Supplementation with creatine monohydrate (CrM) has been consistently shown to promote greater gains in lean body mass (LBM) and strength compared with placebo treated groups (20). However, in most cases, the CrM-treated group was not compared with a group that received a placebo containing protein and an equivalent amount of energy (9,13,25,27). Only one resistance exercise (RE) training study has compared the effects of a CrM-containing supplement (10 g of CrM, 75 g of CHO) with a supplement containing a similar amount of nitrogen (protein) and energy (10 g of milk protein, 75 g of CHO) (24). That study reports that CrM treatment provided no greater gain in strength, LBM, or muscle fiber hypertrophy (24). However, that study used a group of inactive males (exercised less than twice a week before the study). Although the influence of training status on the effects of supplementation is unknown, previous work involving CrM supplementation and RE-trained individuals has shown that treatment enabled the participants to progress at a more rapid rate, which was reflected by the larger strength gains and greater volume of work completed during the workouts (27). Therefore, unlike inexperienced participants, it may be possible that RE-trained individuals experience strength and LBM gains that are of greater magnitude during training.
Longitudinal studies that have attributed changes in LBM to supplementation during RE training seldom report these changes alongside adaptations at the cellular level (i.e., fiber-specific, type I, IIa, IIx hypertrophy) (5,6,8,13,25). Those that have assessed fiber-specific hypertrophy in response to supplementation (24,27) have not confirmed this response with changes at the subcellular level (i.e., contractile protein content). Therefore, the aim of this study was to use a group of RE-trained participants to examine the effects of a CrM-containing protein-carbohydrate (PRO-CHO) supplement in comparison with a supplement containing a similar amount of nitrogen and energy on strength, body composition, and fiber-specific (i.e., type I, IIa, IIx) hypertrophy, as well as muscle Cr and contractile protein content. The hypothesis was that in RE-trained individuals, a CrM-containing PRO-CHO supplement would provide greater benefits (i.e., muscle strength and hypertrophy) compared with a PRO-CHO supplement containing a similar amount of nitrogen and energy.
Thirty-one recreational male bodybuilders met the requirements to commence this study that involved pre-post assessments and supplementation during 10 wk of RE training (baseline characteristics are presented in Table 1). To qualify as participants, the men (a) had no current or past history of anabolic steroid use, (b) had been training consistently (i.e., 3-5 d·wk−1) for the previous 6 months, (c) had submitted a detailed description of their current training program, (d) had not ingested any ergogenic supplement for 12 wk before the start of this investigation, and (e) had agreed not to ingest any other nutritional supplements or nonprescription drugs that might affect muscle growth or the ability to train intensely during the study. All participants were informed of the potential risks of the investigation before signing an informed consent document approved by the human research ethics committee of Victoria University and the Department of Human Services, Victoria, Australia. All procedures conformed to National Health and Medical Research Council guidelines for the involvement of human subjects for research and conformed to the policy statement regarding the use of human subjects and written informed consent published by Medicine and Science in Sports and Exercise®.
After baseline assessments, the men were matched for maximal strength (1RM) in three weight lifting exercises (see strength assessments) and then randomly assigned to one of three supplement groups in a double-blind fashion: protein only (N = 10) (PRO), PRO-CHO (N = 11), or the same PRO-CHO supplement that contained CrM (N = 10) (Cr-PRO-CHO).
