The primary objective of this study was to determine whether consumption of Cr prior to, and following exercise-induced damage, improves force recovery and markers of muscle damage in healthy individuals. Following repeated eccentric exercises, isokinetic knee extension and flexion and isometric knee extension peak torque was significantly reduced, and remained significantly lower than pre-exercise values, for approximately 4 days or longer. Importantly, isometric (21% higher) and isokinetic (10% higher) knee extension strength were both significantly greater during recovery with consumption of a Cr-CHO supplement compared to a supplement with CHO alone.
The observed decrements in muscle strength were in accordance with previous studies, with Brown and colleagues [14
] showing similar reductions, although others demonstrated less reductions in strength [7
]. Such varying responses in the magnitude of strength loss following eccentric exercises are possibly due to the different muscle groups used (i.e. elbow flexors of the forearm vs. knee extensor/flexors muscles groups) and/or the protocol utilized to induce muscle damage [7
]. It should also be noted that muscle strength was expressed as a percentage of pre-exercise strength values and normalised to contralateral (undamaged) controls. This is a common method of analysing loss of muscle strength following exercise-induced damage [7
], which therefore not only normalises data by accounting for any improvements during the recovery period as a result of familiarisation, but more importantly reduces the inter-individual variability in muscle strength between participants.
Extensive literature has examined the effects of Cr supplementation on exercise performance, in particular high intensity exercise [21
]. However, only a few studies have investigated the efficacy of Cr supplementation on muscle recovery after injury [5
]. In 2001 and 2007, Rawson and colleagues examined the effects of Cr supplementation on muscle damage and recovery following 2 different exercise intensities; a high-force, eccentric exercise [7
] and a low force, hypoxic resistance exercise challenge [6
]. In the first study, male participants were supplemented with Cr for 5 days prior to 50 maximal eccentric contractions. Results showed no significant differences in maximal isometric force of the elbow flexors, or serum CK or LDH activity, between the Cr-supplemented and dextrose control group during the 5 days post-exercise [7
]. In the second study, male participants were supplemented with Cr for 5 days prior to, and 5 days following a squat exercise protocol (5 sets of 15–20 repetitions at 50% of 1 repetition maximum [1 RM]). Similar to the first study, oral Cr supplementation had no effect on reducing the extent of muscle damage and/or enhancing the recovery following the resistance exercise challenge [6
In the current study however, the Cr-supplemented group exhibited an enhanced rate of muscle function recovery compared to the placebo group; as evident by the higher muscle strength values for both the isometric and isokinetic knee extension during the recovery period following exercise-induced muscle damage. Such differing observations could be in part due to the length of supplementation period and/or post-exercise supplementation. In the first study by Rawson and colleagues (2001), participants were only supplemented for 5 days prior to the exercise-induced damage protocol; with no continuation of supplementation following the exercise bout [7
]. Willoughby and Rosene [22
] have suggested that by continuing Cr supplementation after a resistance exercise bout (initial stimulus), Cr may act as a co-regulator, or direct manipulator of gene transcription of amino acid pools, thus enhancing myofibrillar protein synthesis during the recovery period post-injury. Indeed Olsen et al. (2006) supported such a suggestion by recently demonstrating for the first time in human skeletal muscle fibres that Cr supplementation amplifies the training-induced increase in satellite cell number and myonuclei concentration [23
], and thus potentially, muscle regeneration.
