postexercise when compared with pre-exercise during session 1. For session 2, VE failed to exhibit any significant changes in SOR from pre-exercise to any postexercise time point, similar to that of session 2 for FE. Additionally, there were no significant group differences for changes in SOR during each individual session. Serum Creatine Kinase
Figure 1 shows the comparison between the first and second session for FE (Figure 2A) and VE (Figure 2B). Neither group exhibited any significant changes in CK over time for any session. There were also no significant differences between FE and VE within the same session.
DISCUSSION The primary aim of this investigation was to determine the effects of muscle-specific exercise variation on the magnitude of the RBE, while controlling for intensity and volume. After each damaging session, both groups demonstrated changes in criterion measures (i.e., MVC-ISO, MVC-CON, ROM, and SOR) that were indicative of exercise-induced muscle damage. Furthermore, a variable degree of protection was rendered by the initial bout whether the exercise was repeated (i.e., FE) or muscle-specifically varied (i.e., VE) in the subsequent session (Figures 1A, B). In addition, this study examined how this session-to-session shift in exercise-induced damage response (i.e., the RBE) compared between FE and VE. It was hypothesized that by varying muscle-specific exercise between bouts, the RBE may be diminished or even eliminated. However, this study’s findings demonstrated that varying muscle-specific exercise between bouts did not diminish the effects of the RBE. This is based on the results that there was no significant group difference for the time-dependent changes in criterion measures after the second resistant exercise session. In summary, the initial bout conferred protection against muscle damage in our subjects regardless of the biomechanical variation of elbow flexor exercise applied in the succeeding session. To the best of our knowledge, this study was the first to examine the effects of exercise variation on the RBE. However, outcomes differed from that of previous research, which showed a change in RBE magnitude with manipulation of exercise variables from 1 session to the next. For instance, a bout-to-bout variation in intensity (6,14), volume (2,5,22), contraction type (26), or ROM (20) has shown to significantly alter the magnitude of the RBE. As for this study, the biomechanical modification applied to the incline curl protocol in session 2 for VE was unable to produce any detectable alterations to the RBE when comparing with the response exhibited by FE. To explain this outcome, we first evaluated parallels between the exercise protocols used in this investigation (i.e., incline vs. preacher curls). Although the 2 sessions were biomechanically varied for VE, intensity, volume, contraction type, and ROM remained constant. As aforementioned, these variables were shown to significantly influence
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the magnitude of the RBE only when varied between consecutive bouts. This would suggest that the lack of discrepancy observed between FE and VE was presumably attributable to the constancy of these variables between sessions and groups. Another possible explanation is that although the incline and preacher curl are different exercises, they may recruit similar populations of motor units, which are less susceptible to muscle damage in subsequent bouts. Evidently, muscle-specific exercise variation between consecutive bouts produces no independent effect on the magnitude of the RBE. It must be noted, however, a potential limitation could account for the outcomes observed in VE. One might refute that the diminished damage response exhibited by VE after the second session was independent of the RBE and rather because of the possibility that the preacher curl exercise protocol simply induced less muscle damage than the incline. However, in an attempt to counter this notion, we refer to a previous study that investigated the effects of short vs. long muscle length during resistance exercise on muscle damage markers (24). In brief, subjects randomly underwent 2 separate exercise sessions, 1 on each leg. For each session, subjects performed maximal eccentric contractions of the knee extensors either in a seated (908 hip angle) or prone (1808 hip angle) position to attain a short or long rectus femoris length, respectively. It was confirmed through anatomical analysis that the initial muscle length was shorter at a 908 hip angle compared with 1808. Results suggested that performing exercise at a shorter initial muscle length induces greater muscle damage and torque deficits than the longer length. To relate these findings to the present results, we evaluated the initial muscle length of the prime elbow flexor, biceps brachii, during the incline and preacher curl exercises. These variations to the standard curl exercise are generally applied to optimize biceps brachii contribution for elbow flexion by fixing shoulder angle at a specific value (23). By performing dumbbell curls in a seated incline position, the shoulder joint is extended, thereby initially lengthening the biceps brachii, whereas the flexed position of the shoulder during the preacher curl shortens the initial muscle length (23). Thus, the exercises and respective joint positions used in this study produced divergent initial muscle lengths, such as the protocols applied by Paschalis et al. If previous outcomes observed by Paschalis et al. are extrapolated to the elbow flexors, it can be implied that the preacher curl exercise, compared with the incline, is a more damaging protocol because of a shorter initial length of the prime elbow flexor. If this holds true for the present model, the outcomes exhibited by VE were indeed attributable to the RBE as the damage response was attenuated after the preacher curl protocol during session 2. Thus, these findings suggest that the protective effect conferred by an initial bout persists even when the muscle-specific exercise is varied in the following session. Support for this result can be partially deduced from previous findings demonstrating the transfer of the RBE even when 2 bouts are biomechanically diverse (i.e., eccentric contractions of the knee extensors to downhill running) (8). VOLUME 29 | NUMBER 8 | AUGUST 2015 |
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RBE, Muscle Damage, Resistance Training In conclusion, this study demonstrated that (a) a single bout of exercise reduced damage symptoms from a subsequent bout of muscle-specific exercise that was biomechanically varied and (b) the level of RBE rendered by the initial session was unaffected by varying muscle-specific exercise selection during the second. It seems that the RBE manifests independent of exercise variation provided that the same muscle group is stressed during the consecutive bouts. However, it is important to note that a relatively minor biomechanical variation was used in this study, which might account for these observations.
