A bout of exercise consisting of maximal eccentric contractions provides a strong protective effect against muscle damage in the subsequent bouts of the same exercise performed within several weeks (8,11,15,27). This protective effect is called the repeated bout effect (RBE) and is characterized by smaller changes in and/or faster recovery of indirect muscle damage markers such as maximal voluntary contraction (MVC) strength, range of motion (ROM), delayed onset muscle soreness (DOMS), plasma creatine kinase (CK) activity, and abnormalities in ultrasound or magnetic resonance images after the second than the initial bout (1,6,15). Some studies have shown that the initial bout of maximal eccentric exercise confers the protective effect on the contralateral limb (17,25,32,35). This type of the RBE is described as the contralateral RBE (CL-RBE), in contrast to the RBE of the same limb muscles termed as the ipsilateral RBE (IL-RBE).
Howatson and van Someren (17) were the first to report that changes in MVC strength, serum CK activity, and DOMS were significantly attenuated when the second exercise bout of three sets of 15 maximal isokinetic (30°·s−1, 0.53 rad·s−1) eccentric contractions of the elbow flexors was performed by the contralateral arm 2 wk later, although the magnitude of the protective effect was less than that of the ipsilateral arm. Starbuck and Eston (32) also showed that the decreases in MVC strength and relaxed elbow joint angle and the development of DOMS were significantly attenuated in the contralateral arm when 60 maximal isokinetic (30°·s−1, 0.53 rad·s−1) eccentric contractions of the elbow flexors were performed with the contralateral arm 2 wk after the first bout. They showed that the magnitude of the protective effect was similar between the contralateral and the ipsilateral arms. Newton et al. (25) found significant attenuation of changes in MVC strength, upper arm circumference, and plasma CK activity after the second bout of 60 maximal isokinetic (90°·s−1, 1.58 rad·s−1) eccentric contractions of the elbow flexors when compared with the first bout of the same exercise that was performed by the other arm 4 wk before. By contrast, Hody et al. (14) found no CL-RBE when two bouts of 90 maximal isokinetic (60°·s−1, 1.05 rad·s−1) eccentric contractions of the knee extensors were separated by 6 wk. This may be due to a different muscle group used in the exercise (i.e., elbow flexors vs knee extensors) but could also be due to the 6-wk interval between bouts. Xin et al. (35) observed the CL-RBE when two bouts of 100 maximal isokinetic (30°·s−1, 0.53 rad·s−1) eccentric contractions of the knee extensors were separated by 4 wk. Thus, it is assumed that the magnitude of the CL-RBE is affected by the time interval between bouts, but this has not been systematically investigated.
Because a crossover design in which one limb receives treatment and the other limb serves as a control or a placebo condition is often used to investigate an intervention effect on muscle damage induced by eccentric exercise, it is important to determine how long and how strong the CL-RBE could last. It is also interesting to examine whether the CL-RBE is evident when the two exercise bouts are performed in the same day such as one after the other or with a relatively short interval (e.g., 6 h and 12 h) between bouts. To understand the underpinning mechanisms of the CL-RBE, which are currently unknown, it is important to clarify how soon and how long the CL-RBE is conferred.
Therefore, the present study set various intervals between bouts (i.e., 0.5 h, 6 h, 12 h, 24 h, 1 wk, 4 wk, and 8 wk) and investigated whether the magnitude of the CL-RBE would be affected by the interval between bouts. It was hypothesized that the CL-RBE would be evident when the second bout was performed within 4 wk, and that the shorter the interval between bouts, the greater the protective effect.
Subjects and Study Design
A total of 104 young healthy men who had not performed regular resistance, aerobic, or flexibility training in the past 1 yr, and who did not carry heavy objects frequently in their daily activities, were recruited for this study. They had no previous musculoskeletal injuries of the upper extremities. They provided an informed consent to participate in this study, which had been approved by a local institutional review board. The study was conducted in conformity with the policy statement regarding the use of human participants by the Medicine & Science in Sports & Exercise® and the Declaration of Helsinki. Their mean ± SD age, height, and body mass were 22.0 ± 1.8 yr, 172.9 ± 4.8 cm, and 68.2 ± 9.7 kg, respectively.