The Cr-PRO-CHO group consumed the exact same supplement as the PRO-CHO group (50% whey isolate; 50% glucose). The only difference was that the Cr-PRO-CHO supplement contained a dose of CrM (0.1 g·kg−1·d−1). Participants were instructed to consume 1.5 g of the supplement per kilogram of body weight per day (1.5 g·kg−1·d−1) for the 10-wk program while maintaining their habitual daily diet. The chosen supplement dose was based on previously reported intakes in this population (14). For example, an 80-kg participant in the PRO-CHO group consumed 120 g·d−1 of a supplement that contained 52 g of protein, 59 g of carbohydrate, < 0.6 g of fat, and 1877 kJ (449 kcal). An 80-kg participant in the Cr-PRO-CHO group consumed 120 g·d−1 of a supplement that supplied 48 g of protein, 53 g of carbohydrate, < 0.6 g of fat, 8.4 g of CrM, and 1710 kJ (409 kcal). Another matched group (PRO) was provided a protein-only supplement (whey isolate) (1.5 g·kg−1·d−1) that provided an 80-kg participant (120-g dose) with 103 g of protein, < 6 g of carbohydrate, < 1.2 g of fat, and 1864 kJ (447 kcal). All supplements were supplied by AST Sport Science (Golden, CO) and were tested to comply with label claims before leaving the place of manufacture. The protein was also independently assessed by Naturalac Nutrition LTD (Level 2/18 Normanby Rd Mt Eden, New Zealand) on two separate occasions, and it matched labeled ingredients on both occasions.
The participants were asked to consume their supplement dose in three equal servings throughout the day (described with measuring scoops provided). For example, one serving was consumed midmorning, another soon after the workout in the afternoon (or similar time on nontraining days), and the final serving in the evening before sleep. The participants were weighed on a Seca 703 stainless steel digital medical scale (Seca, Perth, WA) every week to track body mass, and they were shown how to adjust the supplement dose as required. The supplements were provided in identical containers with sealed, tamper-proof lids. Participants were given an approximately 1-wk supply of the supplement at the start of each week and were asked to return the container before they received the next week's supply as an act of compliance to the dosing procedure. In addition to having to return the container, the participants were asked to document the time of day they took the supplement in nutrition diaries that were provided. The participants' diets were monitored and assessed as previously described (7). In brief, each participant was asked to submit three written dietary recordings: one before and two during the study (each recording consisted of 3 d), for the calculation of macronutrient and energy intake. Energy intake is expressed in kilocalories per kilogram of body weight per day; macronutrients are expressed in grams per kilogram of body weight per day. The participants were asked to report any adverse events from the supplements in the nutrition diaries provided. No adverse events were reported by the participants.
Resistance training protocol.
Questionnaires demonstrated that the participants had been training consistently (i.e., 3-5 d·wk−1) for at least 6 months before expressing interest in this investigation. However, to ensure the participants were trained, and to minimize the impact of a new program on strength and hypertrophy adaptations, all participants underwent a structured RE program for about 12 wk that was very similar to the one used in the study (Max-OT, AST Sport Science, Golden, CO) (8). No supplementation was permitted during this pretrial phase. Once the pretrial training phase was completed, participants underwent baseline assessments. The 10-wk training/supplementation program began the week immediately after baseline assessments. In brief, the program was designed specifically to increase strength and muscle size. It consisted of high-intensity (overload) workouts using mostly compound exercises with free weights. Training intensity for the program was determined initially using repetition maximums (RM) from strength tests. However, once a designated RM was achieved in each phase, the participants were encouraged by the trainer to increase the weight used. This progressive overload program was divided into three phases: preparatory (weeks 1-2) (10RM), overload phase 1 (weeks 3-6) (8RM to 6RM), and overload phase 2 (weeks 7-10) (6RM to 4RM). Qualified personnel supervised each participant on a one-to-one basis for every workout. Aside from the personal training each participant received during the 10-wk program, they also kept training diaries to record exercises, sets, repetitions performed, and the weight used throughout the program; these were viewed by the trainer on a weekly basis. The following assessments occurred in the week before and after the 10-wk RE program.
Strength assessments consisted of the maximal weight that could be lifted once (1RM) in three weight training exercises: barbell bench press, squat, and cable pulldown. Recognized 1RM testing protocol and exercise execution guidelines were followed as previously documented (1). Briefly, the participant's maximal lift was determined within no more than five single-repetition attempts after three progressively heavier warm-up sets. Participants were required to successfully lift each weight before attempting a heavier weight. Each exercise was completed before the next attempt, and in the same order. Reproducibility for these tests was determined on two separate occasions; intraclass correlations (ICC) and standard error of measurement (SEM) for 1RM tests were bench press r = 0.98, SEM 1.0 kg; squat r = 0.99, SEM 2.5 kg; and pulldown r = 0.98, SEM 2.5 kg.
