Resistance training (RT) has become an integral part of the training practices of most athletes. With the increasing popularity of strength and power training, many ergogenic aids and nutritional strategies have been employed in an attempt to improve performance or increase muscle growth. CHO supplementation is one ergogenic aid that is not often associated with RT performance and muscle growth. Strength-power athletes have high training intensities and volumes for most of the training season, so energy intake must be sufficient enough to support recovery and adaptation.
Strength and power training can be characterised by the prominence of the ATP-CP and glycolytic system that are stressed during strength and power based activity (Tipton et al., 2001). These anaerobic activities are usually categorized as any type of activity against an external resistance, using sets and repetitions interspersed with rest periods, which can induce muscular adaptations, including strength and power. It is worth noting that strength and power training principles applied to repetition, sets, and rest are implemented to train for individual aspects within the force-velocity curve depending upon the time of season, phase of training and requirements of the sport (Plisk and Stone, 2003). Strength to maximum strength phases are usually characterized by low volume and increases in intensity (>80% of 1 repetition maximum [RM]) eliciting neuromuscular adaptations with no more than 6 reps, 3-5 sets, and between 3-6 days per week) (Stone et al., 1999). Power training is characterized by a mixed methods approach in which lower intensity (<50% 1RM) and higher intensity (50-70% 1RM) loads and exercise types are used in a periodised fashion to optimize power output (Haff and Nimphius, 2012).
Overview of Carbohydrate Metabolism and Habitual Intakes
CHO provides the majority of the fuel for exercise intensities above 75% VO2max, and is a fuel for both anaerobic glycolysis and ATP-CP system (Jeukendrup and Gleeson, 2004). During exercise of increasing intensity, when adenosine triphosphate (ATP) production from oxidative phosphorylation cannot match the rate of ATP hydrolysis, the shortfall in ATP supply is met by substrate level phosphorylation. This system provides energy via phosphagen utilisation and the metabolism of muscle glycogen and plasma glucose. The availability of CHO as a substrate for the muscle and central nervous system becomes a limiting factor in the performance of intermittent high-intensity exercise, and plays a permissive role in the performance of brief high-intensity work (MacDougall et al., 1977). Insufficient levels of muscle glycogen, caused by inadequate dietary carbohydrate intake, have resulted in accentuated exercise-induced muscle weakness (Yaspelkis et al., 1993) decreased isokinetic force production (Hepburn and Maughan, 1982) and reduced isometric strength (Jacobs et al., 1981).
The current Recommended Daily Allowance (RDA) of CHO for the normal population is described as an energy ratio of 55-60% of total energy (Burke et al., 2001). However, rigid interpretation of dietary guidelines based on energy ratios can prove unnecessary and unfeasible for some athletes, resulting in exceeding combined requirement for daily glycogen storage and training fuel. Therefore, it is preferable to provide recommendations for routine CHO intake in grams (relative to the body mass of the athlete) and allow flexibility for athletes to meet these intakes within the context of their energy needs and other dietary goals.
Habitual CHO intakes of male strength-power athletes range from 4.5 to 6.4 g/kg body mass per day, averaging 5.8 g/kg body mass per day. Female athletes are reported at 4.3 to 5.8 g/kg body mass per day, averaging 4.6 g/kg per day (Burke et al., 2001). Female athletes report lower CHO intakes than male athletes, principally as a result of lower total energy intakes. Many female athletes share similar training loads and energy expenditure to their male counterparts, but undertake periods of energy restriction and negative energy balance in the desire to achieve or maintain low body fat levels believed to be necessary for optimal performance (Burke et al., 2001).
Effects of Resistance Training on Carbohydrate Metabolism
Resistance Training and Glycogenolysis
Traditionally, it has been thought that short-duration high-intensity exercise is primarily supplied with energy from the ATP-CP system, however, glycogenolysis has previously demonstrated to be an important energy supplier during high-intensity intermittent exercise, such as RT (MacDougall et al., 1977; Tesch et al., 1986; Tesch, 1988; Robergs et al., 1991).
Haff et al. (Haff et al., 2000) reported that 3 sets of isokinetic leg extensions performed at 120º/s can reduce the muscle glycogen content of the vastus lateralis by 17%. Additionally, in the same investigation a multiple-set RT session (back squats, speed squats, 1-leg squats) performed at 65, 45, and 10% of 1RM back squat resulted in a 26.7% decrease in muscle glycogen of the vastus lateralis.