The present study used only men as previous CL-RBE studies had conducted (1,13,17,22,25,35), and it was considered to be better to have one gender to reduce variance in a group. The sample size was estimated using the data from a previous study reporting the CL-RBE of the elbow flexors (17). On the basis of an expected 10% difference in MVC strength recovery between the ipsilateral and the contralateral arms with the effect size of 0.8, an α level of 0.05, and a power (1 − β) of 0.80 (9), it was estimated that at least 12 participants per group were necessary by G*Power (G*Power 22.214.171.124, Heinrich-Heine-Universität Düsseldorf, Düsseldorf, Germany; http://www.gpower.hhu.de/). The participants were placed into one of the seven CL-RBE groups based on the time interval between bouts (30 min [0.5 h], 6 h, 12 h, 24 h [1 d], 1 wk, 4 wk, and 8 wk) or a control group (n = 13 per group) by stratifying the participants based on the preexercise maximal voluntary concentric contraction (MVC-CON) torque at the angular velocity of 60°·s−1 (1.05 rad·s−1) for the baseline average and SD of MVC-CON torque to be similar among the groups. The participants in the control group performed two bouts of five sets of six maximal eccentric contractions of the elbow flexors using their nondominant arm separated by 2 wk. The reason why we chose the 2-wk interval between bouts for the control group was based on a previous study (26) showing that the IL-RBE of the elbow flexors attenuated from 4 to 8 wk and another study (6) reporting strong IL-RBE when the interval between bouts was 2–3 wk. It was thought that the 2-wk interval would reflect the magnitude of the RBE between 1 and 4 wk that were included in the CL-RBE groups. Because previous studies (2,6) used nondominant arm for investigation of the IL-RBE, the present study also chose the nondominant arm only for the control group to minimize possible inconvenience to the participants. However, it should be noted that Newton et al. (25) did not find a significant difference between dominant and nondominant arms for their responses to maximal eccentric exercise of the elbow flexors. The participants in the 0.5 h, 6 h, 12 h, 1 d, 1 wk, 4 wk, and 8 wk groups performed the first bout of five sets of six maximal eccentric contractions of the elbow flexors with a randomly chosen arm (either dominant or nondominant arm) followed 0.5, 6, 12 h, 1 d, 1 wk, 4 wk, or 8 wk later, respectively, by the opposite arm (contralateral arm). The use of dominant and nondominant arms was counterbalanced among participants such that six participants used the dominant arm first and the other seven participants used the nondominant arm first.
The participants were asked and reminded to refrain from unaccustomed exercise and/or avoid vigorous physical activity and to maintain their normal dietary habits and not to take any anti-inflammatory drugs (e.g., nonsteroidal anti-inflammatory drugs) or nutritional supplements (e.g., vitamins and amino acids) during the experimental period. However, the participants’ activities and food intake were not monitored nor recorded. The participants were instructed to drink enough water after exercise to avoid a possible risk of acute renal failure due to rhabdomyolysis, refrain from alcohol, and not have any treatments of the exercised muscles (e.g., massage and stretching) during the study.
A familiarization session was held 3 d before the first eccentric exercise bout, in which the participants experienced the measurements of upper arm circumference, muscle soreness, ROM, and MVC-CON torque in this order. The investigator demonstrated the maximal eccentric exercise of the elbow flexors, but no eccentric contractions were performed by the participants because it has been reported that only a few maximal eccentric contractions could confer some protective effect (28). The dependent variables included MVC-CON torque, peak torque angle in MVC-CON, ROM, upper arm circumference, muscle soreness assessed by a visual analog scale (VAS), echo intensity of the B-mode ultrasound images, and plasma CK activity and myoglobin (Mb) concentration. The measurements of MVC-CON torque, peak torque angle, ROM, and upper arm circumference were taken immediately before and after and 1–5 d after each exercise bout. MVC-CON torque was also measured immediately before and after ECC1 for nonexercised arm to examine the effect of the exercise on the opposite arm. Muscle soreness and echo intensity were measured at all time points shown in the previous sentence except immediately after exercise. Plasma CK activity and Mb concentration were assessed before and 1–5 d after each exercise for the control, 1, 4, and 8 wk groups, but for the 0.5 h, 6 h, and 12 h groups, they were measured before and 1–5 d after ECC1. Because ECC2 was performed in the same day as ECC1, it was not possible to distinguish the effect of ECC2 from that of ECC1.