Lean body mass (total fat-free mass), fat mass, and body fat percentage were determined using a Hologic QDR-4500 dual-energy x-ray absorptiometry (DEXA) with the Hologic version V 7, REV F software (Waltham, MA). Whole-body scans were performed on the same apparatus, by the same licensed operator. Quality-control calibrations were performed as previously described (8). Participants were scanned at the same time of the day, that is, in the morning in a fasted state. For longitudinal studies in which relatively small changes in body composition are to be detected, whole-body scanning with this instrument has been shown to be accurate and reliable (CV 0.8-2.8%) (19).
Muscle biopsies for determination of muscle fiber type, cross-sectional area (CSA), contractile protein content, and Cr concentrations were taken in the week before and after the RE program. Biopsies (100-450 mg) were taken using the percutaneous needle technique with suction to ensure adequate sample size (10) at a similar depth in the vastus lateralis muscle by the same medical practitioner. A small part of the sample was immediately frozen for assessment of contractile protein content and Cr. The remaining tissue was mounted using OCT medium and snap frozen in isopentane, precooled in liquid nitrogen, and stored at −80°C for histochemical analysis to classify muscle fiber types I, IIa, and IIx on the basis of the stability of their ATPase activity, as previously described (7). Fiber type percentages and CSA were determined from sections containing a mean of 210 (range 130-400) fibers. Samples were measured on two separate occasions for day-to-day reproducibility. ICC and SEM for fiber type distribution were type I r = 0.82, SEM 1.8%; type IIa r = 0.94, SEM 1.3%; and type IIx r = 0.94, SEM 1.2%. Mean areas were, for fiber type I, r = 0.97, SEM 87 μm2; type IIa r = 0.98, SEM 100 μm2; and type IIx r = 0.97, SEM 141 μm2. Approximately 5 mg of muscle was used to determine contractile protein content as detailed by Beitzel et al. (3) and reported previously (7). Samples were run twice on two separate occasions (ICC r = 0.98, SEM 2.1 mg·g−1). Two milligrams of muscle was used to analyze Cr concentrations, using fluorimetric techniques as in Harris et al. (11), with data expressed as millimoles per kilogram dry weight (ICC r = 0.88, SEM 22).
Statistical evaluation of the data was accomplished by two-way repeated-measures analysis of variance (ANOVA) with group (supplement) and time (training) as the factors, using SPSS statistical analysis software (SPSS v 11.0; Chicago, IL). Where significant main effects were identified by ANOVA, Tukey's post hoc analysis was performed to locate differences. Deltas for each variable were analyzed with a one-way ANOVA. Preliminary power testing of expected changes in strength and body composition were based on previous data obtained by our laboratory (7,8) and others (24,27,29). This testing revealed that eight participants were required per group to obtain significance at an alpha level of 0.05 and a power of 0.8. Test-retest reliability was quantified using the intraclass correlation coefficient (ICC) two-way ANOVA (mixed-effects model) and the SEM (28). Simple regression was used to determine significant relationships among the deltas for selected variables. A P value of less than 0.05 was designated to indicate statistical significance.
Baseline characteristics are presented in Table 1. There were no differences between the groups in any variables at the start of the study (P > 0.05).
Table 2 shows the average of 3-d written dietary recalls for energy (kcal·kg−1·d−1) carbohydrate and protein (g·kg−1·d−1) of the groups before and in the first and last weeks of the training program. The data do not include supplementation. No differences were identified between the groups or across time with regard to energy or macronutrient intake (P > 0.05).
