*4.3. Muscle Recovery Adaptations*

Protein supplementation has been shown to improve muscle function recovery following resistance training [19] and other modes of exercise including cycling, running, eccentric exercise and resistance training [62], ye<sup>t</sup> no review has evaluated the effects following arduous concurrent exercise training specifically. Protein consumption in close proximity to exercise prevents a decrease in myogenin messenger RNA expression, which can accelerate the remodelling and recovery of skeletal muscle [76]. Specifically, leucine

may be a key component at initiating this process post-exercise through the activation of mTOR and MPS [39,75]. One study found post-exercise protein ingestion reduced muscle soreness in U.S. Marines following a loaded march [61]. However, Flakoll et al. [61] failed to provide the total daily protein intake, which therefore, limits our understanding of the impact of protein supplementation specifically on recovery adaptations [77]. The remaining studies all failed to find an effect of protein supplementation on muscle recovery in the days following arduous concurrent exercise [52,59,60]. Specifically, no impact was observed on markers of muscle damage [59], soreness [52] or function [52,60]. Jimenez-Flores et al. [59] observed no differences in markers of muscle inflammation or damage between protein and placebo conditions. However, some of the markers which were chosen may be questionable. For example, cortisol is a stress hormone [78,79] which can indicate changes in whole-body catabolism [79]. C-reactive protein is a marker of whole-body inflammation and is not necessarily specific to skeletal muscle [78]. The data also sugges<sup>t</sup> a large inter-participant variability, which is a known limitation of such markers, particularly creatine kinase [79].

Eccentric exercise initiates a chain of events which leads to myofibrillar damage, degradation of structural proteins and membrane damage, thus inhibiting muscle function especially if individuals are unaccustomed to the exercise bout [80]. The participants in the study by Eddens et al. [52] completed a bout of concurrent endurance and eccentric exercise. The participants consumed a similar total daily protein intake, which corresponds to current recommendations [64]. As such, it is possible that the additional protein consumed post-exercise in the experimental group did not accelerate muscle recovery, due to protein requirements already being met by the participants. It was acknowledged by Eddens et al. [52] that the decrement in muscle function over the 24 h post-exercise was ~15%, which is lower than that observed with other eccentric exercise protocols, with decrements of between 10–65% reported elsewhere [81]. Therefore, it cannot be excluded that the muscle damaging protocol may not have been arduous enough, which might explain the lack of statistical difference between conditions [52]. Blacker et al. [60] found no statistically significant difference between protein and placebo conditions on acute muscle function recovery following arduous concurrent exercise. However, both supplement conditions accelerated recovery of muscle function compared to the control condition. Similarly, participants consumed a standardised total daily amount of protein (0.9 ± 0.3 <sup>g</sup>·kg−1·day−1) across conditions. Although the amount of protein is lower than the current general recommendations (1.2–2.0 <sup>g</sup>·kg−1·day−1) [64], no effect of protein supplementation post-exercise was observed by Blacker et al. [60] when compared to an isocaloric placebo [60]. Jimenez-Flores et al. [59] found no impact of protein supplementation compared to an isocaloric placebo on markers of muscle damage following arduous concurrent exercise, however, high-variability between study participants was observed and likely influenced the ability to detect statistical differences [59]. However, unlike Blacker et al. [60], there was no control group, therefore, it is unknown if the additional energy intake accelerated muscle recovery [59]. The limited number of studies and the differences between methodologies and outcome measures make it difficult to determine whether protein supplementation does improve muscle recovery, thus warranting further research. Additionally, due to the lack of women included in studies to date, future research is needed in women to examine the impact of protein on muscle recovery, particularly given the known difference in the rate of muscle function recovery post-exercise between men and women [82].