All of the dependent variables were measured 2 d and immediately before exercise to establish the test–retest reliability of the measures using these two baseline measures. The test–retest reliability of each dependent variable measurement was determined by an intraclass correlation coefficient (ICC) and the coefficient of variation (CV) using the values taken at 2 d and immediately before the ECC1 for the 0.5 h, 6 h, 12 h, 1 d, 1 wk, 4 wk, 8 wk, and control groups. Thus, the data from the total of 104 subjects (13 subjects × 8 groups) were used for this analysis. The ICC values and the CV for MVC-CON torque, peak torque angle, ROM, upper arm circumference, muscle soreness, plasma CK activity, Mb concentration, and echo intensity were 0.91, 0.92, 0.96, 0.99, 1.00, 0.87, 0.84, and 0.90, respectively, for ICC and 8.9%, 8.7%, 3.7%, 1.9%, 0%, 9.5%, 5.3%, and 7.7%, respectively, for CV.
During the exercise, each participant’s trunk was stabilized by a pelvic strap and two shoulder straps to minimize the involvement of other body parts. The shoulder joint angle was set at 45° (0.79 rad) flexion with 0° (0 rad) abduction, and each participant grasped a hand bar connected to the lever arm attached to an isokinetic dynamometer (Biodex System 3 Pro; Biodex Medical Systems, Shirley, NY) with wrist being supinated. The participants performed five sets of six maximal voluntary isokinetic eccentric contractions of the elbow flexors at 30°·s−1 (0.53 rad·s−1) from an elbow flexed (90°, 1.58 rad) to an elbow fully extended position (0°). Each contraction lasted for 3 s and was repeated every 10 s during which the isokinetic dynamometer passively returned the elbow joint to the flexed position at the velocity of 9°·s−1 (0.16 rad·s−1), with a 2-min rest between sets (2,5,33). Participants were verbally encouraged to maximally resist the movements of the isokinetic dynamometer to extend the elbow joint. The torque and the elbow joint angle position signals of each contraction were saved to the desktop computer connected to the isokinetic dynamometer as described in the previous section, and peak torque and work (the area under the torque curve) of each contraction were calculated using the Biodex software (2,33). The mean torque of each set was obtained and used for subsequent analysis, and the total work was also calculated as the sum of the work of 30 eccentric contractions.
MVC-CON torque and peak torque angle
MVC-CON torque and peak torque angle were measured by the same isokinetic dynamometer, in the same position as those described for the eccentric exercise. MVC-CON torque was measured at the angular velocity of 60°·s−1 (1.05 rad·s−1) for the ROM of 140° for the elbow flexors (0°–140°, 0–2.45 rad) and extensors (140°–0°) for three continuous contractions for both directions (2,3). Verbal encouragement was provided during the tests. Raw torque data were filtered and smoothed using an “isokinetic windowing,” and the optimum angle of each contraction was assessed by a Biodex Medical Systems software. The highest value of the three trials for the elbow flexor and extensor MVC-CON torque was used for further analysis, and the mean of the three trials for the peak torque angle of the elbow flexors was used for subsequent analysis (2,3).
Elbow joint angles and ROM
ROM of the elbow joint was determined as the difference between the elbow joint angles of maximal voluntarily flexion (FANG) and extension (EANG) measured by a manual goniometer (5). Three measurements were taken for each angle, and the mean of the three measurements was used to calculate ROM (2,5).
Upper arm circumference
While each participant was standing, relaxing, and letting the arm hang down by his side, the upper arm circumference was measured at the midportion of the upper arm, between the acromion process of the clavicle to the lateral epicondyle of the humerus, using a Gulick tape measure (Creative Health Products, Plymouth, MI). The measurements were taken three times, by the same examiner, and the mean of the three measures was used for statistical analysis (2).
Muscle soreness of the elbow flexors was quantified using a VAS that had a 100-mm continuous line with “no pain at all” on one end (0 mm) and “unbearable pain” on the other end (100 mm). The participants were asked to rate their perceived soreness rating on the VAS when the elbow joint was passively extended and flexed, respectively, for the ROM that was used for the MVC-CON torque measurements (3,18).
