Introduction
The importance of carbohydrates as a fuel source
during physical activity has been recognized for
many years (Krogh & Lindhard 1920; Levine et al.
1924; Dill et al. 1932). Krogh and Lindhard (1920)
reported that subjects placed on a high fat diet
complained of feeling tired and had difficulty
performing a standardized 2-h exercise protocol
on a cycle ergometer. However, 3 days on a high
carbohydrate diet relieved their symptoms of
tiredness and the subjects were able to complete
the 2-h exercise task without undue stress. Simi-
larly, Christensen and Hansen (1939a, 1939b)
found that the capacity for prolonged exercise
was three times greater after 3–7 days on a high
carbohydrate diet as opposed to a high fat-
protein diet. They also reported that exhaustion
was accompanied by hypoglycaemia, and that
ingestion of a carbohydrate supplement during
recovery rapidly returned the blood glucose con-
centration back to normal and allowed consider-
able additional exercise to be performed. These
results were in agreement with the earlier obser-
vations of Levine et al. (1924), who found that
blood glucose levels of runners fell to very low
levels during a marathon. They also noted that in
conjunction with this hypoglycaemic state, the
participants were physically fatigued and dis-
played neuroglucopenia symptoms such as
muscular twitching and disorientation. Based on
these observations, it was generally accepted
that hypoglycaemia resulting from liver glyco-
gen depletion was responsible for fatigue dur-
ing prolonged strenuous exercise. However, this
view would be modified as technical advances
allowed the direct investigation of muscle
metabolism during and following prolonged
strenuous exercise.
Based on the findings of several Scandinavian
research groups, it became apparent that there
is an increased reliance on muscle glycogen as a
fuel source as exercise intensity increases, and
that perception of fatigue during prolonged
strenuous exercise parallels the declining muscle
glycogen stores. It was also found that aerobic
endurance is directly related to the initial muscle
glycogen stores, and that strenuous exercise
could not be maintained once these stores are
depleted (Hermansen et al. 1965; Ahlborg et al.
1967a; Bergström et al. 1967; Hultman 1967).
The amount of glycogen stored in skeletal
muscle, however, is limited. If muscle glycogen
was the only fuel source available, it could be
completely depleted within 90 min of moderate
intensity exercise. Therefore, because of the
limited availability of muscle glycogen and its
importance during prolonged strenuous exer-
cise, methods for increasing its concentration
above normal prior to exercise and for its rapid
restoration after exercise have been extensively
investigated. This chapter discusses our current
understanding of the regulation of muscle glyco-
gen synthesis, the effect of diet and exercise on
the muscle glycogen concentration prior to exer-
cise, and methods for muscle glycogen restora-
tion immediately after exercise. The chapter
concludes with recommendations for increasing