Muscle Damage and Soreness Following Repeated Bouts of Consecutive Drop Jumps

Muscle Damage and Soreness Following Repeated Bouts of Consecutive Drop Jumps Motoyoshi MIYAMA 1 and Kazunori NOSAKA 2 1 Faculty of Management and Information Science, Josai International University 2 School of Biomedical and Sports Science, Edith Cowan University (AUSTRALIA) Abstract This study investigated the magnitude of muscle damage and soreness induced by consecutive drop jumps (DJ), and whether changes in indicators of muscle damage attenuate when the same DJ exercise is repeated 8 weeks after the first exercise bout. Eight subjects performed two bouts of DJ exercise separated by 8 weeks. Subjects performed five sets of 20 DJ from a height of 0.6 m box with a 10-s interval between jumps, and 2-min rest period between sets. Jump height during DJ, peak vertical ground reaction force (peak VGRF), and ground contact time were determined from the force platform data for each DJ. Heart rate was also measured during exercise. Maximal isometric force (MIF), muscle soreness (SOR), plasma creatine kinase (CK) activity, and vertical jump performance were measured before and immediately after, and 1, 24, and 48 hours after the DJ exercise, and blood lactate concentration was measured before and immediately after the exercise. All indicators of muscle damage changed significantly (P 0.05) after both bouts, whereas no significant differences between bouts were evident for jumping height during DJ, peak VGRF, ground contact time, heart rate, and blood lactate concentration. Compared to the first bout, all indicators of muscle damage resulted in significantly (P 0.05) smaller changes after the second exercise bout. The maximal decline in MIF from pre-exercise value for bout 1 and 2 were 39.6 % and 29.2 %, respectively. SOR peaked 24-48 h after exercise for both bouts, but SOR after bout 2 was significantly (P 0.05) lower than that after bout 1. Peak CK activity was 910 IU L 1 after the first bout, but 391 IU L 1 after the second bout, which was significantly (P 0.05) lower than after the first bout. It was concluded that a consecutive DJ exercise induced severe muscle damage, and adaptation resulting in attenuated responses of MIF, SOR, plasma CK activity, and vertical jump performance lasted for 8 weeks. plyometric exercise, isometric strength, muscle soreness, plasma CK activity, vertical jump Address for correspondence to: Motoyoshi MIYAMA Faculty of Management and Information Science, Josai International University, 1 Gumyo, Togane, Chiba, 283-8555, JAPAN Telephone: +81-475-53-2146 Fax: +81-475-55-8811 E-mail: miyama@violet.plala.or.jp Introduction Eccentric muscle actions are performed in absorbing shocks, deceleration (7), or stretch-shortening cycle (SSC) exercise (10, 11). Drop jump is a typical plyometric or SSC exercise, and has been shown to be effective for improving jumping ability (1, 27). However, there is no or few research demonstrating optimal program variables for the design of plyometric exercise programs (24). It is reported that severe muscle damage occurred after approximately 100 consecutive DJ with maximal intensity (15, 19). However, optimal intensity, frequency, and recovery time between sessions for best training effect by DJ exercise are not well examined (24). Many studies have reported that a single bout of eccentric exercise confers protection against muscle damage following subsequent bouts of a similar exercise (6, 13, 14). This phenomenon, often referred to as a repeated bout effect, is characterized by a faster recovery of strength, a smaller restriction in joint range of motion, reduced swelling and muscle soreness, smaller increases in muscle proteins in the blood, fewer abnormalities on magnetic resonance or ultrasound images, and blunted immune responses after repeated exercise bouts (5, 21, 22). The repeated bout effect has been shown in various types of exercise for different muscle groups such as eccentric exercises of elbow flexors or knee extensors, and downhill running (14). It has been reported that the repeated bout effect lasts for several weeks to several months. Significantly smaller changes and faster recovery of indicators of muscle damage were sustained up to 2-4 weeks (16), 6 weeks-6 months (18), and 6 months (21) for high intensity eccentric exercise of the elbow flexors. For the lower extremity muscles, attenuated changes in indicators of muscle damage have been shown when the second bout was performed 4-13 days (12), or 3 weeks (2) after the first bout. Byrnes et al. (4) reported less muscle soreness and smaller increase in serum creatine kinase (CK) and myoglobin when a second bout of downhill running was repeated up to 6 weeks, but not 9 week, after the first bout. It appears that the lasting period of the repeated bout effect varies among exercise mode and muscle groups used 63