PRACTICAL APPLICATIONS From a practical stance, muscle damage to a target muscle group might be minimized to a similar extent in a succeeding bout whether exercise variation is applied or not. Thus, novice participants may, through the RBE, avoid sustained muscle damage and performance detriments during resistance training while incorporating exercise variations to a specific muscle group during ensuing bouts. Thus, eliciting the RBE may be a beneficial strategy to allow novice trainees to avoid unnecessary muscle damage and fatigue and increase adherence to training. Furthermore, if exercise variation attenuates damage and increases adherence to training, an indirect effect may be enhanced training variables such as frequency and volume. In turn, enhancing frequency and volume should lead to greater skill acquisition on a certain lift and larger improvements in muscular growth and strength. However, caution should be used when extrapolating these findings as future research investigating the effects of more complex variations using compound movements, (i.e., squat, bench press, deadlift, etc.) which target larger muscle groups (i.e., quadriceps, hamstrings, chest, etc.) are required to better infer these findings to practical training conditions.
ACKNOWLEDGMENTS No funding was received for this project. The authors declare that they have no conflict of interest.
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6. Chen, TC, Nosaka, K, and Sacco, P. Intensity of eccentric exercise, shift of optimum angle, and the magnitude of repeated-bout effect. J Appl Physiol (1985) 102: 992–999, 2007.
26. Whitehead, NP, Allen, TJ, Morgan, DL, and Proske, U. Damage to human muscle from eccentric exercise after training with concentric exercise. J Physiol 512: 615–620, 1998.
7. Clarkson, PM, Nosaka, K, and Braun, B. Muscle function after exercise-induced muscle damage and rapid adaptation. Med Sci Sports Exerc 24: 512–520, 1992.
27. Yu, JG, Fu¨rst, DO, and Thornell, LE. The mode of myofibril remodelling in human skeletal muscle affected by DOMS induced by eccentric contractions. Histochem Cell Biol 119: 383–393, 2003.
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IN
MUSCLE-SPECIFIC
MICHAEL C. ZOURDOS,1 PAUL C. HENNING,2 EDWARD JO,3 ANDY V. KHAMOUI,4 SANG-ROK LEE,5 YOUNG-MIN PARK,6 MARSHALL NAIMO,4 LYNN B. PANTON,4 KAZUNORI NOSAKA,7 AND JEONG-SU KIM4 1
Department of Exercise Science and Health Promotion, Florida Atlantic University, Boca Raton, Florida; 2Military Performance Division, U.S. Army Research Institute of Environmental Medicine, Natick, Massachusetts; 3Department of Kinesiology and Health Promotion, California State Polytechnic University, Pomona, California; 4Department of Nutrition, Food, and Exercise Sciences, The Florida State University, Tallahassee, Florida; 5Department of Health and Sport Sciences, The University and Memphis, Memphis, Tennessee; 6Department of Nutrition and Exercise Physiology, University of Missouri, Columbia, Missouri; and 7School of Exercise and Health Sciences, Centre for Exercise and Sports Science Research, Edith Cowan University, Joondalup, WA, Australia ABSTRACT
Zourdos, MC, Henning, PC, Jo, E, Khamoui, AV, Lee, S-R, Park, Y-M, Naimo, M, Panton, LB, Nosaka, K, and Kim, J-S. Repeated bout effect in muscle-specific exercise variations. J Strength Cond Res 29(8): 2270–2276, 2015—A single bout of unaccustomed exercise confers protective effect against muscle damage from a subsequent bout of similar activity, that is, repeated bout effect (RBE). It remains unknown whether varying muscle-specific exercise between sessions alters the magnitude of the RBE. This study examined the effects of musclespecific exercise variation between consecutive sessions on the RBE. Twenty untrained males (21 6 2 years) were assigned to one of 2 groups (n = 10 per group): (a) 2 sessions of incline curls, Fixed Exercise or (b) 1 session of incline curls followed by 1 session of preacher curls, Varied Exercise, with 7 days between sessions. Subjects performed 5 sets of 6 repetitions at ;50% of maximal isometric elbow flexor strength during each session. Changes in maximal voluntary isometric and isokinetic torque, range of motion, muscle soreness, and serum creatine kinase were measured before, immediately after, and 24, 48, 72, and 96 hours after each exercise session, and the changes were compared between bouts and between groups. There were significant time effects (p , 0.05) for isometric maximal voluntary contraction, concentric maximal voluntary contraction, range of motion, and muscle soreness during sessions 1 and 2 with no between-group differences. Both groups demonstrated a significantly faster recovery of range of motion and soreness to baseline levels after session 2 compared with session 1. Overall, our findings suggest that incline curls conferred a protective effect during subsequent
