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The Effect of Pomegranate Juice Supplementation on Strength and Soreness after Eccentric Exercise

Trombold, Justin R; Reinfeld, Ari S; Casler, James R; Coyle, Edward F

The Journal of Strength & Conditioning Research: July 2011 - Volume 25 - Issue 7 - p 1782-1788
doi: 10.1519/JSC.0b013e318220d992
Original Research

Trombold, JR, Reinfeld, AS, Casler, JR, and Coyle, EF. The effect of pomegranate juice supplementation on strength and soreness after eccentric exercise. J Strength Cond Res 25(7): 1782-1788, 2011—The purpose of this study was to determine if pomegranate juice supplementation improved the recovery of skeletal muscle strength after eccentric exercise in subjects who routinely performed resistance training. Resistance trained men (n = 17) were randomized into a crossover design with either pomegranate juice or placebo. To produce delayed onset muscle soreness, the subjects performed 3 sets of 20 unilateral eccentric elbow flexion and 6 sets of 10 unilateral eccentric knee extension exercises. Maximal isometric elbow flexion and knee extension strength and muscle soreness measurements were made at baseline and 2, 24, 48, 72, 96, and 168 hours postexercise. Elbow flexion strength was significantly higher during the 2- to 168-hour period postexercise with pomegranate juice compared with that of placebo (main treatment effect; p = 0.031). Elbow flexor muscle soreness was also significantly reduced with pomegranate juice compared with that of placebo (main treatment effect; p = 0.006) and at 48 and 72 hours postexercise (p = 0.003 and p = 0.038, respectively). Isometric strength and muscle soreness in the knee extensors were not significantly different with pomegranate juice compared with those using placebo. Supplementation with pomegranate juice attenuates weakness and reduces soreness of the elbow flexor but not of knee extensor muscles. These results indicate a mild, acute ergogenic effect of pomegranate juice in the elbow flexor muscles of resistance trained individuals after eccentric exercise.

Human Performance Laboratory, Department of Kinesiology and Health Education, The University of Texas at Austin, Austin, Texas

Address correspondence to Justin R. Trombold,

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Eccentric exercise results in a large reduction in strength of 10-50% with weakness persisting 4-14 days and a delayed-onset of muscle soreness (DOMS) peaking 24-48 hours postexercise (6,7). The causes of these functional alterations are not completely understood but are likely because of a complex interaction of structural damage to the myofiber (13), excitation-contraction coupling dysfunction (16), inflammation (4), and oxidative stress (32). Attenuating inflammation (4) or oxidative stress (32) improves skeletal muscle function after eccentric exercise.

Studies providing dietary supplements in an attempt to reduce the soreness or weakness that is characteristic of DOMS have used traditional antioxidant or anti-inflammatory treatments such as vitamin E (3), vitamin C (7), ibuprofen (10), and nonsteroidal anti-inflammatory medication (11). These studies have yielded equivocal or ergolytic effects. Recently, polyphenol supplementation from pomegranates has been studied for the treatment of clinical conditions characterized by inflammation and oxidant stress such as cardiovascular disease (2), rheumatoid arthritis (26), and type 2 diabetes (22). The anti-inflammatory and antioxidant capacities of polyphenols have been attributed to oxidant scavenging (14) or selective inhibition of the proinflammatory transcription factor, NF-κB (26,28). Attenuation of proinflammatory signaling or improved capacity to scavenge free radicals in skeletal muscle cells may alleviate degenerative muscle damage and improve strength recovery. Additionally, our work (29), and the work of others (8) in untrained men, has reported improved the recovery of strength after a single eccentric exercise bout when untrained subjects consume polyphenols.

It is well established that the performance of a single bout of eccentric exercise provides some protection from weakness and soreness when a second bout of eccentric exercise is performed and that this protection lasts for up to 6 weeks (i.e., repeated bout effect) (12,17). As a result of the protective effect, previous research using eccentric exercise to examine the functional and biochemical changes after acute muscle damage has been performed on untrained individuals. However, a recent study has shown that resistance trained individuals do indeed display some muscle soreness and weakness after an intense eccentric exercise bout (18), although the response is attenuated compared with that of untrained control subjects. Because most training situations for athletes are not in an untrained condition, the description of functional outcomes (i.e., strength and soreness) after an intense exercise stimulus in individuals who are previously resistance trained will provide a more sports-specific experimental approach. Consequently, it is of practical importance to investigate the ergogenic effects of polyphenol supplementation in resistance trained men who may have an attenuated DOMS response to eccentric exercise.