Tesch et al. (Tesch, 1988) have also reported a 40% reduction in muscle glycogen in response to the performance of 5 sets of 10 repetitions of concentric knee extensions performed at 60% 1RM. A 30% decrease in the muscle glycogen content on type IIab and IIb fibres in response to this protocol was reported. Muscle glycogen concentration was also reported to decrease by ~20% in response to the performance of 5 sets of 10 repetitions at 45% 1RM.
Robergs et al. (Robergs et al., 1991) have shown that 6 sets of 6 repetitions of leg extensions performed at 0 and 35% 1RM can elicit a significant glycogenolytic effect resulting in 39 and 38% reductions in glycogen, respectively. In the study type II fibres were also demonstrated to have a greater glycogen loss when compared with type I fibres.
Tesch et al. (Tesch et al., 1986) also reported that a 26% decrease in the muscle glycogen content of the vastus lateralis can occur in response to a RT regimen consisting of front squats, back squats, leg presses, and knee extensions. The above studies utilised a protocol similar to hypertrophy training, however, strength-power athletes commonly perform high-intensity RT bouts which have the potential to further affect the rate and amount of muscle glycogenolysis (Robergs et al., 1991), therefore CHO supplements may be warranted (Tesch et al., 1998).
Recommended Carbohydrate Requirements for Strength and Power Training
Only a few previous reviews have focused on the complexities of strength-power sport athletes (Maughan and Poole, 1981; Stellingwerff et al., 2007; Stellingwerff et al., 2011) and given the diversity of training and competition that these athletes undertake, an individualised CHO intake approach needs to be implemented dependent on various factors such as training status (novice/advanced), duration, intensity, and frequency of the training programme (Kraemer, 1988; Kraemer et al., 1998; Stellingwerff et al., 2007; Burke et al., 2011). During most of the training season, adequate energy must be consumed to support the training volume and intensity. Many strength-power athletes can undertake 9-14 training sessions each week, with workouts ranging from 30 minutes to 3 hours in duration, including resistance and plyometric/neuromuscular training several times per week. The primary focus for strength-power athletes is the recovery of muscle energy stores (primarily glycogen) and the synthesis of new proteins. Previous reports the dietary amount of CHO required to achieve glycogen replenishment on a daily basis in active individuals is 6-8 g/kg body mass in women, and 8-10 g/kg body mass in men (Slater and Phillips, 2011).
Extremely low CHO diets (3-15% CHO) have uniformly been shown to impair high-intensity exercise (Maughan and Poole, 1981).
Maughan & Poole (Maughan and Poole, 1981) investigated the effects of a moderate CHO (42.5% CHO), low CHO (2.6% CHO), and high-CHO (84.2% CHO) diet on exercise performance of 6 physically active men. The results of this study indicate that the high-CHO diet stimulated a significantly longer (+3.3 minutes) period of exercise performed at 104% of VO2max than did the low-CHO diet. The high-CHO regime also resulted in a 1.7 minute increase in exercise duration when compared to the moderate-CHO trial, however this was not significant. These results are difficult to interpret because all participants performed the testing in the same order, which may have resulted in an order effect.
In another investigation, the consumption of a 10% CHO diet (58.5 g CHO) for 3 days resulted in a significant decrease in exercise duration (87s and 124s) at a workload equivalent to 100% of VO2max when compared with exercise duration after consumption of a moderate- CHO diet (46.2% CHO) and high-CHO (65.5% CHO) diet (Greenhaff et al., 1987).
Simonsen et al. (Simonsen et al., 1991) reported that high-CHO diets (70% CHO) can elicit an ergogenic effect during high-intensity rowing. Participants were randomly assigned to a high-CHO diet (70% CHO) or a moderate-CHO diet (42% CHO) over a 4-week training period. The high-CHO diet promoted significantly greater muscle glycogen concentrations and mean power outputs compared with the moderate-CHO diet.