Address correspondence to Jeong-Su Kim, [email protected]. 29(8)/2270–2276 Journal of Strength and Conditioning Research Ó 2015 National Strength and Conditioning Association
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preacher curls in a similar way to repeating incline curls; therefore, the RBE was not exercise specific.
KEY WORDS muscle damage, resistance training, eccentric exercise, incline curls, preacher curls
INTRODUCTION
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single bout of unaccustomed exercise generates a significant degree of skeletal muscle damage, especially when eccentric actions are predominantly stressed (1,3,7,9,16). Exercise-induced muscle damage is directly indicated by morphologic disruptions to normal myofiber ultrastructure (11,27). However, muscle damage can also be evaluated by indirect measures that include localized soreness, swelling, temporary strength, and range of motion (ROM) decrements, as well as elevated muscle-specific proteins in the blood, such as creatine kinase (CK) (5–7,17,19). Previous studies have reported that a single bout of unfamiliar eccentrically biased exercise confers protection against muscle damage from a subsequent session of similar activity (4,6,18,20,21). This phenomenon, known as the repeated bout effect (RBE), has shown, primarily through indirect damage markers, to manifest up to 6 months after the initial bout and is specific only to the exercised muscles (21). The current mechanistic underpinnings for the RBE remain equivocal; however, past research supports the role of myofiber regeneration, changes to connective tissue, or acute increases in motor unit recruitment as contributors to the RBE (12,15,16,25). Several studies have demonstrated that the level of protection afforded by an initial exercise bout was dependent on volume and intensity. For instance, Chen et al reported that eccentric elbow flexion exercise performed within a range of 40–100% of maximal voluntary contraction elicited the RBE, which was evidenced by reduced muscle damage after a similar secondary bout at maximal intensity. Furthermore, the magnitude of the RBE increased in parallel with the intensity of the
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Journal of Strength and Conditioning Research initial exercise session, such that the greater the intensity the larger the degree of protection conferred (6). As for volumedependent effects, previous research investigated the extent to which an initial bout consisting of 2, 6, or 24 maximal eccentric actions protected against muscle damage from a subsequent bout of 24 maximal repetitions and found that regardless of the volume prescribed in the first bout, participants demonstrated the RBE as outcomes from the second session revealed a significantly blunted damage response (22). An important finding in this study was that the magnitude of the RBE was diminished when the initial bout consisted of less volume (i.e., 2 or 6 eccentric actions) than the second (i.e., 24 eccentric actions) (22). Thus, the RBE and thereby the level of protection rendered is maximized when a high-volume exercise bout is preceded by a session of equivalent volume. Nevertheless, these findings from Nosaka et al. (2001b) and other comparable results (5) suggest that the RBE manifests in a volume dependent fashion. Given that training regimens usually incorporate repeated bouts of exercise for a selected muscle group, systematically minimizing muscle damage from session to session through intensity and volume manipulation of the RBE may be beneficial in the context of performance optimization. However, the potential applicability of previous acute findings to practical settings is limited, partly because previous research has only used the same exercise between consecutive bouts. It can be conjectured that by varying muscle-specific exercise from 1 session to the next, the RBE might be diminished or even completely abolished, thereby compromising exercise performance and adaptations because of the adverse effects that sustained muscle damage over the course of time may have on training volume. Therefore, this study was designed to determine whether muscle-specific exercise variation between consecutive exercise sessions would alter the magnitude of the RBE while intensity and volume were controlled. We hypothesized that the magnitude of the RBE would be attenuated in response to an exercise variation for a given muscle group.
