Muscle Glycogen Metabolism and High‑Intensity Exercise Performance: A Narrative Review

Muscle glycogen is the main substrate during high-intensity exercise and large reductions can occur after relatively short
durations. Moreover, muscle glycogen is stored heterogeneously and similarly displays a heterogeneous and fiber-type specific
depletion pattern with utilization in both fast- and slow-twitch fibers during high-intensity exercise, with a higher degradation
rate in the former. Thus, depletion of individual fast- and slow-twitch fibers has been demonstrated despite muscle glycogen
at the whole-muscle level only being moderately lowered. In addition, muscle glycogen is stored in specific subcellular
compartments, which have been demonstrated to be important for muscle function and should be considered as well as
global muscle glycogen availability. In the present review, we discuss the importance of glycogen metabolism for single and
intermittent bouts of high-intensity exercise and outline possible underlying mechanisms for a relationship between muscle
glycogen and fatigue during these types of exercise. Traditionally this relationship has been attributed to a decreased ATP
resynthesis rate due to inadequate substrate availability at the whole-muscle level, but emerging evidence points to a direct
coupling between muscle glycogen and steps in the excitation–contraction coupling including altered muscle excitability
and calcium kinetics.