Conversely, others have suggested that moderate- and low-CHO diets do not impair athletic performance and produce comparable performance results when compared with high-CHO diets (Hargreaves and Briggs, 1988; Lamb et al., 1990; Vandenberghe et al., 1995). Lamb et al. (Lamb et al., 1990) examined the effects of a high-CHO (80% CHO) or moderate-CHO (43% CHO) diet for 9 days on swimming performance. The high-CHO diet did not elicit a significant ergogenic effect when compared to the moderate-CHO diet in interval swims of 50 – 200 m. These results are similar to those reported by Geenhaff et al. (Greenhaff et al., 1987), who reported that moderate-CHO diets did not result in significant decrease in high-intensity exercise performance. Lamb et al. (Lamb et al., 1990) suggested that the CHO content (43%) of the total energy intake of the moderate-CHO diet was sufficient enough to maximise glycogenolysis. A possible limitation of this study was that the swim tests utilised may not have been maximal because they were part of the actual training sessions.
The amount of CHO that is oxidised during exercise depends on both the exercise intensity and duration. Owing to high exercise intensities during the specific preparation and competition phases, the relative dependency on CHO based ATP provision increases throughout training cycles. However, given the large training volumes during the general preparation phase, the absolute requirement for CHO is high, thus CHO-rich foods must provide the majority of energy throughout the training year (Stellingwerff et al., 2011). Thus constantly training in an energy and CHO depleted state may compromise immune function, training staleness, and burnout.
Therefore, depending on individual training volume and intensity, a habitually moderate to high CHO diet of around 4-8 g/kg body mass, with females on the lower end and males on the higher end range, is recommended to maintain immune function, recover glycogen storage, and reduce overreaching (Stellingwerff et al., 2011). While this may appear low relative to endurance athletes, conclusive evidence of benefit from maintaining a habitual high CHO intake among strength-power athletes remains to be confirmed.
Timing and Amount of Carbohydrate
Having established the ranges of CHO requirements for strength-power athletes, it is essential to consider the timing of CHO intake in the hours before exercise, based on the assumption that pre-exercise nutritional strategies can influence exercise performance, specifically acute CHO ingestion prior to strength training (Lambert et al., 1991; Haff et al., 1999; Haff et al., 2001).
Lambert et al. (Lambert et al., 1991) reported that supplemental ingestion before and during RT (1 g/kgˉ¹ before, 0.5 g/kgˉ¹ during) increased total work capacity when performing sets of 10 repetitions of leg extensions at 80% 10RM to muscular failure, a response that has been replicated elsewhere (Haff et al., 1999; Haff et al., 2001).
Haff et al. (Haff et al., 2001) have reported CHO supplementation (1 g/kgˉ¹ before, 0.5 g/kgˉ¹ during) before and after sets 1, 6, and 11 can increase the amount of work performed during 16 sets of 10 repetitions of isokinetic leg extensions performed at 120º/sˉ¹. Additionally, significantly greater torque was generated by the quadriceps when the CHO supplement was consumed. Significant increases in RT performance have been reported after CHO are consumed during and between multiple training sessions in one day (Haff et al., 1999). Subjects ingested either a CHO supplement or placebo beverage during a 1-hour morning RT session (1.2 g/kgˉ¹), 4-hour recovery period (0.38 g/kgˉ¹), and an afternoon performance test (0.3 g/kg ˉ¹) consisting of sets of 10 back squat repetitions performed at 55% 1RM. The authors found the CHO protocol resulted in significantly more repetitions (+67.7) and sets (+7.4) and greater exercise duration (+31.6 minutes) during the afternoon performance test.
It is important to note that these studies required subjects to perform a RT session that required the performance of high volumes of work similar to those performed during the hypertrophy phase of a periodised training programme. Further research is required aimed at understanding the ergogenic role of CHO during high-intensity (≥85% 1RM) resistance exercise in exercises commonly associated with strength-power training programmes (power clean, back squat, snatch).
However not all investigations show performance benefit with acute CHO ingestion (Haff et al., 2000; Kulik et al., 2008). Haff et al. (Haff et al., 2000) found the addition of a CHO supplement (prior to and every 10 minutes during RT) did not elicit an ergogenic effect. This may potentially be a product of the performance test selected (3 sets of 10 repetitions isokinetic leg extension). Leveritt & Abernethy (Leveritt and Abernethy, 1999) have reported that low levels of glycogen seem to impair the performance of back squats but have no effects on isokinetic leg exercise. This may occur because force is only applied through a small range of motion during isokinetic exercise (Murray and Harrison, 1986; Chow et al., 1997), this potentially could decrease the amount of work performed and result in a masking of the ergogenic benefit of CHO supplementation. Additionally, large-mass exercise may stimulate a greater amount of glycogen loss in a number of muscles (not just the prime movers), allowing for an increased ergogenic benefit from CHO supplementation.