METHODS Experimental Approach to the Problem
This study was designed to examine the magnitude of the RBE if muscle-specific exercise was varied between consecutive sessions. Subjects were randomly assigned to one of the 2 groups: fixed exercise (FE) or varied exercise (VE). Each group initially underwent a familiarization session followed by 2 experimental sessions, which were separated by 7 days of rest. During the familiarization session, maximal isometric elbow flexor strength (MIEFS) was determined and used to establish the exercise load to be used for the experimental sessions. Additionally, this familiarization was solely isometric to avoid significant muscle damage and subsequent occurrence of the RBE before the initial experimental session. One week later, subjects returned to the laboratory for the first experimental session. During session 1, both FE and VE performed a damaging exercise bout consisting of 5 sets of 6 repetitions
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(load = 45–55% MIEFS) of the incline dumbbell curl exercise using the nondominant arm with 3 minutes rest between sets. For session 2, FE repeated the exercise protocol used in session 1 (i.e., incline to incline), whereas VE performed the dumbbell preacher curl exercise with the nondominant arm using the same set, repetition, and load configuration as the first session (i.e., incline to preacher). Indirect markers of muscle damage, which have been used frequently in previous research (5,6,10,13,18,20,22) included maximal voluntary isometric contraction (MVC-ISO), maximal voluntary concentric contraction (MVC-CON), ROM, muscle soreness (SOR), and serum CK activity. Furthermore, subjects were also familiarized with the MVC-ISO and isokinetic concentric strength testing during the familiarization session to avoid a learning effect. All indirect muscle damage markers were assessed before and immediately, 24, 48, 72, and 96 hours after exercise for both experimental sessions. Thereby, VE performed varied muscle-specific exercise in the subsequent bout to examine the occurrence and magnitude of the RBE when implementing a variation of exercise selection. Finally, all subjects came to the laboratory at the same time of day for each laboratory visit and were instructed to fast for 2 hours before any blood collection. Subjects
Twenty healthy college aged men participated in this study. Subjects had previous recreational resistance training experience, however, had not engaged in resistance training for at least 6 months before the start of the study. Mean age, body mass, and height were not significantly different (p . 0.05) between FE (n = 10; age = 19.7 6 1.6 years; body mass = 79.5 6 10.5 kg; height = 182 6 7 cm) and VE (n = 10; age = 21.4 6 2.4 years; body mass = 83.2 6 17.8 kg; height = 179 6 6 cm). Subjects were informed of the experimental risks and signed an informed consent form before the investigation. The investigation was approved by the Florida State University Institutional Review Board. Subjects were also instructed to refrain from any nutritional supplementation, additional exercise, and therapy for muscle recovery during the entire experimental period. Procedures
Maximal Isometric Elbow Flexor Strength Testing. Subjects were tested for MIEFS to determine the exercise load administered during subsequent experimental sessions. This testing procedure was administered as an alternative to strength testing for the experimental exercises themselves (i.e., incline and preacher curl) to ensure that the exercises were indeed novel to the subjects. To assess MIEFS, subjects were seated on a standard weightlifting bench with their trunk vertically positioned against an upright back support set at 908. For a warm-up trial, subjects performed a 5-second isometric contraction with the nondominant arm against a dumbbell representing 50% of estimated MIEFS at 908 of elbow flexion. After resting 3 minutes, subjects performed an additional warm-up set with 70% of estimated MIEFS. Subjects rested 3 minutes and then attempted 90% of estimated MIEFS as their first trial. If the load was maintained at 908 of elbow flexion for 5 seconds VOLUME 29 | NUMBER 8 | AUGUST 2015 |