Body mass and DEXA determined body composition are presented in Table 3, with changes from baseline presented in Figure 1. Whereas all groups demonstrated an increase (P < 0.05) in body mass after the training program, a group × time interaction (P < 0.05) was detected; the PRO-CHO and Cr-PRO-CHO groups demonstrated a greater gain in body mass (post hoc P < 0.05) compared with the PRO group. All groups demonstrated an increase (P < 0.05) in lean mass (LBM) after the training program. However, a group × time interaction (P < 0.01) for LBM was detected; the Cr-PRO-CHO group showed a greater gain in LBM compared with the PRO and PRO-CHO groups (post hoc P < 0.05). A group × time interaction (P < 0.05) for fat mass and body fat percentage was also observed. When compared with the PRO-CHO group, the PRO and Cr-PRO-CHO groups demonstrated a significant decrease in fat mass and body fat percentage (post hoc P < 0.05).
Table 3 also presents the results of 1RM strength assessments and changes from baseline are presented in Figure 2. All groups demonstrated an improvement (P < 0.05) in strength in each exercise after the training program. However, a group × time interaction (P < 0.05) was detected for the barbell squat, bench press, and pulldown. The Cr-PRO-CHO group demonstrated a greater gain in strength in each of these exercises compared with the PRO and PRO-CHO groups (post hoc P < 0.05). No other differences between the groups were detected.
There were no changes between the groups or across time with regard to fiber type proportions (Table 4). All groups demonstrated an increase in CSA across all muscle fiber types (P < 0.05) after the training program; however, a group × time interaction (P < 0.05) in CSA of both type II fiber subgroups was detected (Table 4). The Cr-PRO-CHO group demonstrated a greater increase in CSA in the type IIa and IIx fibers compared with the PRO and PRO-CHO groups (post hoc P < 0.05) (Fig. 3). A group × time interaction (P < 0.05) was also observed for contractile protein content. The Cr-PRO-CHO group showed a greater increase in contractile protein content compared with the PRO and PRO-CHO groups (post hoc P < 0.05) (Fig. 4). Table 4 also presents muscle Cr data from samples taken before and after the training program. No differences between the groups or across time were detected.
For all participants combined, positive correlations (P < 0.05) were detected between changes in CSA in the type II fibers and strength gained in the 1RM squat exercise (r = 0.677) (Fig. 5). A correlation was also detected between the changes in contractile content (mg·g−1) and strength gained in squat exercise (1RM) (r = 0.643; P < 0.01) (Fig. 6). For all participants combined, a positive correlation was also detected between the changes in LBM and strength (1RM) in the squat (r = 0.661; P < 0.01) (Fig. 7).
The most important finding of this investigation was that in RE-trained individuals, a CrM-containing PRO-CHO supplement provided significantly greater gains in 1RM strength and muscle hypertrophy compared with supplementation with an equivalent dose of PRO-CHO or PRO during 10 wk of training. A significantly greater muscle hypertrophy response from the addition of CrM was evident at three different levels of physiology. That is, the CrM-treated group demonstrated a greater gain in LBM, hypertrophy of the type IIa and IIx fibers, and increase in contractile protein. This is important because we are aware of no other research that has confirmed improvements in body composition via RE training and CrM supplementation with hypertrophy responses at the cellular (i.e., fiber-specific hypertrophy) and subcellular levels (i.e., contractile protein content). Therefore, these results support the hypothesis that, in RE-trained individuals, a CrM-containing PRO-CHO supplement provides greater adaptations than a PRO-CHO supplement containing a similar amount of nitrogen and energy.
