Location location location: muscle glycogen content and endurance exercise.

The percutaneous muscle biopsy technique was developed in Sweden in the 1960s and spawned a decade of revelatory investigations on the interaction between diet, exercise and muscle glycogen. This culminated in the pioneering study that showed the importance of muscle glycogen content for endurance exercise following dietary carbohydrate manipulation (Bergström et al., 1967). These studies have laid the foundations for scientific work in this area, including the use of carbohydrate loading and periodization strategies to enhance muscle glycogen storage capacities. In particular, the muscle biopsy technique has allowed research to elucidate the relationships between muscle glycogen and exercise intensity and duration, individual training status, carbohydrate feeding and carbohydrate restriction (Hearris, et al., 2018). The muscle biopsy is considered a ‘gold standard’ when it comes to biochemical, molecular, histochemical, and histomorphometric muscle analyses. Traditionally, muscle glycogen content has been evaluated using whole-muscle homogenates from muscle biopsies. However, muscle is not a uniform tissue and is composed of multiple fibre types (type I, type IIa, type IIx) which have distinct glycogen contents while it has been known for some time that glycogen is not only differentially located between different muscle fibres but also within subcellular compartments. Whole-muscle quantification, therefore, might not be the most accurate method to elucidate the complex pathways through which muscle glycogen availability may influence exercise performance, and instead, single-fibre and subcellular localisation may further enhance understanding of this topic.