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Gycogen and Resistance Training
Todd Astorino, M.S. and Len Kravitz, Ph.D.

Studies Reviewed:
Haff, G. G., et al. 1999. The effect of carbohydrate supplementation on
multiple sessions and bouts of resistance exercise. Journal of Strength and Conditioning Research, 13, (2), 111-7.

Leveritt, M. & Abernethy, P. J. 1999. Effects of carbohydrate restriction
on strength performance. Journal of Strength and Conditioning Research, 13, (1), 52-7.

The Role of Glycogen in Aerobic and Resistance Exercise

The role of glycogen (stored carbohydrate in muscle) in aerobic exercise has been clearly shown to be associated with increased work output and duration (Haff et al., 1999). Carbohydrate is the body’s preferred substrate during endurance exercise due to its more efficient energy yield per liter of oxygen consumed. Previous resistance training research suggests that weight training is associated with a consequential depletion of muscle glycogen stores. For instance, Robergs et al. (1991) demonstrated that subjects performing 6 sets of leg extensions at 35% and 70% of 1RM resulted in a decrease in muscle glycogen by 38% and 39%, respectively. This article will review two recent articles that further elucidate the role of glycogen in resistance exercise. It is hoped that the personal trainer will gain a better understanding as to the appropriateness of carbohydrate replenishment recommendations for clients engaged in resistance exercise programs.

Energy for Resistance Exercise
Due to the intense and short-term nature of individual bouts of resistance training, it would seem likely that this activity would be highly dependent upon muscle glycogen for ATP provision. High-intensity exercise of short duration (&Mac178; 30 seconds) is characterized by a rapid breakdown of phosphocreatine for the production and use of ATP, as well as stimulation of glycogenolysis (breakdown of glycogen) and glycolysis (breakdown of glucose), with a lesser contribution of oxidative metabolism.

In a study by Tesch et al. (1986), nine bodybuilders completed five sets each of front squats, back squats, leg presses, and leg extensions to fatigue, comprising 30 minutes of exercise. Biopsies of muscle samples were obtained from the vastus lateralis before and immediately after exercise. Muscle glycogen concentration was 26% lower post-exercise, a rather modest decline considering the demanding exercise protocol completed. This led the authors to conclude that energy sources in addition to muscle glycogen support heavy resistance training. Data from Essen-Gustavsson and Tesch (1990) with nine bodybuilders performing the same exercise regimen (as above) revealed a 28% decrement in muscle glycogen content as well as a 30% decrease in muscle triglyceride content. This suggests that intramuscular lipolysis (breakdown of triglycerides) may also play a role in energy production during repeated high-intensity exercise. Overall, research suggests that intramuscular glycogen is an important fuel supporting weight training exercise, but not the only substrate.

Effects of Carbohydrate Restriction and Aerobic Exercise on Strength Performance
Recent research has demonstrated that depleted muscle glycogen stores in conjunction with aerobic exercise compromises strength performance (Levitt & Abernethy, 1999). Subjects (5 young men and one woman) performed resistance exercise under a control (CON) condition (no strenuous exercise for at least 48 hours prior to testing) and after a carbohydrate restricted program (EXP). The EXP condition included 60 min of submaximal cycling and four 1 minute bouts of maximal exercise, followed by 48 hours of reduced carbohydrate intake. The resistance exercise consisted of three sets of squats (80% 1RM) and 5 sets of isokinetic knee extensions, all at different contractile speeds. In comparing the CON to the EXP testing condition, the most observable difference was noted in squat performance, with no significant differences in the knee extension trials. There was a decrease in the average total number of repetitions in Set 1 (CON=18 reps vs EXP=12 reps) and Set 2 (CON=13.5 reps vs EXP=10.33 reps). However, there was no difference between the CON and the EXP groups at any of the five contractile speeds of isokinetic knee extensions.

In explaining the differing outcomes of the squat sets versus the knee extensions sets (to an aerobic and carbohydrate restricted program), the authors summarized previous research that has depicted substrate utilization differences in the type of exercise. Isometric exercise has been shown to be impaired by reducing glycogen content while no change has been seen in isokinetic exercise. The authors hypothesized the differences in the present study were also due to the type of exercise. The isokinetic exercise bouts consisted of relatively short duration (1.5 to 7.5 seconds) versus the sets of squats (approximately 30 seconds per set). It was felt the energy production of the isokinetic exercise was predominantly due to the breakdown of creatine phosphate while the utilization of glycogen was much more apparent in the longer lasting squat exercise regime.

The Effect of Carbohydrate Supplementation on Multiple Sessions and Bouts of Resistance Exercise
For athletes completing multiple high-intensity strength training sessions per day, maintenance of muscle glycogen stores is critical. In a study by Haff et al. (1999), six resistance-trained men ingested a 250 gram carbohydrate supplement or placebo during a morning training session, rested for 4 hours, and then performed a second session consisting of multiple sets of light-intensity squats (55% 1RM) to exhaustion. During the second training session, the number of sets and repetitions performed were markedly higher with the carbohydrate consumption, and subjects were able to exercise for 30 minutes longer. The authors concluded that athletes engaging in multiple exercise sessions per day (ranging from mild to high intensity) will receive a performance advantage with carbohydrate ingestion via maintenance of intramuscular glycogen stores, due to greater glycogen resynthesis during recovery. In addition, the carbohydrate supplementation not only increased workout performance, it markedly increased workout duration.

Practical Application
For the recreational athlete participating in weight training, consideration of muscle glycogen stores is most satisfactory maintained with a well-balanced and calorically-sufficient diet. It is necessary for personal trainers to consider the exercise habits and goals of their weight training clients before prescribing carbohydrate supplementation to benefit exercise performance. So as not to let clients get carried away, it is meaningful to remind them that an excess of carbohydrate intake, exceeding bodily energy expenditure needs, will result in weight gain. However, it is apparent from these two studies reviewed that individuals doing concurrent aerobic exercise with high-intensity resistance training and/or completing multiple training sessions per day should be concerned with maintenance of glycogen stores, since glycogen depletion may reduce work output and duration.

Essen-Gustavsson, B. & Tesch, P. A. 1990. Glycogen and triglyceride
utilization in relation to muscle metabolic characteristics in men performing heavy-resistance exercise. European Journal of Applied Physiology, 61, 5-10.

Haff, G. G., et al. 1999. The effect of carbohydrate supplementation on
multiple sessions and bouts of resistance exercise. Journal of Strength and Conditioning Research, 13, (2), 111-7.

Leveritt, M. & Abernethy, P. J. 1999. Effects of carbohydrate restriction
on strength performance. Journal of Strength and Conditioning Research, 13, (1), 52-7.

Robergs, R. A., Pearson, D. R., Costil, D. L., Fink, D. D., Pascoe, M. A., Benedict, C. P., Lambert, C. P., and Zachweija, J. J. (1991). Muscle glycogenolysis during differing intensities of weight-resistance exercise. Journal of Applied Physiology, 70, 1700-1706.

Tesch, P. A., Colliander, E. B., & Kaiser, P. 1986. Muscle metabolism
during intense, heavy- resistance exercise. European Journal of Applied Physiology, 55, 362-6.

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