Based on previous studies using polyphenol supplementation in untrained populations (8,20,29), we hypothesize that similar nutritional strategies will attenuate DOMS in recreationally resistance trained individuals. To our knowledge, no study has examined the effects of polyphenol supplementation in resistance trained individuals after they perform intense eccentric exercise. The primary purpose of this study was to determine the effects of twice-daily pomegranate juice supplementation on isometric strength and muscle soreness in resistance trained individuals after bouts of high-intensity eccentric exercise involving both the arms and the legs.

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Experimental Approach to the Problem

This study was a double-blind, counterbalanced, placebo-controlled crossover experiment with 2 treatments, each lasting 15 days with a minimum 14-day washout period separated the 2 treatments. The subjects were randomly assigned to either pomegranate juice or placebo (PLA) for the first treatment, with the opposite treatment provided during the second 15-day testing period. On the eighth day of each treatment, the eccentric exercise was performed to elicit DOMS. Furthermore, for each treatment, the eccentric exercise, isometric strength, and muscle soreness measurements were performed unilaterally, and the ordering of dominant or nondominant limb was randomized. The contralateral limb was used for the second trial. Two sessions of familiarization for maximal isometric strength were performed before the start of the study (FAM1) with an additional familiarization on day 7 of each treatment (FAM2). Additionally, on day 7 of each treatment, unilateral knee extension one-repetition maximum was determined for the leg tested the following day.

On day 8, baseline measures of soreness and isometric strength were collected immediately before eccentric exercise with recovery measurements for both variables made 2 hours (day 8), 24 hours (day 9), 48 hours (day 10), 72 hours (day 11), 96 hours (day 12), and 168 hours (day 15) postexercise. The subjects reported to the laboratory at the same time of the day during each treatment and were instructed to avoid caffeine ingestion before each visit.

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Seventeen healthy, nonsmoking, physically active, resistance trained men (age: 21.9 ± 2.4 years, weight: 80.2 ± 7.5 kg, and height: 179.1 ± 8.4 cm) were recruited from the university community. The subjects were required to have been weight training for at least the last 3 months, exercising both their upper body and lower body, at least 2 d·wk−1. Weight training included performing at least 2 sets of elbow flexion and knee extension at an intensity of 70-90% of one-repetition maximum and continued to ‘failure’ typically reached after performing 4-12 repetitions per set. Throughout the duration of the study, the subjects were instructed to maintain their normal diet and current level of physical activity, refrain from consumption of anti-inflammatory or antioxidant supplements and to not introduce new exercises into their weight training regimen. Other criteria for exclusion were a recent weight change of >5 kg; history of hypertension; the presence of major orthopedic injury; use of anti-inflammatory drugs, over-the-counter pain medicine, vitamin or mineral supplements, angiotensin-converting enzyme inhibitors, lipid-lowering medications, or selective serotonin reuptake inhibitors. The study was conducted under a protocol approved by the University of Texas at Austin Institutional Review Board. Each subject provided written informed consent.

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Supplements of PJ or PLA (250 ml) were ingested twice daily at 12-hour intervals over each 15-day treatment. On the eighth day of supplementation, eccentric exercise was performed, and the subjects consumed an additional bottle of PJ or PLA immediately after eccentric exercise. The PJ or PLA were provided by POM Wonderful (Los Angeles, CA, USA) using Wonderful variety pomegranates grown in California. The products were shipped frozen and stored at 4°C. Each 250-ml bottle of PJ contained approximately 1,979 mg/L of tannins, 384 mg/L of anthocyanins, and 121 mg/L of ellagic acid derivatives (content obtained from the manufacturer). Both PJ and PLA contained 35 g of carbohydrate (maltodextrin and sucralose added to PLA to match PJ) with additional coloring and flavoring used to blind the PLA treatment. The subjects were reminded verbally and through e-mail communication to consume the experimental supplements at the required times. The pomegranate juice used in this study has similar pharmacokinetics of ellagitannin metabolites in human plasma compared with what the pomegranate extract (24) used in previous research had (29).