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RBE, Muscle Damage, Resistance Training while upholding an upright seated position, the trial was deemed successful. If the trial was successful, investigators increased the load based on subject perception of the previous attempt; if unsuccessful, the last successful load defined the subject’s MIEFS. Investigators determined MIEFS within 5 trials, which were each separated by 3 minutes of rest. Incline Curl Exercise Protocol. For session 1, both FE and VE performed the incline curl exercise with the nondominant arm. Only FE repeated the protocol during session 2. Subjects were seated on a standard incline weightlifting bench with 508 of trunk flexion and both arms hanging freely. This resting body position permitted 508 of shoulder extension. Subjects were instructed to maintain this shoulder position throughout the entire exercise. Investigators then provided the subjects with a standard dumbbell bar adjusted with the appropriate load. The load administered was between 45 and 55% MIEFS, which was confirmed, through indirect markers, to induce muscle damage during pilot testing. Within this load range, subjects were able to perform 6 maximal repetitions for each of the 5 sets. Subjects were instructed to perform the exercise with the arm supinated. The tempo for each repetition was 1-second concentric action to 2-second eccentric action through the full ROM and was assisted by use of a metronome. Subjects were provided 3 minutes of rest between each set. An illustration of the incline curl exercise can be seen in Figure 1A. Preacher Curl Exercise Protocol. Varied exercise performed the dumbbell preacher curl exercise with the nondominant arm for session 2. Subjects were seated on a commercially standard preacher curl apparatus with their trunk in a vertical position. The subjects’ nondominant arm was rested supinated on the frontal pad, which was angled to allow 508 of shoulder flexion. Each subject performed the exercise using the same set, repetition, and load configuration as the first session. Rest period between each set and tempo for each repetition was used in similar fashion as session 1. An illustration of the preacher curl exercise can be seen in Figure 1B. Criterion Measures
Maximal Voluntary Isometric Contraction. Subjects were positioned on an isokinetic dynamometer (Biodex Medical Systems; Biodex Multijoint System 3, New York, NY, USA) and secured properly to restrict movement around nontested joints to measure isometric torque. First, 2 isometric contractions at 50% of MIEFS were performed for warm-up using a dumbbell. To assess MVC-ISO, the nondominant arm was positioned and fixed at 908 of elbow flexion and 08 of shoulder abduction and flexion. The center of rotation for both the tested elbow joint and the dynamometer were aligned. Subjects then performed two 5-second maximal isometric contractions with 2 minutes of rest between contractions. The greatest peak torque value achieved during the 2 isometric contractions was recorded as the subject’s MVC-ISO. The position setting for each subject was recorded to minimize variability among subsequent assessments.
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Figure 1. A) Incline curl exercise protocol: during rest, the subjects will be positioned with 508 trunk flexion and 508 shoulder flexion (nondominant arms handing freely). When performing the exercise, subjects will maintain shoulder position while flexing the elbow through the full ROM. B) Preacher curl exercise protocol: Subjects will be positioned on a standard preacher curl bench with 508 shoulder flexion while resting the nondominant arms on the angled pad. When performing the exercise, subjects will maintain shoulder position while flexing the elbow through the full ROM. ROM = range of motion.
Maximal Voluntary Concentric Contraction. Subjects were positioned on the dynamometer similar to the MVC-ISO assessment aforementioned to measure isokinetic torque. To assess MVC-CON, the nondominant arm was positioned and fixed at 458 of shoulder flexion and 08 of shoulder abduction. As a warm-up, subjects performed 2 sets of 5 submaximal repetitions with 2-minute rest intervals. Then, subjects performed 2 sets of 5 maximal isokinetic contractions at 908$s21 with 2-minute rest between sets. The MVC-CON was determined by the greatest peak torque value obtained among all repetitions performed during the test.