64 M. MIYAMA et al. in the exercise. It is also possible that the repeated bout effect depends on the magnitude of muscle damage induced by the initial bout of exercise. Since it appears that the magnitude of muscle damage induced by DJ exercise is greater than that of downhill running, the protective effect is expected to last longer than 6 weeks for DJ exercise. However, no study has investigated the repeated bout effect after DJ exercise. It is important to understand the magnitude and the time course of the muscle damage and the protective effect after a single bout of DJ exercise for designing training programs. Therefore, the purpose of this study was to investigate the magnitude of muscle damage and soreness following consecutive drop jumps, and examine whether attenuation of changes in indicators of muscle damage occurs when the same exercise is repeated after 8 weeks of the first exercise bout. Materials and Methods Subjects Eight male students who had little or no experience in resistance training participated in this study after signing a written informed consent document consistent with ethical standards at Yokohama City University, which were in accordance with the Helsinki Declaration of 1975. The subject number was considered adequate to statistically detect any differences between bouts based on a sample size estimation. Their mean ( SD) age, height, and weight were 23.0 3.4 yrs, 172.2 6.4 cm, and 62.7 9.3 kg, respectively. There was no significant difference in the mean body weight between the first and second exercise bouts. All subjects were free from any musculoskeletal disorders and were not allowed to perform any vigorous physical activities or unaccustomed exercises during experiment period. Subjects were requested to abstain from any medicine and dietary supplements during experiment period and between bouts. Exercise All subjects performed two bouts of drop jump exercise on wood (oak) surface of a force platform (TR61750-103, Sogokeiso Inc., Japan) separated by 8 weeks. Subjects dropped from a height of 0.6-m box, and jumped upward maximally immediately after landing from the box and landed on the surface again after the vertical jump. To perform next drop jump, subjects had to climb four steps onto the box. Five sets of 20 drop jumps were performed with a 10 s interval between jumps, and a 2-min rest period was given between sets. Subjects performed DJ with barefoot, because it has been reported that biomechanical factors are influenced dramatically by shoes (8, 23), and comparisons between bouts were thought to be easier by eliminating the effect of shoes. The investigators carefully supervised the exercise to eliminate the risks for unexpected injury. Criterion Measures The following independent variables were chosen to 1) compare the physiological loads and performance of DJ between bouts, and to 2) compare the magnitude of muscle damage. Data from the force platform were collected during the consecutive DJ by use of the Power Lab system (Power Lab, AD Instruments, Australia) and run on a computer (Macintosh G4, Apple Inc., USA) connected to the force platform (TR61750-103, Sogokeiso Inc., Japan) via a strain amp (AS2102, NEC San-ei, Japan). Sampling frequency of the force was 100Hz, and jump height of the drop jump (DJ height), peak vertical ground reaction force relative to body weight (peak VGRF) and ground contact time were determined from the force data for each DJ by using a software program of the Power Lab system (Power Lab, AD Instruments, Australia). Jump height was calculated as a rise of the center of gravity of the body based on flight time that was calculated as the period between takeoff and subsequent landing from the jump (17). During DJ, there were two peaks of VGRF: the first one was seen immediately after landing from the box, and the second one was seen at landing after jump. Ground contact time was determined as the period between initial landing on the test surface and takeoff in the jump (17). Heart rate (HR) was monitored by means of Polar HR monitors (Vantage XL new). HR was recorded 1 min before the exercise and immediately after each set. Blood lactate concentration was determined by a Biosen 5010 (EKF Industrie, Elektronik GmbH, Barleben, Germany) by obtaining blood sample from the finger tip with a 20- l heparin-coated capillary tube. Blood lactate concentration was measured before (pre) and immediately after (post) the exercise. Several indirect markers of muscle damage that have been used in previous studies (26), and vertical jump performance were assessed before the exercise and immediately, 1, 24, and 48 hours after the exercise. The test-retest reliability for all criterion measures has been examined in our previous study (15) and shown to be consistent. Maximal isometric force of the knee extensors: Maximal isometric force (MIF) was measured at a knee joint angle of 90 (1.57 rad) 3 times for 3 sec (1-min rest period between each measurement) by a load cell (model 1269, Takei Scientific Instruments Co. Ltd., Japan) connected to the Power Lab system via a strain amp (AS2102, NEC Sanei, Japan). The mean value of the 3 measurements was used for the analysis. Muscle Soreness (SOR): A visual analog scale (VAS) that had a 100-mm line with no pain on one end (0) and extremely painful on the other end (100) was used to evaluate SOR upon palpation (SOR-Pal) of the knee extensor, back, gluteal, and triceps surae muscles. Additionally, perceived soreness ratings were obtained for the knee extensors during a squat movement (SOR-Squat).

