solution during HIIE but were subject to methodologic limitations. Of particular concern was the use of exercise protocols that employed the same exercise intensity, the lack of maximal- or high-intensity work, and long periods of seated recovery, which failed to replicate the pattern and physiological demands of ex- ercise during team games, thus limiting their applicability to actual team games.118 Bangsbo and colleagues reported that by increasing the carbohydrate content of the diet from 39% to 65%


before a soccer match, muscle glycogen levels increased and resulted in a higher work rate and improved in- termittent endurance.119


In a study of Swedish elite ice hockey players, Akermark and colleagues compared


the performance of ice hockey players given a carbohydrate-enriched diet vs a mixed diet. During a 3-day period between two games, players were fed a mixed diet of approximately 40% carbohydrate or a diet of ap- proximately 60% carbohydrate. Distance skated, number of shiſts skated, amount of time skated within shiſts, and skating speed improved with the higher carbohydrate intake.28


Glycogen depletion may seriously limit


players’ ability to maintain high-intensity work output, distance covered, and sprinting frequency, particularly during the later stages of the game. Te glycogen utilization in leg muscle is markedly increased during bouts of intense exercise. During periods of rest and low-intensity play, glycogen is resynthesized.120 In two studies, researchers found low levels of muscle glycogen at the end of a soccer match and report- ed higher glycogen use in the first half compared with the second half.69,121 was only 50% of the prematch values 2 days aſter the match.69


Te difference in muscle glycogen measured


before and aſter a match represents the net glycogen utilization and does not fully reflect total glycogen turnover during play.11,49


Toward the end of a match, players experienced fatigue and a decrease in perfor-


mance. Although an exact mechanism for this phenomenon has not been identified, low levels of muscle glycogen may be a factor. Saltin observed that players with low levels of glycogen covered 24% less distance, 50% of which was covered by walking.121


the intensity of that exercise is typically less than 50% of capacity.67,122


Without sufficient muscle glycogen, exercise is fueled by fat, and Krustrup and colleagues measured


a 150 to 350 mmol/kg dry weight drop in muscle glycogen (42% ± 6%) as a result of repeated sprinting: Postgame measures were 225 ± 22 mmol/kg dry weight compared with 449 mmol/kg dry weight) at rest. Sprint performance was reduced both temporarily aſter intense exercise and at the end of the match. Half of the glycogen in individual muscle fibers of both types were either depleted or almost depleted, possi- bly preventing maximal sprint effort.76


consequence in intermittent sports where endogenous stores of glycogen are insufficient to meet energy demands for the full duration of play, during overtime, or during tournament competition. It is, therefore, important that athletes begin exercise with full glycogen stores and replenish glycogen stores as soon as possible aſter glycogen-depleting exercise. In the absence of severe muscle damage, an adequate carbohy- drate intake can restore muscle glycogen stores within 24 hours of reduced exercise.123


BLOOD GLUCOSE


Te role of liver glycogen is to regulate blood glucose to meet the needs of the whole organism, including the central nervous system. Te stored glycogen in skeletal muscle provides a major portion of carbohy- drate used during a soccer match, but blood glucose may also be utilized by exercising muscles and may, in fact, spare muscle glycogen.11,124


Muscle contraction causes an increase in the uptake of glucose from the


blood. As exercise duration increases, the carbohydrate from muscle glycogen decreases while that from blood glucose increases.67


Blood glucose concentrations increase during HIIE, as long as there are sufficient


liver glycogen and glucose precursors.9,11,124 Hypoglycemia and depletion of glycogen are associated with fatigue and reduced performance. Ferrau-


ti and colleagues assessed the incidence of hypoglycemia in tennis players during repeated tournament and practice matches by examining the change in glucose concentrations over the test period.125


Te results indi-


cate that glucose homeostasis is disrupted several times during tennis tournaments, leading to a decrease in glucose concentration and an increased risk of hypoglycemia, especially aſter a long match with only short breaks. In the absence of carbohydrate supplementation during extended play, blood glucose can decrease. To reduce the risk of hypoglycemia, a continuous intake of carbohydrate with a low glycemic index is recom- mended beginning with the third set through the end of the match and during changeovers.


CHAPTER 21: NUTRITION FOR HIGH-INTENSITY, INTERMITTENT SPORTS 475


Muscle glycogen concentration


Te effect of low muscle glycogen is more likely to have greater


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