Unaccustomed exercise, particularly eccentric exercise in which the muscle lengthens, is the most common method used to elicit muscle damage. Damaged muscle fibers initiate a cascade of reactions that result in a prolonged and complex interaction between protein synthesis and degradation [1]. However, while protein turnover is elevated substantially, degradation usually exceeds synthesis, and thus, protein breakdown results, leading to muscle degeneration and atrophy [2]. These changes in muscle protein ultrastructure normally result in physiological symptoms such as reductions in muscle strength, increased muscle soreness and impaired muscle function [3, 4].

Stimulating protein synthesis and minimizing protein breakdown (proteolysis) are the two cellular processes that are essential for muscle recovery after damage [5]. While protein breakdown may be an important process involved in the adaptive response during recovery [6], increasing protein synthetic rates within the muscle during the recovery period is vital for muscle regeneration and hypertrophy. Therefore, strategies that can promote a positive net muscle protein balance during the days following muscle injury are likely to increase the rate of protein synthesis, satellite cell proliferation, but more importantly, enhance the regenerative processes that would benefit athletes and others that perform strenuous/unaccustomed physical activity.

Dietary proteins have an important role in regulating protein metabolism in skeletal muscle [7–9]. Whey protein isolate supplementation has been used effectively to increase muscle size and strength after resistance training [10], with some of these improvements thought to come from improved recovery from the exercise sessions. Compared to regular protein supplements, whey isolate is more effective at increasing blood amino acids and protein synthesis due to its different absorption kinetics and amino acid profile [11]. The high availability of amino acids in whey protein isolate, especially branched chain amino acids (BCAA), is important for protein synthesis in the hours immediately after ingestion. White et al. [12], examined the ingestion of a whey protein after an exercise bout which consisted of 50 maximal isokinetic eccentric quadricep contractions. Muscle strength, muscle soreness and CK were all measured at 6, 24, 48, 72 and 96 hours post exercise, with ingestion of whey protein having no significant effects on these variables implying no change in the rate of muscle recovery. Conversely, Buckley et al., [13] showed whey protein hydrolysate ingestion in the days following an intense exercise bout (100 maximal knee extensions of the knee extensors) improved muscle strength recovery. The authors suggested that the use of partially hydrolysed (pre-digested) form of whey protein isolate may provide quicker delivery of amino acids to the muscle, and ultimately, more rapid recovery of force-generating capacity following muscle injury. The administration of whole proteins in the study by White et al. [12], may explain the lack of improvement in force recovery following damage. Furthermore, only a single dose was given to participants, whereas Buckley et al. [13] continued supplementation following the exercise bout and during the recovery period. It could be suggested that for optimal ergogenic effects and recovery within the muscle, a hydrolysed form of whey protein (or free amino acids) needs to be ingested both immediately following the exercise bout, and in the days during recovery. However, this concept, particularly with eccentric contractions, has not been extensively investigated, as Buckley et al. [13] only followed recovery for 24 hours post-exercise. As such, whether the effects observed were related to muscle damage/regeneration, or simply faster recovery from fatigue, are difficult to determine. Jackman and colleagues [14] supplemented a controlled diet with BCAA and ameliorated the soreness following eccentric exercise. While they did not observe changes in strength measurements, ingestion was on the day of damage and for another 3 days afterwards, rather than for the whole regeneration process.

In our previous study [15], ingestion of creatine monohydrate prior to and following a resistance exercise session indicated a possible attenuation of the amount of damage, and an increase in the rate of functional recovery, compared to a CHO placebo. Similarly, in the current study, given the equivocal data on protein supplementation and muscle recovery, we were interested in establishing whether a commercially available protein supplement can improve recovery from exercise-induced muscle damage, and thus used a CHO placebo as the comparison group. Thus, we supplemented the diet of a group of participants with a hydrolyzed whey protein isolate for 14 days during recovery from an identical resistance training session as used in our previous study [15]. We hypothesized that supplementation with hydrolyzed whey protein isolate will accelerate muscle strength recovery compared to an iso-energetic CHO control after a single bout of eccentric exercise.