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Journal of Strength and Conditioning Research Range of Motion. Elbow joint ROM was assessed through goniometry and determined by the difference in the subject’s relaxed joint angle and flexed joint angle in the nondominant arm. Reference points for goniometer placement (proximal apex of deltoid, axis of rotation of the elbow, styloid process, and dorsal tubercle of the radius) were determined by palpating the arm and marked with semipermanent ink to ensure consistency among subsequent assessments. The relaxed joint angle was assessed with the tested arm rested along the side of the body, whereas the flexed joint angle was measured by fully flexing the subject’s elbow. Range of motion was calculated by subtracting the flexed angle from the relaxed angle. Muscle Soreness. To determine SOR, the investigator applied palpations at 4 different locations on the subject’s nondominant arm using the index and middle fingers. One palpation was applied at 3 sites on the upper arm: 25, 50, and 75% of humerus length (i.e., distance from the distal lateral epicondyle of the humerus to the lateral border of the acromium). The final palpation site was on the lower arm, 3 cm below the elbow crease. To maintain homogeneity among assessments, the same investigator conducted all measurements throughout the experiment. Also, each site was marked by semipermanent ink so that palpations could be administered on the same anatomical location on each subject for all assessments. After each palpation, subjects were instructed to indicate a level of perceived soreness on a 100-mm visual analog scale (VAS). On the VAS, 0 mm indicated “no pain” and 100 mm represented “extreme pain.” After administering the palpations, the investigator flexed and extended the corresponding elbow joint, and subjects specified the perceived soreness level on the VAS. Muscle soreness was determined as the mean of all VAS values for each assessment. Serum Creatine Kinase Activity. A 10 ml sample of venous blood was collected from the antecubital vein through sterile venipuncture techniques. For each draw, blood was collected in a serum separating tube. The blood was allowed to clot for 30 minutes at room temperature and then centrifuged for 10 minutes to obtain serum. After separation, all serum samples were aliquoted and stored at 2208 C until analysis. Creatine kinase activity was measured by an automated analyzer (Sirrus Clinical Chemistry Analyzer; StanBio Laboratory, Boerne, TX, USA) using a commercial enzymatic assay kit (CK Liqui-UV; StanBio Laboratory, Boerne, TX, USA). Samples were analyzed in duplicate of which the means were used for subsequent analysis. Our coefficient of variation between duplicates was less than 10%. Statistical Analyses
A 1-way analysis of variance (ANOVA) was used to determine whether there were baseline differences between groups. A 2 (group) 3 2 (session) 3 6 (time) multifactorial ANOVA was used to analyze all dependent variables. In the event of a significant main or interaction effect, Tukey’s post hoc test was used to detect time points showing significant
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mean differences between sessions or between groups. Level of significance was set at p # 0.05.
RESULTS Maximal Voluntary Isometric Contraction
Fixed exercise demonstrated a significant (p # 0.05) decrease in MVC-ISO from pre-exercise to immediately (217%), 24 (28%), 48 (210%), 72 (28%), and 96 (28%) hours postexercise in session 1 (Figure 2A). For session 2, FE showed a significant decline (p # 0.05) from pre-exercise to immediately postexercise (210%); however, MVC-ISO fully recovered to baseline by 24 hours postexercise (Figure 2A). In contrast, VE demonstrated significantly decreased MVC-ISO immediately postexercise only (time effect, 210%) without significant differences between sessions 1 and 2 (Figure 2B). When FE was compared with VE within each session, MVC-ISO was not significantly different (p . 0.05). As expected, significant time effects were noted as MVC-ISO significantly (p # 0.05) decreased from pre-exercise to immediately (216%), 24 (27%), 48 (28%), and 72 (27%) hours postexercise in session 1. For session 2, MVC-ISO significantly decreased (217%, p # 0.05) pre-exercise to immediately postexercise but showed full recovery by 24 hours postexercise. Maximal Voluntary Concentric Contraction
Comparison between sessions 1 and 2 for FE (Figure 2A) and VE (Figure 2B) individually are shown in Figure 1. Fixed exercise and VE each failed to show a significant change in MVC-CON from pre-exercise to any postexercise time point regardless of session. However, VE showed a significant time effect as MVC-CON decreased (28%, p # 0.05) from preexercise to immediately postexercise, with values returning to pre-exercise levels by 24 hours postexercise. Additionally, when comparing FE and VE within each session, there were no significant differences in MVC-CON (i.e., no group 3 time interaction). A main time effect, however, was observed in session 2 as MVC-CON significantly decreased (26%, p # 0.05) from pre-exercise to immediately postexercise. Range of Motion
Figure 1 shows the comparison between the first and second session for FE (Figure 2A) and VE (Figure 2B) individually. Fixed exercise showed a significant (p # 0.05) session by time interaction for ROM, which demonstrated a 48-hour earlier recovery of ROM to pre-exercise values in session 2 compared with session 1. In detail, FE demonstrated a significant (p # 0.05) decrease in ROM from pre-exercise to immediately (27.978), 24 (27.678), 48 (26.708), 72 (24.678), and 96 (23.608) hours postexercise in session 1. In session 2, FE showed a significant decline in ROM from pre-exercise to immediately (26.048), 24 (24.008), and 48 (23.938) hours postexercise; however, ROM fully recovered to baseline by 72 hours postexercise. Varied exercise demonstrated a significant (p # 0.05) session by time interaction for ROM, which indicated a 48-hour faster recovery of ROM to pre-exercise values VOLUME 29 | NUMBER 8 | AUGUST 2015 |
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RBE, Muscle Damage, Resistance Training after the second session compared with the first. Specifically, VE showed significant (p # 0.05) reduction in ROM from pre-exercise to immediately (210.078), 24 (27.428), 48 (27.428), and 72 (24.178) hours postexercise in session 1, as ROM fully recovered to baseline by 96 hours postexercise. For session 2, VE exhibited a significant (p # 0.05) decrease in ROM from pre-exercise to immediately (26.268) and 24 hours (22.768) postexercise; ROM returned to baseline by 48 hours postexercise. No significant differences between FE and VE were evident during each individual session (i.e., no group 3 time interaction). Muscle Soreness
Figure 2. Comparison between the first and second session for changes in maximal isometric strength, maximal isokinetic strength, ROM, SOR, and serum CK activity for the fixed exercise (A) and varied exercise (B) groups before (pre), immediately after (post), and 24–96 hours after exercise. Error bars are expressed in SD. ROM = range of motion; CK = creatine kinase; SOR = muscle soreness.
