The purpose of this study was to determine if a combined protein and antioxidant supplement (PRO + AO) was more effective than either protein (PRO) or carbohydrate control (CHO) alone in restoring muscle soreness and muscle function following fatiguing eccentric contractions (ECC) during the acute 24 h phase. The primary findings of this study demonstrate that during the acute 24 h period following fatiguing ECC exercise: 1) muscle function is significantly reduced and muscle soreness and thigh circumference are increased; 2) both groups that supplemented with PRO tended to have better isometric muscle function, and significantly greater isokinetic muscle function over CHO control; and 3) combined protein and antioxidant supplementation (PRO + AO) tended to have better absolute isokinetic torque and significantly less perceived soreness over time when compared to protein alone (PRO) or a carbohydrate (CHO) control. Collectively, the current findings support protein supplementation to enhance recovery of muscle function and the addition of antioxidants to act synergistically to reduce perceived muscle soreness, in the hours immediately following ECC exercise. Thus, the combination of protein and antioxidants may be an important consideration in developing and implementing strategies to aid in muscle recovery during the acute (0-24 h) time period following a bout of exhaustive eccentric exercise.
Indeed many athletic events that involve high intensity and/or eccentric contractions often require short turnaround times to subsequent events (0–6 h), which does not allow for adequate muscle recovery. For instance, the CrossFit games, wrestling tournaments, powerlifting competitions, tennis tournaments, etc., all require multiple high intensity bouts within a given day, and perhaps on successive days. Thus, developing supplementation strategies to improve recovery and reduce muscle soreness, might improve performance in such settings. In terms of aiding recovery of muscle function from resistance exercise or eccentric exercise, supplementation with protein or amino acids post-exercise has been demonstrated to enhance recovery [17, 19, 20, 22]. Regarding isometric muscle function, previous work by Buckley and colleagues  found that consumption of whey protein hydrolysate resulted in a quicker recovery in peak isometric torque following ECC, with complete recovery at 6 h post exercise. Similarly, the current study found at 24 h post ECC, peak isometric torque was higher, on average, though not statistically, in both PRO + AO and PRO groups compared to the CHO group, by 16 and 11 ft lb., respectively (Fig. 2a), or approximately a 38% difference in peak isometric torque (% of initial) between protein supplemented groups and CHO control (Fig. 2b). However, unlike the Buckley et al. study , and others , which found supplementation with protein completely restored and even surpassed baseline peak isometric torque , we did not find resolution of force within the same observed timeframe (Fig. 2), though the PRO supplemented groups seemed to have a positive slope towards recovery, where the CHO did not. Although total work between groups was not reported in these earlier studies, even concentric muscle force contractions are suppressed following high intensity work . In any case and even though our measurements did not extend to 48 h post ECC, the pattern of our reported muscle function data (Figs. 2 and 3) seems to suggest we observed a similar pattern as others in this regard [23, 28, 33].
As muscle function, broadly defined, is impaired following ECC, the magnitude and temporal pattern may depend upon contraction type; specifically, static versus dynamic, and concentric versus eccentric [5, 6]. Thus, it’s paramount to examine both static and dynamic muscle function, with the latter likely being more important for performance. Examination of dynamic peak isokinetic torque from the current study, we found a group by time interaction when expressed as absolute torque, whereby CHO control experiences the most dramatic decline in torque, followed by PRO, whereas PRO + AO appears least affected (Fig. 3a). Similarly, when expressed as percent of initial peak torque, there was a group effect with PRO and PRO + AO groups having a higher on average peak isokinetic torque (Fig. 3b), or approximately a 17% difference between PRO supplemented groups and CHO control group (Fig. 3b). Thus, in agreement with previous studies , the present study suggests protein supplementation following ECC does, in fact, improve recovery of dynamic muscle function. However, it is unlikely that dynamic muscle function returns to baseline levels within an acute time frame (<48 h), independent of supplementation [23, 28, 33].
Furthermore, previous research has demonstrated that supplementation with antioxidants derived from fruit improves recovery of muscle function following fatiguing eccentric exercise [7,8,9], though not all agree . These studies contained either whole fruit or pasteurized fruit juices containing a combination of anthocyanins with phenolic compounds, flavonoids, ellagitannins, and/or ellagic acid. In the current study, using an antioxidant derived from anthocyanin extract from mixed berries, we found no synergistic effect of supplementing with both protein and antioxidants, or the antioxidant offered no obvious additional benefit beyond that of protein alone (Figs. 2 and 3). In agreement, previous studies have also found no effect of antioxidant supplementation (using either vitamin C or fruit, berry and vegetable concentrate) on muscle recovery [10, 11, 34]. In contrast, previous studies using either a berry or fruit derived antioxidant supplement demonstrate an interaction between groups with no clear direction of improved muscle function  or show enhanced recovery of muscle function with a pre-loading period prior to ECC . Collectively, future work is needed to determine if optimal dosing strategy, types of antioxidants, may influence the impact of antioxidant supplementation on muscle function following fatiguing eccentric exercise.
Decline in muscle function, athletic performance, and intensification of muscle soreness are all correlated to the damaging effects of eccentric exercise on muscle. Studies investigating the effect of protein or amino acids on muscle soreness following ECC have yielded both positive [19, 20, 22] and null [17, 18, 23, 28] effects. In the present study, using both unweighted and weighted conditions to assess perceived muscle soreness, we show only protein and antioxidant supplementation (PRO + AO) favorably impacts perceived muscle soreness (via visual analog scales) for both weighted and unweighted conditions (Fig. 4a and b). While there was a significant time effect for increased thigh circumference indicative of swelling and localized inflammatory responses, no group or group by time effects were found.
Our finding of significantly lower perceived muscle soreness over time in combined protein and antioxidant (PRO + AO) supplementation compared to PRO and CHO groups is intriguing (Fig. 4a). The current finding supports prior work suggesting antioxidant supplementation may reduce muscle soreness following ECC [7, 11, 34], although the positive effect of antioxidants on muscle soreness is not unanimous [8,9,10]. Although the precise mechanism(s) for reduced muscle soreness is unknown, it may be related to either the high leucine or essential amino acid content within whey protein , antioxidants potential to reduce muscle soreness following ECC exercise , or may be related to the many bioactives present within various forms of whey protein. In any case, this finding may prove beneficial in scenarios where subsequent bouts with inadequate recovery are necessitated (e.g. tournament play, multiple sport events, military or occupational scenarios).
While the findings of this study demonstrate improved muscle function, and attenuated muscle soreness with combined protein and antioxidant supplementation, further evaluation of this recovery strategy is necessary. Specifically, the type, dose, and dosing strategy (pre-loading, frequency of dosing, etc.) likely needs optimization. Additionally, while muscle function is paramount, analysis of blood markers of muscle damage and oxidative stress, could provide insight into the underlying mechanisms of protein and antioxidant supplementation in the recovery of muscle soreness and function following eccentric exercise. On the contrary, previous studies which have measured blood markers of oxidative stress or muscle damage (e.g. creatine kinase) that were discordant with function [3, 8, 9, 11, 34], in that muscle function would be improved but not oxidative markers, or vice versa, calling into question the value of such measurements. Additionally, observing the effect of the supplement across a larger time period (up to 92 h or resolution of soreness/function), as well as in trained, competing athletes in which this is primarily intended to benefit, are suggested.