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Eccentric Exercise

On the morning of the eighth day of each treatment, the subjects performed the unilateral eccentric exercise bout 15 minutes after the baseline measurements of strength and soreness. A Biodex isokinetic dynamometer (Cybex International, Medway, MA, USA) was used to perform the eccentric elbow flexor exercise protocol. Previous research has validated the isokinetic dynamometer as an appropriate tool for induction of eccentric muscle damage (3,29). The subjects performed 3 sets of 20 maximal eccentric elbow extensions during which the elbow flexor muscle resisted these repetitions through a full range of motion to complete extension, with 180 seconds of rest between sets. The axis of rotation was aligned with the lateral epicondyle of the humerus. Repetitions were performed once every 15 seconds, with each repetition lasting 3 seconds at a velocity of 0.7 rad·s−1. During the 3-second contraction, the subjects were verbally encouraged to maximally resist the motion of the lever arm. A Cybex leg extension machine (Cybex International) was used for the unilateral eccentric contraction of the knee extensor muscles. The subjects performed 6 sets of 10 eccentric repetitions at 110% of their unilateral 1RM through a full range of motion. The subjects were instructed to lower the weight at a 3-second count and were verbally encouraged to maintain the appropriate velocity, with 180 seconds of rest given between each set.

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Isometric Strength Test

Before the start of the study, and on the seventh day of supplementation in both treatments, familiarization sessions were performed to practice the isometric strength tests. This was conducted to ensure that each subject was comfortable and able to generate maximal isometric force for each arm and leg.

Peak isometric elbow flexion and knee extension strength tests were performed using a modified preacher curl bench and modified knee extension machine, respectively. Force was recorded using a load cell (LC101-500, Omega Engineering, Stamford, CT, USA) secured to the ground using a steel cable and a strap secured to the wrist or lower shin of the subject. The subject performed 4 trials, 2 at both 30° and 45° above elbow extension and 45° and 90° below knee extension, with 180 seconds of rest in between each trial. Strength for both tests was reported as the average of the peak value of each joint angle, as results were similar at both angles. Strength was measured before the eccentric exercise on day 8 (i.e., BASELINE), and 2, 24, 48, 72, 96, and 168 hours posteccentric exercise for both treatments, using only the arm or leg that performed the eccentric exercise. All strength values are normalized to the baseline strength measurement. One subject did not complete the knee extension testing because of knee pain from a prior injury that was not disclosed until the day of the eccentric exercise session.

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Soreness of the elbow flexor and knee extensor muscles was determined before each isometric strength test by having the subject subjectively rate the degree of soreness using a visual analog scale of 0-10 (19). ‘No soreness’ was described as 0, and ‘Unbearable soreness’ was described as 10. This rating was obtained before measurement of strength while performing unloaded elbow flexion and knee extension of the tested arm and leg.

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Exercise Standardization

Each subject was instructed to follow his normal weight training routine throughout the entire study, with the exception of days 7-11 of each treatment, when no exercise was permitted outside the laboratory. Additionally, standardized resistance exercise was performed (days 4-6 and 12-14) to ensure no detraining occurred during the duration of the study. On days 4 and 12, each subject performed upper body “pulling” exercises (lat pulls, etc.), on days 5 and 13, lower body exercises were performed, and on days 6 and 14, upper body “pushing” exercises were performed (bench press, etc.), with additional instruction to match the lifts (type, volume and intensity) for each standardized exercise period. Within each standardized day, subjects were instructed not to perform any unfamiliar lifts to ensure that no additional soreness would be present during either treatment period. On day 12, each subject performed the standardized exercise after the 96-hour measurements were taken. Each subject was provided with a personalized calendar with specific activity and supplementation guidelines for each day of both treatment periods.

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Statistical Analyses

Two-way analysis of variance with repeated measures was used for treatment × time, time and treatment analysis for isometric strength (%), and muscle soreness (0-110). If treatment × time interactions were found to be significant on a specific dependent variable, then t-tests were used to make pairwise comparisons between treatments at each time point. If Mauchly's test of sphericity was violated, the Greenhouse-Geisser correction was used for the assessment of the treatment effect for each dependent variable. When significance was found, Fisher's least significant difference post hoc test was applied. SPSS software was used for all statistical tests. Intraclass coefficients of variation for isometric elbow flexion and knee extension strength were 3.8 and 6.4%, respectively. Significance was assessed at alpha level of p ≤ 0.05. Values are reported as mean ± SD.