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Comparison between sessions for FE (Figure 2A) and VE (Figure 2B) individually are shown in Figure 1. Fixed exercise exhibited a significant (p # 0.05) session by time interaction, which indicated a 96hour faster recovery of SOR to baseline after the second session compared with the first. In detail, FE showed a significant (p # 0.05) increase in SOR from pre-exercise to 24 (+1.44 mm), 48 (+1.60 mm), 72 (+1.01 mm), and 96 (+0.47 mm) hours postexercise in session 1. For session 2, FE failed to exhibit any significant changes in SOR from preexercise to any postexercise time points. Varied exercise also demonstrated a significant (p # 0.05) session by time interaction demonstrating a more rapid recovery of SOR to baseline after the second session compared with the first by 72 hours. Specifically, VE indicated a significant (p # 0.05) increase in SOR from preexercise to 24 (+0.26 mm), 48
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Journal of Strength and Conditioning Research (+0.16 mm), and 72 (+0.12 mm) hours postexercise when compared with pre-exercise during session 1. For session 2, VE failed to exhibit any significant changes in SOR from pre-exercise to any postexercise time point, similar to that of session 2 for FE. Additionally, there were no significant group differences for changes in SOR during each individual session. Serum Creatine Kinase
Figure 1 shows the comparison between the first and second session for FE (Figure 2A) and VE (Figure 2B). Neither group exhibited any significant changes in CK over time for any session. There were also no significant differences between FE and VE within the same session.
DISCUSSION The primary aim of this investigation was to determine the effects of muscle-specific exercise variation on the magnitude of the RBE, while controlling for intensity and volume. After each damaging session, both groups demonstrated changes in criterion measures (i.e., MVC-ISO, MVC-CON, ROM, and SOR) that were indicative of exercise-induced muscle damage. Furthermore, a variable degree of protection was rendered by the initial bout whether the exercise was repeated (i.e., FE) or muscle-specifically varied (i.e., VE) in the subsequent session (Figures 1A, B). In addition, this study examined how this session-to-session shift in exercise-induced damage response (i.e., the RBE) compared between FE and VE. It was hypothesized that by varying muscle-specific exercise between bouts, the RBE may be diminished or even eliminated. However, this study’s findings demonstrated that varying muscle-specific exercise between bouts did not diminish the effects of the RBE. This is based on the results that there was no significant group difference for the time-dependent changes in criterion measures after the second resistant exercise session. In summary, the initial bout conferred protection against muscle damage in our subjects regardless of the biomechanical variation of elbow flexor exercise applied in the succeeding session. To the best of our knowledge, this study was the first to examine the effects of exercise variation on the RBE. However, outcomes differed from that of previous research, which showed a change in RBE magnitude with manipulation of exercise variables from 1 session to the next. For instance, a bout-to-bout variation in intensity (6,14), volume (2,5,22), contraction type (26), or ROM (20) has shown to significantly alter the magnitude of the RBE. As for this study, the biomechanical modification applied to the incline curl protocol in session 2 for VE was unable to produce any detectable alterations to the RBE when comparing with the response exhibited by FE. To explain this outcome, we first evaluated parallels between the exercise protocols used in this investigation (i.e., incline vs. preacher curls). Although the 2 sessions were biomechanically varied for VE, intensity, volume, contraction type, and ROM remained constant. As aforementioned, these variables were shown to significantly influence
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the magnitude of the RBE only when varied between consecutive bouts. This would suggest that the lack of discrepancy observed between FE and VE was presumably attributable to the constancy of these variables between sessions and groups. Another possible explanation is that although the incline and preacher curl are different exercises, they may recruit similar populations of motor units, which are less susceptible to muscle damage in subsequent bouts. Evidently, muscle-specific exercise variation between consecutive bouts produces no independent effect on the magnitude of the RBE. It must be noted, however, a potential limitation could account for the outcomes observed in VE. One might refute that