Although Cr supplementation was continued following the exercise bout in the second study by Rawson and colleagues [6
], it is possible that the resistance exercise session, which was designed to be hypoxic in nature, as opposed to high force, eccentric exercise, may not have elicited enough muscle damage to unmask the anabolic effects of Cr supplementation [24
]. Taken together, it is evident from the current study that Cr supplementation prior to, and during recovery from, an eccentric exercise resistance training session, provides faster recovery in strength. It is not uncommon for resistance trained athletes to undertake subsequent training sessions 2 to 3 days following a previous training session. Such an increase in strength output during recovery would presumably allow for a higher training load during subsequent training sessions in the days following the initial exercise bout. Indeed, this may be one of the explanations behind greater mass and strength gains observed in resistance trained participants ingesting Cr-containing supplements [25
While the majority of studies have examined the role of Cr during the recovery period post exercise [25
]; a number of studies have suggested a possible beneficial role during exercise [28
]. The sarcoplasmic reticulum (SR) Ca2+
pump derives its ATP preferentially from PCr via the CK reaction [28
]. Local rephosphorylation of ADP by the CK-PCr system maintains a low ADP/ATP ratio within the vicinity of the SR Ca2+
pump and ensures optimal Ca2+
pump function (i.e. removal of calcium from the cytoplasm) [31
]. However, when rates of Ca2+
transport are high (as seen in muscle damage), there is a potential for an increase in [ADP], thus creating a microenvironment (i.e. high [ADP]/[ATP] ratio) that is unfavourable for ATPase function, and as a consequence, SR Ca2+
pump function may be diminished [28
]. Furthermore, a decrease in [PCr] below 5 mM, which is characteristic of this increased ATPase activity; reduces local ATP regeneration potential of the CK/PCr system [29
]. Thus, by supplementing with Cr prior to, but also following exercise-induced muscle damage, PCr concentrations within the muscle will be increased, and therefore could theoretically improve the intracellular Ca2+
handling ability of the muscle by enhancing the CK/PCr system and increase local rephosphorylation of ADP to ATP, thus maintaining a high [ATP]/[ADP] within the vicinity of SR Ca2+
-ATPase pump during intense, eccentric exercise. However, this concept requires further investigation.
Myofibrillar enzymes CK and LDH are widely accepted as markers of muscle damage after prolonged exercise [32
]. Due to the different clearance rates of these enzymes, plasma CK and LDH were measured at 1, 2, 3, 4 hours following exercise and on days 1, 2, 3, 4, 7, 10, and 14 post-exercise. Plasma CK and LDH activity significantly increased during the days post-exercise, and remained elevated above baseline until day 10 post-exercise. The time course and magnitude of increased CK and LDH in plasma following the resistance exercise session was in accordance with previous work [7
], with maximum CK and LDH activity occurring approximately 72 to 96 hours after the resistance exercise. The delay in maximal elevation of CK and LDH activity is most likely caused by the increasing membrane permeability due to secondary or delayed onset damage as a result of increasing Ca2+
leakage into the muscle [36
]. Additionally, the differences in the magnitude of CK and LDH present in plasma following eccentric exercise (as seen in Figures and ) is possibly due to the larger molecular weight of LDH compared to CK and hence a decreased ability to diffuse from the muscle cell following injury [37
In the current study, plasma CK activity was significantly lower (~84% on average) at day 2, 3, 4, and 7 in the Cr-CHO supplemented group compared to the CHO group following exercise-induced muscle damage, with a similar trend (~12% lower) in LDH activity. Less leakage of muscle enzymes from the muscle may be indicative of less damage to the muscle, which may be due to improved Ca2+ buffering capacity of the muscle (i.e. the rate of Ca2+ removal from the muscle cytoplasm) and thus less Ca2+ accumulation within the cell and subsequent proteolytic activation.
In summary, the major finding of this investigation was significantly higher muscle strength after Cr supplementation during recovery from a muscle damaging exercise session. While this may be due in part to a faster muscle growth during the recovery period, significantly lower plasma creatine kinase activity in the days after injury is indicative of less muscle damage.
It is clear that a limitation to the current study is that the exact mechanisms by which Cr exerts its effects were not examined, and thus, further research is needed. However, it is evident from other studies that Cr is perhaps working via two possible mechanisms in the current study. Firstly, Cr supplementation prior to eccentric-induced damage may be enhancing the calcium buffering capacity of the muscle by fuelling the SR Ca2+-ATPase pump, thereby decreasing intracellular calcium concentrations and activation of degradative pathways such as calpain. Thus, a reduction in calcium-activated proteases will minimise additional damage to the sarcolemma, but more importantly, further influxes of calcium into the muscle. Secondly, Cr supplementation post-exercise may enhance one or more of the key phases involved in the regenerative response to exercise-induced damage, such as increasing protein synthesis, reducing protein degradation, and thus, creating an environment that facilitates enhanced satellite proliferation and hence formation of new muscle fibres. This combination is likely to allow a higher training volume to be maintained during subsequent exercise sessions during resistance training.