Muscle Damage Following Drop Jumps 65 Plasma CK activity : Blood was taken from the fingertip and 34 l of blood was analyzed using a Reflotron S System (Roche Diagonostics GmbH Mannheim, Germany). The normal reference ranges of plasma CK activity were 24-195 IU L 1 for this method. Vertical Jump Performance: Two types of jump performance tests were conducted; squat jump (SJ) and counter movement jump (CMJ). SJ was performed from a squatting position with a knee joint angle of approximately 90 (1.57 rad). Subjects were asked to hold this position for 3 seconds, and jump vertically to reach maximal height with no counter movement after a verbal command of Go. CMJ was performed from an erect standing position with knees fully extended, and the subjects made a downward counter movement to the same starting position as the SJ, and jumped vertically for maximal height upon the verbal command Go (3). Each test was performed twice, and the mean value of the 2 measurements was used for the analysis. Statistical Analysis Changes in the criterion measures were compared between the first bout (bout 1) and the second bout (bout 2) using a repeated-measures analysis of variance (ANOVA). In the event with a significant F ratio, Tukey s post hoc test was used to compare means. Overall relative changes in MIF were compared using a paired t-test. Statistical significance was set at P 0.05. Data are presented as means SEM. Results Jump height during DJ (DJ height) DJ height decreased by approximately 10% for bout 1 (1 st, 0.39 0.02; 5 th, 0.35 0.02 m), 12% for bout 2 (1 st, 0.40 0.03; 5 th, 0.35 0.01 m) throughout the exercise (Figure 1). DJ height decreased gradually from the first to the 20th jumps in a set, and significant (p 0.01) decreases in the height were also evident from the first to the fifth set for both bouts. There was no significant difference in the jump height and the rate of decrease in height from the first to fifth set between bouts. Peak vertical ground reaction force (VGRF) Peak VGRF recorded immediately after landing from the box increased for bout 1 (1 st set, 2399 185; 5 th set, 2522 185 N), and for bout 2 (1 st set, 2300 121; 5 th set, 2542 182 N). As shown in Figure 2, there was no significant difference in the first peak VGRF between the bouts throughout the DJ exercise (approximately 4.0 times the body weight for both exercise bouts). Similarly, the second peak VGRF after the vertical jump was not significantly different throughout the DJ exercise. Ground contact time Ground contact time increased from the first set (bout 1, 554 39 ms; bout 2, 590 45 ms) to the last set (bout 1, 601 42 ms; bout 2, 619 49 ms) for both exercise bouts, and the rate of increase from the first to the last set was approximately 10% for both exercise bouts. No significant differences in the ground reaction time were evident between bouts 1 and 2. HR HR increased markedly from pre to the first set, and increased progressively toward the last set for both bouts. The peak HR recorded immediately after the fifth set was 181 4 bpm for the first bout, and 170 6 bpm for the second bout. No significant difference in HR responses was found between bouts. Blood lactate concentration Blood lactate concentration increased significantly Fig. 1 Changes in mean jump height of 20 drop jumps in each set over 5 sets of drop jump exercise during bout 1 and 2. Mean ( SEM) values of 8 subjects are shown. Fig. 2 Change in peak VGRF in each set over 5 sets of drop jump exercise during bout 1 and 2. Mean ( SEM) values of 8 subjects are shown.