Transverse B-mode ultrasound images were taken using a Terason t3000 Ultrasound System with a 7.5-MHz linear probe (Terason Co., Burlington, MA) and saved in a computer (HP Workstation xw4400; Hewlett-Packard Company, Singapore). The probe was placed at the midportion of the biceps brachii (the same site of the circumference measurement), and the placement of the probe and the volume of the ultrasound gel were standardized. The echo intensity was analyzed by computer image analysis software (ULT File Reader for Windows; Broadsound Co., Taiwan), and the mean echo intensity of a histogram of gray scale (0, black; 256, white) was calculated for a region of interest (2 × 2 = 4 cm2) set at 5 mm adjacent to the humerus, including both biceps brachii and brachialis, based on the previous study (2,5). It has been reported that increases in echo intensity after eccentric exercise reflect inflammation of muscle, and the changes in echo intensity are similar to those of magnetic resonance T2 images (26). The relative change in the echo intensity from the preexercise value was calculated.
Plasma CK activity and Mb concentration
Approximately 10 mL of venous blood was drawn from the antecubital vein using a standard venipuncture technique and centrifuged for 10 min to separate the plasma, and plasma samples were stored at −80°C for later analyses. Plasma CK activity was assayed via spectrophotometry by an automated clinical chemistry analyzer (Model 7080; Hitachi, Co. Ltd., Tokyo, Japan) using a commercially available test kit (Roche Diagnostics, Indianapolis, IN). Plasma Mb concentration was measured by an automated clinical chemistry analyzer (Model Elecsys 2010, Roche Diagnostics GmbH, Mannheim, Germany) using a test kit (Roche Diagnostics, Indianapolis, IN). Each sample was analyzed in duplicate, and the average value of two measures was used for subsequent statistical analysis. The normal reference range for adult men provided by the manufacturers of the kits was 38–174 IU·L−1 for plasma CK activity and <110 μg·L−1 for Mb concentration. The main reason why both CK and Mb were included in this study was that a large variability among participants for the magnitude of changes in serum/plasma CK activity after eccentric exercise has been reported (8,26). It was thought that it would be better to include other typical blood marker of muscle damage: myoglobin to investigate the RBE on blood markers on muscle damage.
A Shapiro–Wilk test assessed the normality and a Levene test examined the homogeneity of variance assumption. These tests showed that the data of all dependent variables were normally distributed, and the variance for homogeneity was assumed. Baseline values for all dependent variables before each exercise bout (ECC1 and ECC2) were compared among the groups by a one-way ANOVA. Initially, a three-way repeated-measures ANOVA (group  × bout  × time [6 or 7]) was performed for each dependent variable and found a significant three-way interaction effect for all variables. Because the most important comparison in the present study was between the first and the second bouts for each group separately to see if any RBE was evident for each group, changes in the dependent variables over time were compared between ECC1 and ECC2 for each group by a mixed-design two-way ANOVA first. Second, changes in the dependent variables after each exercise bout were compared among the groups by a mixed-design two-way ANOVA. The baseline value was not included when normalized data to the baseline value were analyzed by the ANOVA. Changes in peak torque and work during each bout and the first versus the second exercise bouts were also compared among the groups by a mixed-design two-way ANOVA. When a significant interaction effect was found, a series of two-way ANOVA were performed to compare between two groups to examine which group was different from which other group for the pattern of the changes. When the ANOVA found a significant interaction effect, a Tukey’s post hoc test was performed for the comparison between groups for each time point. A significant level was set at P ≤ 0.05. The data are presented as mean ± SD, unless otherwise stated.
No significant differences in the baseline values of any of the dependent variables were seen among the groups before ECC1 (Table 1). All dependent variables except CK and Mb were not different between ECC1 and ECC2 for the control and all CL-RBE groups. For CK and Mb, because ECC2 was performed before plasma CK activity and Mb concentration returned to the baseline after ECC1 for the 0.5 h, 6 h, 12 h, and 1 d groups, it was not possible to separate the effect of ECC1 and ECC2 for these groups.