Several RE training studies have reported greater increases in strength and LBM in participants who consumed CrM as compared with a placebo (5,6,9,13,25). However, only one has compared the effects of a CrM-containing supplement with a supplement containing a similar amount of protein and energy (24). Tarnopolsky et al. (24) used previously inactive participants and daily supplementation with either 10 g of CrM + 75 g of CHO 1252 kJ (300 kcal) or 10 g of protein + 75 g of CHO 1420 kJ (340 kcal). When compared in this manner, Tarnopolsky et al. (24) conclude that CrM supplementation provided no greater gains in strength, LBM, or muscle fiber hypertrophy. However, whereas Tarnopolsky et al. (24) used previously inactive participants, the present study used RE-trained participants and demonstrates significantly greater improvements in strength (three of three assessments) and muscle hypertrophy (three of three assessments) from treatment with CrM. Generally, untrained participants experience strength and lean mass changes that are of greater magnitude compared with RE-trained athletes (9). However, the influence of training status on the effects of supplements such as CrM is unknown. Previous work involving CrM supplementation and RE-trained individuals has shown that treatment enabled the participants to progress at a more rapid rate (27). This was reflected by the larger 1RM strength gains and greater volume of work completed during the workouts, that is, more repetitions completed with heavier weight (27). Therefore, unlike inexperienced participants, it may be possible that RE-trained individuals experience strength and LBM gains that are of greater magnitude during training. Additionally, muscle Cr uptake is shown to be enhanced by macronutrient consumption (23) and postexercise supplementation (21). In the present study, the CrM-treated participants consumed CrM with protein and carbohydrate, and one of these servings was taken immediately after each workout. The results of this trial would seem to support the suggestion that CrM supplementation provides greater benefits in RE-trained individuals. However, a clear mechanism underlying these benefits remains somewhat elusive.
Improvements in muscular performance during high-intensity contractions are associated with ATP resynthesis as a consequence of increased PCr availability in muscle via CrM supplementation (9,11). Increasing the availability of PCr via supplementation is not only thought to enhance cellular bioenergetics of the phosphagen system but also the shuttling of high-energy phosphates between the mitochondria and cytosol to increase the availability of energy for contractile protein synthesis (2). Creatine is taken up by muscle, where it seems to stimulate transcription factors that regulate the synthesis of contractile proteins (29). Willoughby and Rosene (29) have reported an enhanced hypertrophy response from RE and supplementation (i.e., increase in strength, LBM, and thigh volume) as well as alterations at the molecular level that may explain these benefits. Supplementation with CrM (6 g·d−1 for 12 wk) resulted in a greater increase in LBM (assessed by skinfold caliper), thigh volume, (relative) muscle strength, and contractile protein content as well as upregulation of the genes and myogenic regulatory factors associated with (myosin heavy chain) contractile protein synthesis (29). An analytical review of 22 studies involving supplementation during RE training has demonstrated that CrM clearly enhances maximum strength and weightlifting performance (maximal repetitions at a given percentage of maximal strength), and this benefit was attributed to increased Cr availability during intense muscle contraction (20). More recently, Olsen et al. (16) have reported that CrM supplementation during 16 wk of RE amplified the training-induced increase in satellite cell number and myonuclei concentration in human skeletal muscle fibers, thereby allowing an enhanced muscle fiber growth in response to strength training. Therefore, supplementation with CrM may result in superior strength and hypertrophy responses (20) by inducing greater satellite cell number and myonuclei concentration (16) alongside transcriptional changes in muscle gene expression (29), which may contribute to, or be a product of, CrM's ability to enhance the bioenergetics of the phosphagen system (2,11). Despite the clear, beneficial effect of CrM observed in this study, metabolite assessments revealed no significant change in muscle Cr content at the end of the program. The CrM dose used in this study was based on others that have reported improvements in muscle hypertrophy and strength performance with small daily doses (6 g·d−1) (with no loading phase) similar to the dose used (0.1 g·kg−1·d−1) in this study. However, it may be that small daily doses of CrM for a prolonged duration (10 wk) may not promote elevated muscle Cr concentrations during intense RE training. For instance, despite a loading phase (20 g·d−1, 5 d) that provided a 25% increase in resting muscle Cr concentrations in the first week, Volek et al. (27) report that supplementation (5 g·d−1) for a further 11 wk resulted in an increase of only about 10% by the end of a 12-wk training/supplementation program. Van Loon et al. (26) have demonstrated that a small maintenance dose (2-3 g·d−1 for 6 wk) in sedentary individuals failed to maintain high Cr muscle concentrations that were achieved by a CrM loading phase (20 g·d−1, 5 d). In fact, after the 6-wk maintenance phase, muscle Cr levels had returned to presupplementation values (26). Although the results of the current investigation show clearly that CrM provided significantly greater muscle hypertrophy and strength, metabolite assessments revealed no significant change in muscle Cr content at the end of the program. The benefits of CrM are thought to be dependent on its accumulation within the cell (5,9,11,20). Because the advantages of supplementation may be applicable to a wide sector of the population, further studies should investigate strategies that create and maintain high muscle Cr concentrations during exercise training.
