GLOSSARY

Gross Efficiency usually relates external work to total metabolic energy expenditure during the task. It differs from net efficiency, which handles baseline expenditure differently.

Exercise economy describes oxygen or energy cost at a specified task output, while mechanical efficiency relates external work to metabolic energy input under a stated convention. Gross, net and other efficiency definitions differ in how baseline costs are handled. A lower oxygen cost is not automatically the same as higher maximal aerobic capacity. Compare the same workload, conditions and calculation method, and distinguish a measured physiological change from a demonstrated improvement in race performance.

Exercise uses a mixture of fuels whose proportions change with intensity, duration, training and diet. The crossover concept describes a shift toward greater carbohydrate contribution as intensity rises under specified conditions. It is a model of relative fuel use, not an on-off switch that turns fat metabolism off. The crossover point is not fixed across all individuals or protocols. Interpret substrate patterns with the actual workload and available fuel rather than a universal heart-rate zone.

A sustained low-carbohydrate, high-fat diet can increase reliance on fat and change fuel handling. Nutritional ketosis is a related but more specific state and is not produced by every higher-fat diet. Increased fat oxidation does not establish better performance, particularly when high rates of carbohydrate-supported work are required. Some athlete studies have found impaired exercise economy or high-intensity performance. Restoring carbohydrate after fat adaptation does not guarantee immediate reversal of every effect or the advantages of both approaches.