Glucose is stored in large quantities as glycogen in the liver and skeletal muscles. There have been numerous theories on what governs athletic performance and many studies have reported that endurance performance is limited with depleted or low glycogen levels in muscle (Lima-Silva et al., 2009; Coyle and Coggan, 1984; Hermansen et al., 1967). Decreased glycogen content (within the muscle) is linked to decreased contractile force which is detrimental to certain sport performances (Ortenblad et al., 2011). During exercise, energetic demands increase substantially and the need for ATP increases to sustain muscle contractions. Glycogen serves as the main substrate for adenosine triphosphate (ATP) synthesis during moderate- to high-intensity exercise (van Loon, et al., 2000; 2001). Theories attempting to explain mechanisms of fatigue involve glycogen depletion, concluding low metabolic carbohydrate fuel causes fatigue (Williams et al., 2013; Coyle and Coggan, 1984; Hermansen et al., 1967). Low glycogen levels have been linked to decreased sarcoplasmic reticulum [Ca.sup.2+] release detrimentally altering muscle fiber contractility (Ortenblad, et al., 2011; 2013). Individuals may have compromised contractile abilities if muscle glycogen levels are low.
Most sports teams aim to improve physical ability by utilizing some form of resistance training. The primary bioenergetic pathways involved with anaerobic exercise are intramuscular ATP hydrolysis, creatine phosphate (CrP)-ATP/phosphagen system, and anaerobic glycolysis primarily involving glycogen (Jacobs et al., 1982). Teams use multiple training sessions in a day to maximize adaptations and increase performance. Completing multiple training sessions in a 24-hour period requires glycogen replenishment to allow for continued exercise intensity in later training bouts. The amount of glycogenesis possible during recovery is directly related to the total availability of carbohydrates post-exercise (Bergstrom and Hultman, 1966; van Loon et al., 2000; Costill, 1991). Consequently, limited carbohydrate intake may decrease performance, training adaptations, and recovery.
Carbohydrate ingestion with aerobic-based exercise has been investigated over the past century (Hearris et al., 2018). Studies have shown that carbohydrate consumption can increase time to exhaustion and delay fatigue in extended aerobic exercise (Coyle et al., 1985). In resistance exercise, positive ergogenic effects have been seen over longer durations of resistance exercise (>50min) with moderate loads and high volume, while negligible findings have been found when exercise is shorter (
Ten healthy, recreationally resistance-trained men [regular (i.e. thrice weekly) for at least 1 year], who had a mean [+ or -] SD body mass of 90 [+ or -] 18.2 kg, height 1.79 [+ or -] 0.06 m, age 21.6 [+ or -] 2.27 yr, 5.98 [+ or -] 1.55 leg press strength-to-body weight ratio, and body fat 21.3 [+ or -] 8.1%, participated in this study. Only participants considered low risk for cardiovascular disease with no contraindications to exercise outlined by the American College of Sports Medicine, and who had not consumed any nutritional supplements (excluding multi-vitamins) one month prior to the study could participate. This study was approved by the Institutional Review Board for Human Subjects at Baylor University. Additionally, all experimental procedures involved in the study conformed to the ethical consideration of the Declaration of Helsinki.
Participants were familiarized to the study protocol via a verbal and written explanation outlining the study design and then read and signed a university-approved informed consent document. In addition, each participant was instructed to refrain from exercise for 48 hours before each testing session, eat a light, low carbohydrate meal 3 hours prior to reporting for each testing session, and record their dietary intake for two days (including the light meal the morning of testing) prior to each of the three testing sessions. Diets were not standardized but participants were asked not to change their dietary habits. The MyFitnessPal mobile application (Under Armor Inc., Baltimore, MD, USA) was used to determine the average daily macronutrient consumption of fat, carbohydrate, and protein. Participants completed a medical history questionnaire and underwent a general physical examination to determine whether they further met eligibility criteria.
At session 2, participants performed angled leg press one-repetition maximum (1-RM) tests with the National Strength and Conditioning Association (NSCA) recommendations (Moir, 2010). Foot placement was recorded and held constant over all testing conditions. A goniometer was used to establish 90[degrees] of knee flexion while positioned on the leg press machine and safety catches were adjusted just below this point for all tests to standardize the range of motion to 90[degrees]. The participants were instructed to lower the weight just above the catches before pressing the weight upward.
Participants warmed up with 10 repetitions at approximately 50% estimated 1-RM, rested 1 minute, and completed 5 repetitions at approximately 70% estimated 1RM followed by 2 minutes rest. The weight was then increased conservatively, and the participants attempted the first 1-RM. If the lift was successful, the participant rested for 2 minutes before attempting the next 1-RM. The 1-RM of each participant was compared to male 90% rank normative values of age specific strength-to-body weight ratios (2.27) (Hoffman, 2007). This substantiated that all participants were trained for at least a year as stated in the exercise questionnaire. Thirty minutes following the 1-RM test, participants completed 4 sets of maximal repetitions as outlined in the resistance exercise protocol.
During testing sessions 3 and 4, participants completed a warm-up set of 10 repetitions and 5 repetitions at 50% and 70% of the testing load, respectively. Each set was followed by two minutes of rest and then the exercise testing session began. Participants performed 4 sets of repetitions to volitional fatigue with 70% of the 1-RM on the angled leg press. A 45-second rest interval was provided between sets. When the subject was not able to perform another repetition in the set, study personnel assisted to help re-rack the weight safely. The total number of repetitions performed at each testing session was recorded.
At session 3 and 4, 30 minutes before resistance exercise, in a randomized, doubled-blind fashion, participants ingested either a placebo or a carbohydrate supplement. The 30min time point was selected in an attempt to maximize the bio-available glucose in the blood that could be used at the onset of exercise (Pannoni, 2011). The placebo consisted of a flavored, non-caloric beverage [(Crystal Light) Kraft Foods, Chicago, IL, USA] mixed with water and orally ingested. The carbohydrate supplement was maltodextrin (Carbo Gain, NOW Sports, Bloomingdale, IL, USA) at a dose of 2 g/kg body mass, mixed with Crystal Light in water, and orally ingested (Wax et al., 2012). Both supplements were of similar color, taste, volume, and consistency. A period of 7 to 10 days separated sessions 3 and 4 to allow for adequate supplement washout and muscle recovery.
Venous blood samples were obtained from an antecubital vein into 10 ml vacutainer tubes before supplement ingestion, immediately before resistance exercise (30 minutes following supplement ingestion), immediately post-exercise, and 1-hr post exercise. Blood samples stood at room temperature for 10 minutes and then centrifuged at 2500 rpm. The serum was removed and frozen at -80[degrees]C for later analysis.
Blood was drawn into microhematocrit tubes (in...