Changes in peak torque over five sets were not different among the groups for ECC1 (F28,384 = 0.86, P = 0.68) and ECC2 (F28,384 = 1.05, P = 0.08), and the average peak torque of six contractions per set decreased similarly over sets for ECC1 (e.g., 40.7 ± 6.1 N·m at the first set and 30.7 ± 5.9 N·m at the fifth set) and ECC2 (40.5 ± 6.8 N·m at the first set and 31.6 ± 6.1 N·m at the fifth set). This was also the case for changes in work over sets, which decreased from 302 ± 22 J at the first set to 206 ± 22 J at the fifth set in ECC1 similarly among the groups (F28,384 = 0.59, P = 0.96), and from 303 ± 21 J at the first set to 212 ± 20 J at the fifth set in ECC2 (F28,384 = 0.52, P = 0.98). Both peak torque (ECC1: −24% ± 6%, ECC2: −22% ± 6%) and work (ECC1: −31% ± 7%, ECC2: −29% ± 6%) decreased significantly over five sets without a significant difference between ECC1 and ECC2 (peak torque: F4,828 = 0.12, P = 0.73; work: F4,828 = 0.23, P = 0.92). No significant difference in the total work produced during ECC1 (15242 ± 269 J; F7,84 = 0.11, P = 0.99) and ECC2 (15170 ± 264 J, F7,84 = 0.77, P = 0.61) was evident among the groups, and the total work was not significantly different between ECC1 and ECC2 either (F7,168 = 0.50, P = 0.83).
Changes in dependent variables after ECC1
No significant changes in the elbow extensor MVC-CON torque were evident after ECC1 (F35,480 = 0.66, P = 0.68) and ECC2 (F35,480 = 1.08, P = 0.34). However, all of the other dependent variables showed significant changes over time after ECC1 without significant differences among the groups (MVC-CON: F35,480 = 0.64, P = 0.95; peak torque angle: F35,480 = 0.23, P = 0.99; ROM: F35,480 = 1.41, P = 0.15; upper arm circumference: F35,480 = 0.87, P = 0.68; muscle soreness: F28,384 = 1.25, P = 0.07; echo intensity: F28,384 = 0.58, P = 0.96; plasma CK activity: F28,384 = 0.47, P = 0.99; plasma Mb concentration: F28,384 = 0.36, P = 0.25), as shown in Figures 1–4.
Repeated bout effect
When comparing the changes in the dependent variables between ECC1 and ECC2 for each group separately, a significant bout–time interaction effect (F and P values are shown on the top of Figs. 1–4) was found for all variables for the control, 1 d, 1 wk, and 4 wk groups, but this was not the case for the other groups (i.e., 0.5 h, 6 h, 12 h, and 8 wk). When comparing the changes in each variable except for CK and Mb after ECC2, a significant group–time interaction effect was found for MVC-CON torque (F35,480 = 2.47, P < 0.001), peak torque angle (F35,480 = 2.49, P = 0.02), ROM (F35,480 = 3.23, P = 0.03), upper arm circumference (F35,480 = 4.90, P < 0.001), muscle soreness (F28,384 = 4.69, P < 0.001), echo intensity (F28,384 = 2.05, P = 0.002), plasma CK activity (F12,192 = 1.85, P = 0.04), and plasma Mb concentration (F12,192 = 1.60, P = 0.03). The subsequent ANOVA analyses to compare two groups revealed that the changes in all dependent variables were significantly smaller for the control group than all other groups, and no significant difference was evident among the 0.5 h, 6 h, 12 h, and 8 wk groups. When comparing the 1 d, 1 wk, and 4 wk groups, changes in all dependent variables were significantly smaller, or the recovery was significantly faster for the 1 d group than the 1 wk and 4 wk groups (e.g., 1 d vs 1 wk, MVC-CON torque: F5,120 = 3.19, P = 0.009; muscle soreness: F5,120 = 6.01, P < 0.001; 1 d vs 4 wk: MVC-CON torque: F5,120 = 2.97, P = 0.01; muscle soreness: F5,120 = 4.15, P = 0.002), and for the 1 wk than the 4 wk group (e.g., MVC-CON torque: F5,120 = 2.72, P = 0.02; muscle soreness: F5,120 = 3.54, P = 0.005).
