Resistance exercise is a powerful stimulus to induce gains in strength and hypertrophy [1]. Although such gains can lead to enhanced athletic performance and may reduce the demands of activities of daily living, paradoxically resistance exercise increases markers of oxidative stress [2, 3], skeletal [4] and cardiac [5] muscle damage, and transiently reduces physical performance [6]. These responses to exercise are not necessarily negative, but rather are a necessary part of the adaptation process, providing the level of stress does not exceed the ability of the body to recover.
However, if the stress caused by exercise is too great and the body is unable to recover adequately, long term reductions in performance level can occur, such as in non-functional overreaching, or overtraining [7], or in much more severe cases, can be life threatening in the form of rhabdomyolysis [8]. Supplementation with plant compounds has been suggested to be able to reduce inflammation, oxidative stress and subsequent skeletal and cardiac muscle damage associated with intense exercise, thereby enhancing recovery and helping to stimulate optimal adaptation.
One such group of plant compounds are polyphenols, which are naturally high in fruits and vegetables. Although polyphenols themselves are radical scavengers, they exist in such low concentrations in human blood that their ability to reduce markers of oxidative stress are more likely due to their capacity to enhance endogenous anti-inflammatory and antioxidant mechanisms through the nuclear factor erythroid 2-related factor 2 (Nrf2) pathway than by directly scavenging themselves [9].
One source of polyphenols that has received considerable attention in this area is tart cherries [3, 6, 10,11,12,13,14,15,16]. Following various forms of intense physical activity, such as resistance exercise [3, 6, 13], long distance running [14, 16], cycling [11, 12], repeated sprints [10], or sport [15], consumption of tart cherries reduces markers of oxidative stress [3, 11, 16], muscle soreness [10, 13, 14] and attenuates reductions in physical performance [3, 10, 12, 13, 16]. These results are not unequivocal, however, likely due to differences in dosing and damage protocol [9]. Complicating the interpretation further, the enhanced recovery is not always seen as some studies failed to show either an increase in muscle damage [11], or a reduction in performance [15] from the chosen protocol, giving the supplement no damage/performance measure to recover from.
While there are many ways to assess oxidative stress, one particular method that has been used to demonstrate the effectiveness of a polyphenol supplement to reduce its effects following exercise has been by a decrease in serum protein carbonyls (PC), a marker of oxidative damage [2]. For example, Bowtell et al. [3] saw a significant reduction in PC following 10 sets of 10 knee extensions at 80% 1RM following a Montmorency cherry supplementation when compared to an isoenergetic fruit concentrate. Also, Chang et al. [17] demonstrated a reduction in PC following 1 h of treadmill running with the use of a polyphenol supplementation in the form of purple sweet potato when compared to control.
One variable not measured in prior polyphenol research, but has been assessed in other antioxidant literature, is creatine kinase myocardial band (CK-MB), a marker typically associated with cardiac damage [5] that has recently been proposed as a marker specifically for type I muscle fiber damage [18]. An increase in CK-MB is a typical response to exercise and is associated with the structural disruption of muscle fibers and the leaking of the contents on these fibers into the plasma [5]. Strenuous exercise, such as long distance running [18], professional soccer [19] and also resistance exercise [5] significantly increases CK-MB. These increases have been attenuated by other antioxidant supplements, such as Vitamin E under hypoxic conditions [20] and the drug Allopurinol [19], and thus antioxidants could potentially enhance recovery from strenuous exercise in the context of CK-MB.
While it stands to reason that aerobic activity, when compared with anaerobic activity, would be the more likely beneficiary of an antioxidant supplementation due to its reliance on oxidative metabolism, oxidative stress has been demonstrated following resistance training and there are alternative ways that oxidative stress can impact recovery. For example, many studies have assessed the impact of polyphenol supplementation on skeletal muscle damage in the form of changes in serum creatine kinase (CK) and/or muscle soreness. The prevailing rationale for such analyses is that while mechanical stress is a major contributor to skeletal muscle damage, an increase in reactive oxygen species could cause oxidative modifications of skeletal muscle proteins. Thus, an antioxidant could potentially reduce biochemical damage [9]. This rationale has received equivocal support, with some studies showing a benefit of polyphenol supplementation [6, 14, 21,22,23] and others not [3, 11, 12, 16, 24,25,26]. In terms of the studies that utilized resistance exercise, outside of Levers et al. [6], all utilized unilateral strength exercise or parallel group design, which are susceptible to high inter-individual differences and could explain the lack of agreement in the literature [9].
While changes in oxidative stress and/or markers of muscle damage would be promising to demonstrate whether polyphenol supplementation is efficacious, it would be important to demonstrate a measureable improvement in physical performance level resulting from that reduced oxidative stress. While the majority of research has demonstrated some form of enhanced performance level during the recovery process as a result of polyphenol supplementation, such as maximal voluntary contraction [3, 12, 24, 27], isokinetic performance [21, 25] or time trial performance [10], some studies have failed to show performance benefits [6, 15, 26]. However, it is important to note that neither Peschek et al. [26] nor McCormick et al. [15] successfully reduced their performance measures with their exercise protocol, thus providing no performance decrement for the supplements to attenuate. The final study that failed to show performance improvement by Levers et al. [6] saw a significant reduction in performance at only 1 of the 4 time points that were measured, again providing the supplement little opportunity to demonstrate effectiveness. Overall, it appears that when performance level is reduced by strenuous exercise, the overwhelming majority of studies do demonstrate an enhanced recovery of performance with polyphenol supplementation over placebo.
Even when reductions in muscle damage and soreness are observed in conjunction with enhanced recovery of physical performance measures with the use of a nutritional supplement, an individual would still need consider the benefits of the supplement over the consumption of the additional energy and whether that would fit with their intended dietary intake. On this note, the overwhelming majority of the literature pertaining to the effects of cherry derived antioxidants on muscle damage and performance have used fruit juices, containing at least 100 kcal of energy, with all using multiple doses (2–3 per day) over many days (up to 10 days) [3, 11,12,13,14,15,16]. Very few studies have assessed the effects of the antioxidant containing extracts in the absence of any macronutrients, although they have suggested potential benefits [6, 28]. The use of tart cherry supplementation in the absence of added macronutrients has involved consuming the supplement in powder form [6, 28], as opposed to juice form containing significant amounts of sugar. Interestingly, these powder studies used considerably lower doses of polyphenols (500 mg vs. > 1000 mg) but still demonstrated ergogenic effects [6, 28].
The primary purpose of this study, using a randomized crossover design, was to assess whether polyphenol supplementation in the form of a powdered tart cherry extract reduces oxidative stress, skeletal and cardiac muscle damage, and muscle soreness following intense bilateral resistance exercise. The secondary purpose was to determine whether the recovery of physical performance was enhanced as a result. We hypothesized the polyphenol supplementation would result in reduced muscle damage and soreness and the recovery would be enhanced following intense resistance exercise.