The studies by Lambert et al. (Lambert et al., 1991), Haff et al. (Haff et al., 1999) and Haff et al. (Haff et al., 2001) showed ergogenic effects when the exercise bouts lasted 56, 77, and 57 minutes, respectively. In contrast , the studies that failed to demonstrate an ergogenic effect lasted 39 (Haff et al., 2000) and 29 (Kulik et al., 2008) minutes. Thus it is possible that the duration of the activity influenced the ergogenic effectiveness of the CHO supplement. Secondly, the volume of work performed may be a significant factor mediating the ergogenic effect of the CHO supplement. It is possible that high volumes of work performed for a duration greater than 40 minutes stimulate a greater stress on the glycolytic system. The consumption of a CHO supplement during this scenario could possibly spare muscle glycogen or result in blood glucose (BG) becoming the predominant fuel source as glycogen becomes depleted (Hargreaves and Briggs, 1988; Slater and Phillips, 2011). Liquid CHO ingestion during the exercise may shift the exercise-induced hormonal milieu toward a profile more favourable for anabolism (Tarpenning et al., 2001).
At present, a specific recommendation for an optimum rate or timing of CHO ingestion for strength-power athletes before and during any given training session cannot be determined. As with all athletes, strength-power athletes should be encouraged to optimise muscle glycogen stores to support training and recovery.
General sports nutrition guidelines advocate the ingestion of CHO at a rate of 1.0-1.2 g/kgˉ¹ body mass in the immediate post-exercise period (Burke et al., 2004). However this has no influence on muscle protein metabolism in the absence of dietary protein ingestion (Koopman et al., 2007). Thus the combined ingestion of CHO and protein acutely following RT results in more favourable recovery outcomes, including restoration of muscle glycogen stores and muscle protein metabolism, than ingestion of either nutrient alone (Miller et al., 2003; Børsheim et al., 2004; Bird et al., 2006a; Bird et al., 2006b). Post-exercise protein ingestion also lowers CHO intake requirements in the acute recovery period, with an energy matched intake of 0.8 g/kg/hˉ¹ CHO plus 0.4 g/kg/hˉ¹ protein resulting in similar muscle glycogen resynthesis over 5 hours as 1.2 g/kg/hˉ¹ CHO alone following intermittent exercise (van Loon et al., 2000), with a similar response evident following RT (Roy and Tarnopolsky, 1998).
Knowing that the habitual CHO intake of strength-power athletes are between 3.0-5.6 g/kg body mass per day (Burke et al., 2001; Slater and Phillips, 2011), it is reasonable to assume that CHO requirements centred around exercise aimed at promoting muscular adaptations to RT are being met. Evidence has reported that actual CHO requirements for strength-power training may be sufficient at moderate intakes of ~4 g/kg body mass per day for female and ~7 g/kg body mass per day for male athletes (Conley and Stone, 1996; Volek, 2004; Volek et al., 2006; Slater and Phillips, 2011; Stellingwerff et al., 2011).
Furthermore, there is a lack of research in understanding the ergogenic roles of CHO during various types of RT bouts. However, it is important to note that, based on previous literature a high-intensity RT session can result in a significant glycogenolytic effect. If several days of high-intensity RT are coupled with inadequate dietary CHO consumption, muscle glycogen stores may be negatively affected, which could result in a performance decrement (Haff and Whitley, 2002). It would be interesting to study the chronic effect of CHO on strength-power training, rather than the acute effects of a single RT.
Nevertheless, despite the lack of research, there remain some key strength-power nutritional concepts that impact upon the effectiveness of CHO supplementation strategies to enhance training adaptations for strength-power athletes. First, liquid CHO ingestion before RT may reduce muscle and liver glycogen loss associated with an acute bout of RT, and this may be of importance for athletes involved in multiple training bouts per day (Haff et al., 1999). Second, liquid CHO ingestion during RT may shift exercise-induced insulin and cortisol responses towards a profile more favourable for anabolism, regulating CHO metabolism and protein turnover. Lastly, CHO ingestion (1.0-1.2 g/kg body mass) combined with 20 g of protein within the first 30 minutes post exercise has shown to increase recovery and maximise adaptive process within the muscle.