The CrM-treated group demonstrated a significantly greater increase in contractile protein content (milligrams per gram of muscle) compared with the other groups after the training program (Fig. 4). This result reflects the changes in CSA and LBM that were also detected. An increase in contractile protein is thought to be an important stimulus that results in an increase in muscle fiber CSA (17). RE-induced muscle fiber hypertrophy is thought to be primarily responsible for improvements in force production and strength that are observed in RE-trained participants (22). When all participants were combined, a strong relationship between changes in muscle fiber CSA of the type II fibers (IIa and IIx grouped) and strength improvements in the squat exercise were evident (Fig. 5). A similar relationship between changes in contractile protein content or LBM and strength improvements in the squat was also detected (Figs. 6 and 7). The r values obtained suggest that a substantial portion (at least 40%) of the strength improvements observed across all groups could be attributed to the changes in skeletal muscle morphology. These correlations reflect a direct relationship between muscle adaptation (hypertrophy) and an improvement in functional strength. Obviously, the barbell squat exercise was the focus of these correlation assessments, simply because, unlike the bench press and pulldown exercise, the vastus lateralis is recruited heavily during this exercise and was the muscle from which the biopsy samples were obtained.
Aside from skeletal muscle morphology, the improvements in 1RM strength observed in this trial must also be attributed to the benefits of personalized coaching/supervision. Although the participants in our study were experienced, none had ever received personal training by a qualified instructor (the personal training only occurred during the 10-wk trial, not the training program before the study). Personalized instruction of the participants was a major strength of this study, because this level of supervision is shown to provide better control of workout intensity and greater strength improvements during training (15). This level of supervision was important to our hypothesis because it would ensure the best chance of enhanced physiological adaptations from an interaction between training and CrM supplementation. This is based on the premise that those taking the CrM would obtain a greater anabolic response from each workout and progress at a faster rate. It is important to remember that the instructor was blinded to the supplement groups, yet the CrM-treated group demonstrated significantly greater gains in 1RM strength (in three of three assessments) and greater muscle hypertrophy responses (in three of three assessments), thus supporting the hypothesis presented.
Another interesting finding from this study was the influence of the different supplements on body composition. Whereas all groups demonstrated a gain in body mass after the training program, the Cr-treated group demonstrated a significantly greater gain in body mass compared with the PRO group, but not the PRO-CHO group. However, there were differences in the composition of these changes in mass. Compared with the PRO-CHO group, the Cr-PRO-CHO group and the PRO groups demonstrated decreases in fat mass and body fat percentage (Table 3 and Fig. 1). The exact reasons for these different responses to the various supplements are not clear. A decrease in body fat and/or body fat percentage in response to whey protein supplementation (6-10 wk) is a phenomenon that has been reported previously in rodents (4) and humans undertaking RE training (7). Whey protein supplementation has been shown to induce greater lipid oxidation during and after exercise compared with casein and CHO, a response that resulted in a greater use of body fat for fuel and a reduction in body fat (4). However, this does not explain the contrasting body composition changes observed in the Cr-PRO-CHO and PRO-CHO groups. Both of these groups consumed the same supplement; the only difference was the relatively small amount of CrM present in the Cr-PRO-CHO supplement (approximately 7%). Despite this, the Cr-treated group demonstrated a reduction in fat mass (and body fat percentage) when compared with the PRO-CHO group. CrM does not seem to provide any benefit with regard to fat metabolism (12). Therefore, the improvement in body composition observed from CrM-supplementation is most likely attributable to the large accretion of LBM that was observed in this group, which was, on average, 6 kg. This extra muscle mass would almost certainly have had a positive influence on resting metabolic rate and, therefore, fat metabolism, particularly in active individuals that consume the same relative energy intake (per kilogram of body mass) for a prolonged period of time (18), as was the case in this study. If the addition of CrM to a PRO-CHO supplement does enhance LBM gains and improve body composition during training, as observed in this study, this may have specific implications for some populations. For example, those who desire maximum gains in LBM, strength, and muscle hypertrophy without an increase in fat mass will benefit from a CrM-containing PRO-CHO supplement. However, for those who desire a gain in body mass in general, CrM may not be required. Alternatively, athletes who desire strength and muscle hypertrophy with only a relatively modest increase in body mass may opt for supplementation with whey protein alone.