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Isometric Elbow Flexion Strength and Soreness

Analysis of variance indicated that isometric elbow flexion strength was significantly greater (main treatment effect, 93.6 vs. 88.9%; p = 0.031) with the PJ treatment compared with that with the PLA treatment during the 2- to 168-hour period after eccentric exercise (Figure 1). At 2 hours, isometric strength was significantly reduced in both PJ and PLA treatments compared with BASELINE (75.5 ± 13.3 and 65.0 ± 16.9% of BASELINE, respectively; p < 0.0001). There was no treatment × time interaction between treatments (p = 0.158).

Figure 1

Figure 1

Analysis of variance indicated that the soreness of the elbow flexor muscles was significantly reduced (main treatment effect; p = 0.006) in the PJ treatment compared with that in the PLA treatment during the 2- to 168-hour period after eccentric exercise (Figure 2). In addition, there was a significant treatment × time interaction (p = 0.004), and the elbow flexor soreness was reduced in PJ compared with that in PLA at 48 and 72 hours (2.71 ± 2.11 vs. 3.77 ± 1.68; p = 0.003 and 1.65 ± 2.09 vs. 2.41 ± 1.70; p = 0.038), respectively. In both treatments, muscle soreness was significantly increased from BASELINE at 2, 24, 48, and 72 hours (p < 0.01). There were no ordering effects for either isometric strength measurement.

Figure 2

Figure 2

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Isometric Knee Extension Strength and Soreness

Two hours posteccentric exercise, isometric strength was significantly (p < 0.001) reduced to 80.0 ± 10.8 and 82.1 ± 11.3% of BASELINE in PJ and PLA, respectively (Figure 3). The rating of soreness of the knee extensor muscle increased after the eccentric exercise at 2, 24, 48, and 72 hours (p < 0.01), yet no significant differences were observed when comparing PJ and PLA (Figure 4). Treatment × time interaction was not significantly different for isometric strength or soreness of the knee extensor muscles (p = 0.116 and p = 0.390, respectively). There were no ordering effects of the treatments for both upper and lower body strengths or soreness measurements.

Figure 3

Figure 3

Figure 4

Figure 4

There were no differences between FAM2 for either treatments and BASELINE in both elbow flexor and knee extensor isometric strength, indicating sufficient familiarization.

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The primary finding of this study was that twice-daily pomegranate juice supplementation in resistance trained subjects attenuated the reduction in isometric strength of the elbow flexor muscles after an intense bout of eccentric exercise. This was also associated with a significant attenuation of the rating of muscle soreness in the elbow flexors during the 2- to 168-hour period after eccentric exercise. The secondary finding of this study is that resistance trained individuals who perform bouts of very intense eccentric exercise can indeed experience a loss of isometric strength and develop muscle soreness in both the elbow flexor and knee extensor muscles. Additionally, it appears that PJ supplementation did not attenuate muscle soreness and weakness in the leg muscles of recreationally resistance trained individuals.

Many types of tissue damage because of injury or disease are characterized by acute and chronic elevations in local and systemic markers of inflammation and oxidative stress. In animal models of muscle damage, genetic manipulation aimed at attenuation of inflammation or oxidant stress decreases muscle damage after stretch injury (4,32). Furthermore, pomegranate juice has been shown to possess potent antioxidant and anti-inflammatory effects (23), with daily consumption shown to attenuate the development of diseases characterized by local and systemic inflammation and oxidant stress (2). In this study, postexercise strength (main treatment effect 93.6 vs. 88.9% of baseline; p = 0.031) and soreness (main treatment effect 1.54 vs. 2.03; p = 0.006) of the elbow flexor muscles was improved with PJ supplementation. These findings are in agreement with those of previous human studies showing that polyphenol supplements from tart cherry juice (8) and pomegranate extract (29), when provided to untrained subjects, improves the recovery of strength in the days after eccentric exercise. This provides further evidence that polyphenol supplementation has an acute ergogenic effect on muscle strength and soreness in the elbow flexors after eccentric exercise, an effect that extends to individuals that are previously resistance trained. Despite this finding, there was no ergogenic effect in the knee extensors.

Supplementation with polyphenols has been shown to attenuate postexercise inflammation in mice (9) and human models of eccentric exercise (15,20). Despite this, local inflammation is minimal 2 hours postinjury (1); however, in this study, the significant improvement in isometric strength in the elbow flexor muscles seems to be largely driven by the 2-hour postexercise measurement. Although the exact mechanism for the timing of this attenuated weakness is unknown, polyphenols have been shown to promote membrane stability (25) and attenuate propagation of lipid peroxidation by scavenging peroxyl radicals (5). In light of this, the high phenolic content in the PJ supplement used in this study (23) may have stabilized membranes in the myocytes and preserved excitation-contraction coupling immediately postexercise, an effect that may have attenuated the weakness at 2 hours in the PJ group.