66 M. MIYAMA et al. from pre to post-exercise for bout 1 (pre, 2.0 0.2; post, 6.4 0.4 mmol L 1 ), and for bout 2 (pre, 2.3 0.3; post, 6.4 0.8 mmol L 1 ). No significant difference was evident in blood lactate responses between bout 1 and 2. Maximal Isometric Force There was no significant difference in the pre-exercise MIF between bout 1 and 2. MIF decreased significantly (p 0.01) immediately after exercise for both bouts. The maximal decline in MIF from pre-exercise value for bout 1 and 2 were 39.6 % (24 h post-exercise) and 29.2 % (immediately after exercise), respectively (Figure 3). MIF was still 63.3 8.2 % of the pre-exercise value for bout 1 and 77.4 9.2 % for bout 2 at 48 h after exercise, and these values were significantly lower (P 0.05) than preexercise value. The mean values of MIF relative to pre-exercise value from immediately to 48 h after DJ exercise was significantly (p 0.01) higher for bout 2 (75.0 5.7 %) compared to bout 1 (63.0 5.7 %). Muscle soreness Muscle soreness was evident for the knee extensors, back, gluteal, and triceps surae muscles after the exercise for both bouts. However, the peak SOR-Pal of the back (bout 1, 40.9 11.1 mm), gluteal (bout 1, 40.9 11.6 mm), and triceps surae (bout 1, 30.4 12.0 mm) muscles were significantly less than that of the knee extensors for bout 1. As shown in Figure 4, muscle soreness developed 24 h after exercise and showed a peak 24-48 h after exercise for both bouts. The peak SOR-Pal of the knee extensors was significantly (P 0.05) different between bout 1 (71.1 7.5 mm) and bout 2 (39.8 9.9 mm). Compared to bout 1, SOR-Squat was also significantly (P 0.05) lower than bout 2. Plasma CK activity There was no significant difference in the pre-exercise plasma CK activity between bout 1 (115 20 IU L 1 ) and bout 2 (97 28 IU L 1 ). Plasma CK activity increased significantly after DJ for both bouts, and the magnitude of increase was significantly (P 0.05) larger for bout 1 compared to bout 2 (Figure 5). Plasma CK activity showed a peak 24-h after exercise in both bouts, and peak values for bout 1 and bout 2 were 910 273 IU L 1 (791 % of the pre-exercise value), and 391 140 IU L 1 (403 % of the pre-exercise value), respectively. Vertical Jump Performance There was no significant difference in SJ between bouts (Bout 1 & 2, 0.38 0.02 m) and CMJ (Bout 1 & 2, 0.41 0.02 m) before exercise, and CMJ was 7.9 % higher than SJ. SJ and CMJ height decreased significantly (P 0.05) after DJ exercise in both bouts, and the magnitude of Fig. 4 Changes in muscle soreness upon palpation of the knee extensors (a) and during a squat movement (b). Before (pre), immediately after (post), and 1-48 hours after bout 1 and 2. Mean ( SEM) values of 8 subjects are shown. *P 0.05. Fig. 3 Maximal isometric force expressed relative to the pre-exercise value (100%) before (pre), immediately after (post), and 1-48 hours after bout 1 and 2. Mean ( SEM) values of 8 subjects are shown. Fig. 5 Plasma CK activity before (pre), immediately after (post), and 1-48 hours after bout 1 and 2. Mean ( SEM) values of 8 subjects are shown. *P 0.05.

Muscle Damage Following Drop Jumps 67 decrease was significantly different between bout 1 and 2 (Figure 6). Reductions in SJ and CMJ persisted for 48 h (P 0.05) after DJ exercise for bout 1. 48 h after exercise, SJ and CMJ recovered to 90.2 4.7 %, and 93.6 3.3 %, respectively, of the pre-exercise value in bout 2, which were significantly (p 0.01) greater than in bout 1 (SJ, 69.9 4.7 %; CMJ, 73.7 4.4 %). Fig. 6 Jump height of squat jump (a) and counter movement jump (b) expressed relative to the pre-exercise value (100%) before (pre), immediately after (post), and 1-48 hours after bout 1 and 2. Mean ( SEM) values of 8 subjects are shown. *P 0.05. Discussion Muscle damage after the initial exercise bout The present study investigated the development of muscle damage following 5 sets of 20 consecutive drop jumps from a height of 0.6 m. Based on the HR, blood lactate, and VGRF (Figure 2) data, it seems reasonable to assume that this exercise was strenuous and stressful for leg muscles. In fact, all indicators of muscle damage changed significantly after the DJ exercise bout. The maximal decline in MIF from the pre-exercise value following the first DJ exercise was 39.6 % at 24 h, and 36.7 % at 48 h post exercise (Figure 3). Warren et al. (26) reported that prolonged strength loss after eccentric exercise is considered to be one of the most valid and reliable indirect measures of muscle damage. It is reported that concentric exercises decrease strength by 10-30 % immediately after exercise, but strength returns to baseline within hours after exercise (5). In the present study, MIF did not recover 48 h after exercise, suggesting that not only fatigue but also muscle damage caused the strength loss (Figure 3). Strength loss after a bout of eccentric exercise could be due to 1) an inhabitation of motor unit activation secondary to pain or damage, 2) physical disruption of the force-generating structures or 3) a failure to activate forcegenerating structures within the muscle fiber (13). Clarkson & Hubal (5) reported that the extent of muscle soreness differed between downhill running and strenuous eccentric exercise, but its time course was similar. The magnitude of SOR was largest for the knee extensors, with a peak value of 71.1 mm at 48 h post-exercise