In conclusion, this study used a group of RE-trained participants to examine the effects of a CrM-containing (0.1 g·kg−1·d−1) PRO-CHO supplement in comparison with the same PRO-CHO supplement (without CrM) during 10 wk of RE training. Although both supplements were similar in energy and nitrogen content, the group that received CrM demonstrated greater gains in 1RM strength in three exercises, and these improvements were supported by a greater hypertrophy response that was apparent at three different levels: LBM, muscle fiber CSA, and contractile protein content. Therefore, in RE-trained individuals, the presence of CrM in a PRO-CHO supplement results in significantly greater adaptations during RE training than supplementation without CrM.
All supplements were kindly supplied by AST Sport Science, Golden, CO. The lead investigator is a consultant to AST Sports Science. The results of the present study do not constitute endorsement of the product by the authors or ACSM.
1. Baechle, T. R., R. W. Earle, and D. Wathen. Essentials of Strength and Conditioning: National Strength and Conditioning Association (NSCA)
. 2nd ed. T. R. Baechle and R. W. Earle (Eds.). Champaign, IL: Human Kinetics, 2000.
2. Bessman, S., and F. Savabi. The role of phosphocreatine energy shuttle in exercise and muscle hypertrophy. In: Creatine and Creatine Phosphate: Scientific and Clinical Perspectives
, M. A. Conway and J. F. Clark (Eds.). San Diego, CA: Academic Press, 1988, pp. 185-198.
3. Beitzel, F., P. Gregorevic, J. G. Ryall, D. R. Plant, M. N. Sillence, and G. S. Lynch. Beta 2-adrenoceptor agonist fenoterol enhances functional repair of regenerating rat skeletal muscle after injury. J. Appl. Physiol.
4. Bouthegourd, J. J., S. M. Roseau, L. Makarios-Lahham, et al. A preexercise α-lactalbumin-enriched whey protein meal preserves lipid oxidation and decreases adiposity in rats. Am. J. Physiol.
5. Brose, A., G. Parise, and M. A. Tarnopolsky. Creatine enhances isometric strength and body composition improvements following strength exercise training in older adults. J. Gerontol. A Biol. Sci. Med. Sci.
6. Burke, D. G., P. D. Chilibeck, K. S. Davidson, D. G. Candow, J. Farthing, and T. Smith-Palmer. The effect of whey protein supplementation with and without creatine monohydrate combined with resistance training on lean tissue mass and muscle strength. Int. J. Sport Nutr. Exerc. Metab.
7. Cribb, P. J., and A. Hayes. Effect of supplement-timing and resistance training on skeletal muscle hypertrophy. Med. Sci. Sports Exerc.
8. Cribb, P. J., A. D. Williams, M. F. Carey, and A. Hayes. The effect of whey isolate on strength, body composition and plasma glutamine. Int. J. Sports Nutr. Exerc. Metab.
9. Earnest, C. P., P. G. Snell, R. Rodriguez, A. L. Almada, and T. L. Mitchell. The effect of creatine monohydrate ingestion on anaerobic power indices, muscular strength and body composition. Acta Physiol. Scand.