Magnitude of protective effect
Figure 5 presents the magnitude of protective effect based on the values at 5 d postexercise for MVC-CON torque, peak torque angle, ROM and circumference (representing the recovery rate), and peak values for plasma CK activity and Mb concentration, echo intensity, and muscle soreness (representing the maximal change) using the following equation: (ECC1 − ECC2)/ECC1 × 100. If the responses were the same between ECC1 and ECC2, it is considered as no protection (the index is 0%). The protective effect was greatest for the control group, and the protection index for MVC-CON torque, peak torque angle, ROM, circumference, muscle soreness, echo intensity, CK, and Mb were 100%, 98%, 100%, 82%, 84%, 80%, 100%, and 97%, respectively. For the 0.5 h, 6 h, 12 h, and 8 wk groups, the protective index was low for all variables. The protective index for 1 d, 1 wk, and 4 wk groups were smaller than that of the control group for all variables (P < 0.05), and the average of the index of the six variables excluding CK and Mb for the 1 d (70% ± 5%), 1 wk (55% ± 8%), and 4 wk (36% ± 7%) groups was smaller (P < 0.001) than that of the control group (91% ± 10%), and it became smaller (P < 0.05) with increasing the interval between bouts from 1 d to 4 wk.
The present study showed that 1) changes in all muscle damage markers were significantly smaller after ECC2 than ECC1 for the groups that performed ECC2 with the opposite arm at 1 d, 1 wk, or 4 wk after ECC1, demonstrating the CL-RBE; 2) the magnitude of the RBE was smaller for the CL-RBE groups that performed ECC2 at 1 d, 1 wk, or 4 wk after ECC1 when compared with the control group that used the same arm for the two bouts separated by 2 wk; and 3) the magnitude of the CL-RBE decreased as the interval between bouts from 1 d to 4 wk increased, but no CL-RBE was observed when ECC2 was performed 0.5, 6, or 12 h or 8 wk after ECC1. These results supported the hypotheses that the CL-RBE would be evident when the second bout was performed within 4 wk, and that the shorter the interval between bouts, the greater the CL-RBE, but it had not been expected that no protective effect was observed when ECC2 was performed within 12 h after ECC1.
The changes in the muscle damage markers after ECC1 in all groups were similar to those reported in previous studies in which a similar eccentric exercise of the elbow flexors was performed by non-resistance-trained individuals (2,3,17,30,32), suggesting that responses to ECC1 of all groups were typical. The total work and changes in peak torque and work over sets during ECC1 and ECC2 were similar among all groups; therefore, it seems reasonable to assume that all CL-RBE groups (i.e., 0.5 h, 6 h, 12 h, 1 d, 1 wk, 4 wk, and 8 wk) should have shown similar responses to those observed after ECC1 if no RBE had been produced. Thus, the differences in the changes in the variables between ECC1 and ECC2 for the 1 d, 1 wk, and 4 wk groups and the differences in the changes after ECC2 between the control and the three CL-RBE groups were most likely due to the RBE conferred to the contralateral arm. Thus, the present study confirmed the existence of the CL-RBE shown in the previous studies (17,25,32,35).
The recovery of MVC-CON torque and ROM was significantly faster at 1–5 d postexercise, and changes in other variables (peak torque angle, circumference, muscle soreness, and echo intensity) were significantly smaller after ECC2 than ECC1 for the 1 d, 1 wk, and 4 wk groups; however, this was not the case for the 0.5 h, 6 h, 12 h, and 8 wk groups (Figs. 1–4). These suggest that the CL-RBE was not induced when the interbout interval was less than 12 h or more than 8 wk. Previous studies (17,25,32,35) observed that CL-RBE was evident when ECC2 was performed 2–4 wk after ECC1, but no CL-RBE was observed when ECC2 was performed 6 wk after ECC1 for the knee extensors (14). The present study was the first to investigate the CL-RBE for the interval between bouts of less than 12 h, 1 d, 1 wk, and 8 wk for the elbow flexors. It has been reported that the IL-RBE lasts more than 6 months for the elbow flexors, although the magnitude of the effect is attenuated over time (29). The present study clarified that the magnitude of CL-RBE was also attenuated when the interval between ECC1 and ECC2 increased from 1 d to 1 wk and from 1 wk to 4 wk, and the effect was abolished between 4 and 8 wk.