for bout 1, and the time course of SOR change in the present study appeared to be similar to that shown by previous studies (Figure 4). It is reasonable to assume that the knee extensor muscles are subjected to eccentric actions at the moment of landing (7), and these repetitive eccentric muscle actions caused muscle damages. It is reported that plasma CK activity showed a peak 12-24 h post-exercise with increases ranging from 100 to 600 IU L 1 after downhill running (4, 9, 25), whereas peaked 4-6 days (2,000-10,000 IU L 1 ) after high-force eccentric exercises for the forearm flexors (5). In the present study, plasma CK activity showed faster and smaller increases than that reported after eccentric exercises for forearm flexors (5), although the reason for this has been unclear. The time course of changes in plasma CK activity in the present study was similar to that after downhill running, but, the peak value seemed to be larger than after downhill running (Figure 5). It has been suggested that mechanical factors rather than metabolic factors are important in determining the magnitude of eccentric exercise-induced muscle damage (20). The maximal decline in SJ and CMJ performance in bout 1 were 30.1 and 26.3 % from the pre-exercise value, respectively (Figure 6). The decreases in the jump performance seem to be correlated with the decreases in MIF of the knee extensors, although the magnitude of decrease in jump performance was not as large as that of MIF. This suggests that not only isometric muscle force but also other factors such as muscle power or timing of muscular activation determine the jump performance. In addition, vertical jump performance was mainly determined not only by the knee extensors, but also by the hip and ankle extensors (1, 27). It seems likely that the smaller decline in SJ and CMJ than in MIF is related to the smaller damages to the hip and ankle extensors, which were indicated by the smaller SOR for these muscles than for knee extensors. The repeated bout effect It should be noted that the amount of exercise volume was similar for bouts 1 and 2, and muscles were similarly stressed by the exercise, because there were no significant differences between bouts in DJ height, peak VGRF, ground contact time, HR, and blood lactate concentration (Figures 1 & 2). It is also important to note that the pre-exercise values of all the criterion measures did not differ significantly between bouts. Therefore, changes in indicators of muscle damage after the second bout should have been similar to those after the first bout if no adaptation would occur. However, changes in MIF, SOR, plasma CK activity and vertical jump performance were found to be signifi-

68 M. MIYAMA et al. cantly smaller after the second bout than after the first bout (Figures 3-6). These data suggest that the repeated bout effect lasts, at least, for 8 weeks after the DJ exercise. Byrnes et al. (4) reported that a protective effect against muscle protein release and the generation of SOR was observed for 6 weeks but not 9 weeks after a downhill running for 30 minutes at 10 slope. The present findings indicate that the repeated bout effect is produced by a different type of exercise for the lower limb. Nosaka et al. (21) has shown that a protective effect against the muscle damage lasts at least for 6 months after 24 maximal eccentric actions of the elbow flexors, and the magnitude of the repeated bout effect decreases with an increase in interval between bouts and the effect is totally lost between 9 and 12 months after the first bout. However, it has been reported that there are different responses of indicators of muscle damage to a bout of eccentric exercises between lower limb muscles (downhill or marathon running, DJ) and the elbow flexors (5, 15). Moreover, different duration of the protective effect between downhill running (4) and eccentric exercise for the elbow flexors (21) may be due to different methodologies used and/or the extent of muscle damage (21). The maximal duration of the repeated bout effect after DJ exercise is to be determined separately. DJ exercises have often been used as one of the plyometric exercises to develop power in many sports. It is suggested that recovery time required between successive sessions of plyometric training should be 48-72 h, and complete and adequate recovery between sessions is required in a guideline (24). The present study showed that MIF (Figure 3), SOR (Figure 4), and vertical jump performance (Figure 6) did not recover to the pre-exercise level by a 48 h rest period after the first exercise bout. This suggests that the recovery time for 48 h between sessions is not sufficient after a bout of 100 consecutive DJ exercise, and might bear a risk for overtraining, especially at an initial phase of training. However, it should be noted that the recovery of MIF and vertical jump was significantly faster (Figure 3 & 6), and development of SOR and increases in plasma CK activity were smaller (Figure 4 & 5) after the second bout compared to the first bout, even when the second bout was performed 8 weeks after the first bout. Although full recovery of criterion measures was not achieved even 48 h after second bout in this study, it seems reasonable to assume that the recovery process is enhanced as training proceeds. 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