the diminished damage response exhibited by VE after the second session was independent of the RBE and rather because of the possibility that the preacher curl exercise protocol simply induced less muscle damage than the incline. However, in an attempt to counter this notion, we refer to a previous study that investigated the effects of short vs. long muscle length during resistance exercise on muscle damage markers (24). In brief, subjects randomly underwent 2 separate exercise sessions, 1 on each leg. For each session, subjects performed maximal eccentric contractions of the knee extensors either in a seated (908 hip angle) or prone (1808 hip angle) position to attain a short or long rectus femoris length, respectively. It was confirmed through anatomical analysis that the initial muscle length was shorter at a 908 hip angle compared with 1808. Results suggested that performing exercise at a shorter initial muscle length induces greater muscle damage and torque deficits than the longer length. To relate these findings to the present results, we evaluated the initial muscle length of the prime elbow flexor, biceps brachii, during the incline and preacher curl exercises. These variations to the standard curl exercise are generally applied to optimize biceps brachii contribution for elbow flexion by fixing shoulder angle at a specific value (23). By performing dumbbell curls in a seated incline position, the shoulder joint is extended, thereby initially lengthening the biceps brachii, whereas the flexed position of the shoulder during the preacher curl shortens the initial muscle length (23). Thus, the exercises and respective joint positions used in this study produced divergent initial muscle lengths, such as the protocols applied by Paschalis et al. If previous outcomes observed by Paschalis et al. are extrapolated to the elbow flexors, it can be implied that the preacher curl exercise, compared with the incline, is a more damaging protocol because of a shorter initial length of the prime elbow flexor. If this holds true for the present model, the outcomes exhibited by VE were indeed attributable to the RBE as the damage response was attenuated after the preacher curl protocol during session 2. Thus, these findings suggest that the protective effect conferred by an initial bout persists even when the muscle-specific exercise is varied in the following session. Support for this result can be partially deduced from previous findings demonstrating the transfer of the RBE even when 2 bouts are biomechanically diverse (i.e., eccentric contractions of the knee extensors to downhill running) (8). VOLUME 29 | NUMBER 8 | AUGUST 2015 |
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RBE, Muscle Damage, Resistance Training In conclusion, this study demonstrated that (a) a single bout of exercise reduced damage symptoms from a subsequent bout of muscle-specific exercise that was biomechanically varied and (b) the level of RBE rendered by the initial session was unaffected by varying muscle-specific exercise selection during the second. It seems that the RBE manifests independent of exercise variation provided that the same muscle group is stressed during the consecutive bouts. However, it is important to note that a relatively minor biomechanical variation was used in this study, which might account for these observations.
PRACTICAL APPLICATIONS From a practical stance, muscle damage to a target muscle group might be minimized to a similar extent in a succeeding bout whether exercise variation is applied or not. Thus, novice participants may, through the RBE, avoid sustained muscle damage and performance detriments during resistance training while incorporating exercise variations to a specific muscle group during ensuing bouts. Thus, eliciting the RBE may be a beneficial strategy to allow novice trainees to avoid unnecessary muscle damage and fatigue and increase adherence to training. Furthermore, if exercise variation attenuates damage and increases adherence to training, an indirect effect may be enhanced training variables such as frequency and volume. In turn, enhancing frequency and volume should lead to greater skill acquisition on a certain lift and larger improvements in muscular growth and strength. However, caution should be used when extrapolating these findings as future research investigating the effects of more complex variations using compound movements, (i.e., squat, bench press, deadlift, etc.) which target larger muscle groups (i.e., quadriceps, hamstrings, chest, etc.) are required to better infer these findings to practical training conditions.
ACKNOWLEDGMENTS No funding was received for this project. The authors declare that they have no conflict of interest.
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