As shown in Figure 5, the magnitude of the protective effect was smaller for the CL-RBE groups than that induced by the control group that performed ECC1 and ECC2 by the same arm separated by 2 wk. Although the magnitude of the protective effect varied among the dependent variables, the average of the protective effect roughly demonstrated that the protective effect of the control group was approximately 90%, showing that the recovery of MVC-CON torque, peak torque angle, and ROM was enhanced by 90%, and increases in upper arm circumference, muscle soreness, echo intensity, CK activity, and Mb concentration were reduced by 90%. This was similar to that reported in a previous study in which the second bout of maximal eccentric exercise of the elbow flexors was performed 2 or 3 wk after the initial bout (6). When comparing the magnitude of the protective effect between the control and the 4 wk groups, it was significantly smaller for the 4 wk group (36%) than the control group (91%). However, it should be noted that ECC2 was performed 2 wk after ECC1 for the control group, but the interval between bouts was 4 wk for the 4 wk group. It would have been better to include an ipsilateral group that performed ECC2 at 1 or 4 wk after ECC1 to compare the magnitude of RBE between the ipsilateral and the contralateral conditions. Because the present study did not include such a group, a direct comparison was not possible. However, it can be assumed that the magnitude of the IL-RBE is greater if the interval between bouts was 1 wk but smaller if the interval between bouts was 4 wk, when compared with that shown by the control group in which the interval between bouts was 2 wk. Hirose et al. (13) used a similar elbow flexor eccentric exercise to that of the present study and reported that the magnitude of the protective effect conferred by the first eccentric exercise bout on the subsequent bout of the same exercise performed by the same arm 4 wk later varied among the variables, but their average was approximately 80%. These data suggest that the magnitude of the protective effect in the CL-RBE (approximately 40%) is approximately 50% of that of the IL-RBE (approximately 80%) for the same interval up to 4 wk. It appears that the time course of the attenuation of the CL-RBE is similar to that of the IL-RBE in this period.
It is interesting to note that the CL-RBE was not evident when ECC1 and ECC2 were separated by 0.5, 6, or 12 h. It appears that more than 12 h are necessary for the CL-RBE to be conferred after the initial eccentric exercise. This appears to be important for speculating the underlying mechanisms of the CL-RBE. It has been stated in review articles (19,24) that the mechanisms underpinning the RBE are complex, and probably a combination of neural (e.g., increased recruitment of slow-twitch motor units and activation of larger motor unit pool), mechanical (e.g., increased dynamic and passive muscle stiffness), cellular adaptations (e.g., longitudinal addition of sarcomeres, adaptation in inflammatory response, and strengthened extracellular matrix), and others (e.g., heat shock protein [HSP] and strengthened weak sarcomeres) is involved in them. Regarding the cellular adaptations, Hyldahl et al. (20) reported that extracellular matrix was strengthened at 27 d after 300 maximal isokinetic eccentric contractions of the knee extensors, and they stated that this would be associated with the RBE. Paulsen et al. (30) reported that HSP27, HSP70, and αB-crystallin accumulated in cytoskeletal/myofibrillar structures (e.g., Z disks and desmin) at 2–7 d and 2 d after the first and the second bouts, respectively. They speculated that small HSP possibly stabilized and protected myofibrillar structures to reduce the magnitude of muscle damage. It is interesting to examine whether these adaptations are induced within 24 h after the initial bout for the contralateral muscle. Xin et al. (35) observed the CL-RBE when two bouts of 100 maximal eccentric contractions of the knee extensors were separated by 4 wk, and they found the attenuation of nuclear factor κB DNA-binding activity in vastus lateralis muscle after the second bout performed by the opposite limb (ECC1: 123% ± 3% vs ECC2: 109% ± 3%). Because nuclear factor κB is a key regulator of muscle inflammation, it might be that the first eccentric exercise affected the inflammation of the nonexercised contralateral muscle. Thus, it seems possible that some of cellular adaptations play a role in inducing the CL-RBE. Other peripheral adaptations such as mechanical changes in muscle stiffness are unlikely to be involved in CL-RBE because it is difficult to imagine that eccentric exercise of one arm can produce muscle stiffness changes in the other arm. If cellular changes are associated with the CL-RBE, the signal(s) responsible for the cellular adaptations in the contralateral muscle or other muscles should be investigated further.