Traditional isotonic weight training, and performance of eccentric exercise that is submaximal, is characterized by providing protection from weakness (17) and soreness (12) when a subsequent eccentric bout is performed up to 6 weeks later (i.e., repeated bout effect). Furthermore, recent research has shown improved isometric and isokinetic strength recovery from eccentric exercise in individuals that are resistance trained compared with that in untrained controls (18). Despite previous resistance training, the trained group in the previous study (18), and in this study, experienced significant weakness and soreness (∼70-80% of pre-exercise isometric strength at 1-2 hours of recovery). Because skeletal muscle can generate up to 50% greater torque during the eccentric compared with that during the concentric phase of movement (21), the loading of skeletal muscle during the eccentric portion of traditional isotonic training is limited by the concentric 1RM. The presence of DOMS symptoms in the previously isotonic resistance trained subjects used in this study is probably because of the atypical intense loading of the muscles during the bouts of eccentric exercise. In support of this, Warren et al. demonstrated that the magnitude of eccentric-induced muscle damage is dependent on the peak force development during muscle lengthening (30). Furthermore, Clarkson and Tremblay (6) reported that the protective effect of prior eccentric exercise for both strength and soreness was volume dependent. Taken together, attenuation of classic DOMS symptoms may be dependent on the type (eccentric vs. isotonic), volume, and intensity of the initial exercise. This indicates that the introduction of an intense novel exercise stress, even in previously trained individuals, results in weakness and soreness.

An interesting finding in this study was that the postexercise strength loss was more prominent in the elbow flexor muscles compared with that in the knee extensors. This phenomenon may result from the daily use of quadriceps for ambulation, offering added protection from weakness or soreness. Alternatively, the protocol used for eccentric exercise was different between the elbow flexors and knee extensors because maximal eccentric torque of the knee extensors exceeded the torque limit of the isokinetic dynamometer. Consequently, the eccentric knee extension protocol used a fixed resistance set at 110% of the unilateral concentric 1RM. However, during the elbow flexion protocol, the subjects were instructed to maximally resist the lengthening of the isokinetic dynamometer arm, generating eccentric torque that is likely to have exceeded 110% of the concentric 1RM (27). In this study, despite similar soreness between the knee and elbow, isometric strength at 2 hours was lower in the elbow flexors compared with that in the knee extensor muscles (70.2 ± 15.9 vs. 81.1 ± 10.1%; p < 0.05). As isometric strength is considered to be the most valid and reliable indicator of muscle damage (31), relatively greater weakness at 2 hours in the elbow flexors may indicate more extensive myofibrillar disruption. Although we cannot definitively conclude from the results of this study, more strength loss in the arms might increase the therapeutic potential of PJ supplementation compared with the leg muscles. Therefore, it is unclear if the lack of benefit of PJ for attenuating soreness and weakness of the knee extensor muscles was because of an inherent feature of the knee extensors or if it was because of inadequate loading during eccentric exercise.

This study demonstrates that twice-daily pomegranate juice supplementation attenuates weakness, improves strength recovery, and reduces muscle soreness of the elbow flexor but not of the knee extensor muscles in resistance trained individuals who perform intense bouts of eccentric exercise.

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Practical Applications

Maintenance of muscular strength after soreness-inducing exercise is of great importance to athletes, both recreational and competitive, who are required to perform repeated bouts of strenuous exercise. Sporting events for which successive days of competition are required (i.e., weekend tournaments) may benefit from PJ supplementation to maintain performance. Additionally, the supplementation of PJ during the early stages of new training regimens may offer benefits by decreasing soreness and preventing weakness. Although the ergogenic effect of PJ supplementation during periods of chronic training is unknown, supplementation with PJ in resistance trained individuals appears to lessen the weakness and soreness of the arms during the 2- to 168-hour period after performing intense eccentric exercise.

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We would like to thank Moses Khan for his involvement in subject coordination and the daily operations of data collection. There were no conflicts of interest. This study was funded by POM Wonderful, L.L.C.

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polyphenols; isometric strength; delayed-onset muscle soreness

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