by NSCA’s Guide to Sport and Exercise Nutrition
Kinetic Select July 2020
The following is an exclusive excerpt from the book NSCA’s Guide to Sport and Exercise Nutrition, published by Human Kinetics. All text and images provided by Human Kinetics.
Strength performance, as well as training to improve muscular strength, muscular endurance, and muscular power, consists of repetitive bouts of high-intensity work with relatively short rest intervals. Therefore, carbohydrate is the primary fuel source over the course of a resistance-type exercise session. As with anaerobic exercise, the intensity of the bout dictates the level of fast-twitch muscle fiber recruitment, which in large part determines the performance capabilities of a muscle or muscle group in resistance-type exercise. During high-intensity (i.e., >60% 1-repetition maximum [1RM]) resistance exercise, fast-twitch fibers are heavily recruited, and they quickly fatigue as their glycogen content is utilized. Not surprisingly, the recruitment of fast-twitch Type IIx fibers (formerly referred to as Type IIb) is increased during eccentric and high-speed contractions (Nardone, Romano, and Schieppati 1989; Tesch, Colliander, and Kaiser 1986). However, several studies have demonstrated that the faster-twitch fiber types are recruited at moderate- (i.e., 60% 1RM; Tesch et al. 1998) and even lower-intensity (i.e., 20-40% 1RM; Gollnick et al. 1974; Robergs et al. 1991) muscle contractions.
These findings suggest that the fatigue associated with muscular endurance-type exercise, such as that performed in many individual and team sport training sessions and competitions, could be limited by the initial glycogen content and rate of depletion in the recruited fast-twitch fibers. Because many strength and power athletes perform intense training several times a week, adequate carbohydrate intake is necessary to prevent a gradual depletion of glycogen in trained muscle over time. Further, the amount of glycogen used in their resistance training sessions also appears to be related to the total amount of work accomplished and the duration of the resistance training bout.
Because of the cumulative glycogen use in resistance-trained muscle during a workout or competition, including warm-up, stretching, and cool down sessions, it has been suggested that consumption of higher levels of carbohydrate in the diet would improve muscle performance in these types of activities (Balsom et al. 1999; Casey et al. 1996; Maughan et al. 1997; Robergs et al. 1991; Rockwell, Rankin, and Dixon 2003; Tesch, Colliander, and Kaiser 1986). However, research studies have provided varied results with respect to specific nutritional carbohydrate practices and acute strength training performance (Haff et al. 1999, 2000; Kulik et al. 2008; Robergs et al. 1991).
Another factor to consider regarding ingestion of carbohydrate in the time period surrounding a resistance training bout is its effect on increasing insulin. Carbohydrate ingestion (particularly high-glycemic types) dramatically increases endogenous insulin secretion. The hormone insulin enhances the anabolic stimulus that resistance exercise produces. Specifically, insulin acts as a powerful anabolic hormone in previously exercised muscle in multiple ways, including
Two of these effects of insulin release—increasing protein synthesis and decreasing protein breakdown—may improve the chronic anabolic adaptations of resistance exercise, particularly if insulin is elevated surrounding the time frame of each resistance exercise bout via carbohydrate consumption. Associated with this is the recommendation to ingest liquid carbohydrate before, during, and after exercise to promote a faster recovery and gains in lean body mass (Haff et al. 2003). Chapter 9 expands on this concept of nutrient timing and the impact that carbohydrate ingestion has on endogenous insulin secretion, as well as the exercise performance improvements observed with such practices.
NSCA’s Guide to Sport and Exercise Nutrition will lead you through the key concepts of sport and exercise nutrition so that you can assess an individual’s nutrition status and—if it falls within your scope of practice—develop customized nutrition plans. The book is available in bookstores everywhere, as well as online at the NSCA Store.