As for neural adaptations, it is highly possible that adaptations at the cortical and spinal levels are involved in the CL-RBE. Starbuck and Eston (32) found significant increases in recruitment of slow-twitch motor units in the contralateral elbow flexors during the second eccentric exercise bout. Howatson et al. (16) compared eccentric and concentric contractions of the left wrist flexors for changes in corticospinal and spinal excitability in the relaxed right flexor carpi radialis (FCR). They showed that corticospinal excitability in the relaxed right FCR increased more during eccentric than concentric contractions, intracortical facilitation decreased during concentric contractions but increased during eccentric contractions, and interhemispheric inhibition to the “nonactive” motor cortex diminished during concentric contractions but became nearly abolished during eccentric contractions. They did not find any differences in the decreased amplitude of the H-reflex in the relaxed right FCR between eccentric and concentric contractions of the left wrist flexors. These data suggest that the ipsilateral motor cortex output and the shift to lower interhemispheric and intracortical inhibition are greater during eccentric than concentric contractions. Kidgell et al. (21) also showed that eccentric training modulated corticospinal excitability and inhibition of the untrained limb to a greater extent than concentric training. It may be that the first eccentric exercise can affect motor control used in the second eccentric exercise even for the opposite arm, but it should be noted that this process requires more than 12 h after the first exercise bout. Future research is required to understand the mechanisms underlying the CL-RBE by investigating the changes occurring between 12 and 24 h after the initial exercise bout.
Regardless of the underlying mechanisms, the present study provides important information for future muscle damage studies and clinical implications. When considering a study design, the results of the present and previous studies (16,17,25,32,35) suggest that it is more appropriate to use a between-subject design than a within-subject design to investigate the effect of an intervention in comparison with a control condition. For example, many studies have used an arm-to-arm comparison design to investigate an effect of an intervention (e.g., supplementation and therapeutic modality) on DOMS and/or other muscle damage markers. The present study showed that if the arm-to-arm comparison design is used, it would be better to have at least 8 wk between bouts to eliminate potential “carry over” effect. If blood samples are not taken, or systemic responses are not necessary to be considered, it is also possible to set two bouts (e.g., control vs intervention) separated by less than 12 h. It can also be said that the RBE is a good model to investigate muscle adaptation mechanisms because the adaptation is rapid, and it seems possible that the underlying mechanisms of the RBE overlap with the mechanisms underpinning muscle adaptations after resistance training. As one of the possible practical applications, the CL-RBE should be considered for rehabilitation after immobilization and/or disuse. It is possible that the remobilization of an immobilized or disused limb induces muscle damage when eccentric contractions are included because disused muscles are susceptible to eccentric exercise-induced muscle damage (10,31,34). To minimize this kind of muscle damage, it can be recommended that before retraining the injured limb muscles, eccentric exercise should be given to the contralateral “healthy” limb muscles. As shown in several previous studies (12,17,23,25,32,35), cross-education effects are useful for rehabilitation for recovery from a unilateral injury, and the use of eccentric contractions in such occasion appears to be effective (21). Kidgell et al. (21) have suggested that unilateral eccentric contractions provide a greater stimulus in cross-education paradigms and should be an integral part of the rehabilitative process after unilateral injury to maximize the response.
In conclusion, the present study showed that CL-RBE lasted 4 wk but not 8 wk; thus, the magnitude of the effect was smaller than that of the ispilateral RBE that has been reported to last more than 8 wk. Importantly, when the two bouts were performed within 12 h, the CL-RBE was not evident. Thus, it requires a day for the CL-RBE to be conferred. Further studies are necessary to investigate how CL-RBE is produced and lost. It has been shown in the previous studies (2–4,7,22) that low-intensity eccentric contractions or maximal isometric contractions at a long muscle length of one limb can confer a preconditioning effect to attenuate muscle damage after a subsequent bout of more demanding eccentric exercise performed with the ipsilateral limb. Thus, it is interesting to investigate whether this is also the case for the CL-RBE. Moreover, the present study only focuses on men, and it is necessary to investigate women whether they will show similar results to men.
The authors thank Miss. Jui-Hsin Lin, Mr. Yu-Lei Su, Mr. Kuan-Chieh Chen, and Miss. Guan-Ling Hwang for their assistance with the data collection or data analysis. This research was supported by the National Science Council (NSC102-2628-H-415-036-MY3), “Aim for the Top University Plan” of National Taiwan Normal University and the Ministry of Education, Taiwan.
The authors declare that they have no conflict of interest, and the results of the present study do not constitute endorsement by the American College of Sports Medicine.
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