The major finding of this study was that whey protein isolate supplementation resulted in an attenuation of the exercise-induced force reduction (isometric knee extension) compared to the carbohydrate control during the recovery period following exercise-induced muscle damage. A similar trend was also observed in isokinetic strength, with a further, tendency for lower LDH levels in the WPH group compared to the CHO group following the resistance exercise session. Most previous research into whey protein supplementation has examined its effects on muscle strength gains after resistance training. However, improved recovery from the acute bouts of exercises performed during the training sessions has been suggested as a possible mechanism for the beneficial effects observed in those studies [23]. The current study demonstrates that whey protein in a partially hydrolysed (pre-digested) form improves strength recovery rates, possibly due to an increase in the rate of repair processes and/or a reduction in the extent of damage, from intense training, in particular, eccentric exercise that is commonly used in weight training.

Following the eccentric contraction-based exercise session, isokinetic and isometric knee extension peak torque was significantly reduced and remained significantly lower than pre-exercise values for at least 4 days. In support of muscle damage producing these force decrements, plasma CK and LDH activity was increased during the days post resistance exercise, being significantly elevated above baseline 2 - 4 days into recovery. These observations were comparable to previous studies utilizing similar protocols to induce muscle damage [24–26].

In support of our hypothesis, WPH ingestion during recovery attenuated the decline in isometric extension strength compared to CHO group, with a similar trend in isokinetic knee extension. Interestingly, isokinetic knee flexion peak torque was not significantly affected by the resistance exercise session. This was primarily due to the very minimal decrements in muscle strength observed in the WPH group (close to 100% of pre-exercise values), such that the WPH group tended to have higher isokinetic knee flexion strength compared to the CHO group. Recent studies have confirmed that resistance exercise stimulates an increase in myofibrillar and sarcoplasmic proteins [27, 28] as well as connective tissue proteins [29]. A single bout of resistance exercise results in the acute stimulation of muscle protein synthesis (up to 50-100% above basal values) that peaks within 3-24 hours, and can remain elevated, although at a diminishing rate, for up to 48 hours post-exercise [30–32]. Studies that have assessed both the rate of muscle protein breakdown and synthesis in response to a bout of resistance exercise have demonstrated that in a fasted state [31, 32] the net muscle protein balance remains slightly negative. However, providing exogenous amino acids, especially within the first 4 hours after resistance exercise (as implemented in the present study), increases protein synthesis, decreases protein breakdown, and produces a positive protein balance [31, 33], thus providing an environment for muscle growth. Although the aforementioned observations were not made with whey protein ingestion, a later study from the same laboratory confirmed the positive impact of whey protein supplementation on protein metabolism after resistance training exercise [34].

In the present study, oral ingestion of whey protein after the resistance exercise session most likely increased delivery of amino acids to the muscle, thus augmenting muscle protein synthesis and minimising protein degradation, thus producing the smaller reduction in force and/or faster recovery observed in the WPH group. Since neither muscle protein synthesis nor breakdown rates were measured, the relative balance cannot be determined. However, increased muscle protein synthesis is likely due to increased delivery of amino acids. Though not measured in the current study, recent results comparing protein fractionation on the bioavailability of amino acids clearly demonstrated significantly greater increases in the plasma concentrations of amino acids (and dipeptides) following protein hydrolysates compared to non-hydrolysed proteins [35],

Recent literature suggests that ingesting pre-digested proteins or free amino acids may be more advantageous during times of recovery from muscle damage compared to whole intact, slow absorbing proteins [12]. Indeed, Nosaka et al. [36], and more recently, White et al. [12] and Buckley et al. [13] clearly support this concept and findings observed in the current study. However, a limitation of the current study was the absence of another protein group (for example, whole intact protein such as milk) to make comparisons of this nature. Given the equivocal data on protein supplementation and muscle recovery, it can only be speculated that the beneficial effects of the protein source used in the current study was due to its hydrolysed, pre-digested form, and further research to clearly establish any difference is clearly warranted. Notwithstanding this, the positive protein balance created by increasing dietary intake of WPH following a single resistance exercise session would help to aid in recovery before subsequent exercise challenge during a resistance training program, thus allowing higher forces and hence training volumes to be achieved, eliciting greater strength benefits and muscle adaptations over time, as has been previously observed with WPH supplementation [23, 37].