10. Evans, W. J., S. D. Phinney, and V. R. Young. Suction applied to a muscle biopsy maximizes sample size. Med. Sci. Sports
11. Harris, R. C., K. Söderlund, and E. Hultman. Elevation of creatine in resting and exercised muscle of normal subjects by creatine supplementation. Clin. Sci.
12. Huso, M. E., J. S. Hampl, C. S. Johnston, and P. D. Swan. Creatine supplementation influences substrate utilization at rest. J. Appl. Physiol.
13. Kreider, R. B., M. Ferreira, M. Wilson, et al. Effects of creatine supplementation on body composition, strength, and sprint performance. Med. Sci. Sports Exerc.
14. Leutholtz, B., and R. Kreider. Exercise and sport nutrition. In: Nutritional Health
, T. Wilson and N. Temple (Eds.). Totowa NJ: Humana Press, pp. 207-239, 2001.
15. Mazzetti, S. A., W. J. Kraemer, J. S. Volek, et al. The influence of direct supervision of resistance training on strength performance. Med. Sci. Sports Exerc.
16. Olsen, S., P. Aagaard, F. Kadi, et al. Creatine supplementation augments the increase in satellite cell and myonuclei number in human skeletal muscle during strength training. J. Physiol.
17. Phillips, S. M. Short-term training: when do repeated bouts of resistance exercise become training? Can. J. Appl. Physiol.
18. Poehlman, E. T., and C. Melby. Resistance training and energy balance. Int. J. Sport Nutr.
19. Prior, B. M., K. J. Cureton, C. M. Modlesky, et al. In vivo validation of whole body composition estimates from dual-energy x-ray absorptiometry. J. Appl. Physiol.
20. Rawson, E. S., and J. S. Volek. Effects of creatine supplementation and resistance training on muscle strength and weightlifting performance. J. Strength Cond. Res.
21. Robinson, T. M., D. A. Sewell, E. Hultman, and P. L. Greenhaff. Role of submaximal exercise in promoting creatine and glycogen accumulation in human skeletal muscle. J. Appl. Physiol.
22. Shoepe, T. C., J. E. Stelzer, D. P. Garner, and J. J. Widrick. Functional adaptability of muscle fibers to long-term resistance exercise. Med. Sci. Sports Exerc.
23. Steenge, G. R., E. J. Simpson, and P. L. Greenhaff. Protein-and carbohydrate-induced augmentation of whole body creatine retention in humans. J. Appl. Physiol.
24. Tarnopolsky, M. A., G. Parise, N. J. Yardley, et al. Creatine-dextrose and protein-dextrose induce similar strength gains during training. Med. Sci. Sports Exerc.
25. Vandenberghe, K., M. Goris, P. Van Hecke, M. Van Leemputte, L. Vangerven, and P. Hespel. Long-term creatine intake is beneficial to muscle performance during resistance training. J. Appl. Physiol.
26. van Loon, L. J., A. M. Oosterlaar, F. Hartgens, M. K. Hesselink, R. J. Snow, and AJ. Wagenmakers. Effects of creatine loading and prolonged creatine supplementation on body composition, fuel selection, sprint and endurance performance in humans. Clin. Sci.
27. Volek, J. S., N. D. Duncan, S. A. Mazzetti, et al. Performance and muscle fiber adaptations to creatine supplementation and heavy resistance training. Med. Sci. Sports Exerc.
28. Weir, J. P. Quantifying test-retest reliability using the intraclass correlation coefficient and the SEM. J. Strength Cond. Res.
29. Willoughby, D. S., and J. Rosene. Effects of oral creatine and resistance training on myosin heavy chain expression. Med. Sci. Sports Exerc.
Keywords:©2007The American College of Sports Medicine
WHEY PROTEIN; HISTOCHEMISTRY; SKELETAL MUSCLE STRENGTH; FIBER AREA; CONTRACTILE PROTEIN; BODY COMPOSITION