Whether WPH was also able to decrease the amount of damage produced by the eccentric exercise session is difficult to ascertain. Both groups exhibited increased CK and LDH loss from the muscle into the plasma, peaking 48 - 96 hours after exercise. The pattern of change in CK and LDH in the current study was similar to that following high force, eccentric exercise reported by [38]. However, plasma LDH levels were generally lower during recovery in the WPH group compared to the CHO group (P = 0.064), which may be indicative of less muscle fibre damage. Whey protein supplementation had no significant effect on plasma CK response after exercise which could be due to the extreme variability in CK response after exercise compared to the LDH response. Although CK is used as an indirect marker of muscle damage, there is a larger inter- and intra-participant variability in the CK response after exercise because blood concentrations reflect what is being released from damaged tissue as well as what is taken up by the reticuloendothelial system [39, 40].

The beneficial effect from the whey protein supplement is likely due to its amino acid content, in particular the high essential amino acids (EAA) content, as opposed to any other constituents in the supplement. For instance, a carbohydrate drink with the same energy content as the protein supplement produces dramatic increases in blood glucose and insulin, but fails to stimulate protein synthesis [41, 42]. Borsheim et al. [8] demonstrated that essential amino acids alone (without addition of carbohydrate) are an effective method for stimulating muscle protein synthesis following resistance training. Furthermore, in a later study by the same laboratory [43], adding 35 grams of carbohydrate to the amino acid mixture did not cause a greater stimulation of net muscle protein synthesis compared to the amino acids alone [43], showing that the stimulation of protein synthesis was clearly not a caloric effect of the supplement. Interestingly, since both groups were consuming the current recommended dietary allowance (RDA) for protein (0.8 g/kg/day) in sedentary individuals, the improvements in force recovery and reduced extent of damage can be attributed to the extra protein provided by the whey protein supplement.

However, increased protein synthesis is not likely to be the only contributing factor for the effects observed, particularly in the early stages of recovery. Nosaka et al. [36], showed that a mixture of amino acids was effective in reducing muscle soreness following eccentric exercise. A more recent study utilised only leucine, valine and isoleucine ingestion and observed the same effect 2-3 days following an eccentric exercise session [14], thus demonstrating the effectiveness of BCAA's in decreasing muscle soreness following exercise. Presumably, a maximal force effort would be more likely to be achieved if a person did not feel as much muscle soreness. Although Jackman et al. [14] did not observe improvements in muscle strength, perhaps the whey protein hydrolysate used in the present study not only supplied the BCAA's to reduce muscle soreness (although this was not measured), but also supplied all essential amino acids to maximise the increase in protein synthesis during recovery.

In summary, the major finding of this investigation was that whey protein isolate supplementation elicited better maintenance of muscle strength in the days following contraction-induced eccentric muscle damage. This is likely due to increased protein synthesis due to the EAA contained within the WPH supplement, but could also be somewhat attributed to less damage to the muscle, as suggested by the trend for lower plasma LDH activity in the WPH group. Since the amino acid composition of whey proteins is very similar to that of skeletal muscle, whey protein supplementation may be providing amino acids essential for optimal muscle remodelling. Although the improvements elicited by whey protein supplementation appear small, an aggregation of those benefits with sustained, repeated training over time could still be of immense benefit for an athlete, providing even the smallest advantage, and may be the difference between winning and losing, or a faster return to competition. However, since untrained individuals were utilized in the current study (to ensure a robust damage response), any transferable benefits to the athletic population is speculative, although our previous research with recreational resistance-trained individuals does lend some support for this notion [10, 22]. Future research should examine how different forms/fractions of proteins influence the rate of recovery and/or extent of damage following injury, and if training status plays an important role. Research into promoting functional recovery would not only have potential benefit for athletes, but could be of considerable benefit to a variety of populations, including those suffering from muscle wasting conditions, weakness associated with aging, neuromuscular disorders, acquired immunodeficiency syndrome, burn injury, cancer cachexia and prolonged sepsis.