These five factors interact to provide a balance of fuels to supply energy for working muscles. Although the most extensive research on fuel use during exercise has been done on endurance athletes (distance runners, cross country skiers, cyclists, and swimmers), data is also available for athletes in strength and power sports such as ice hockey and for resistance exercise (weight training).
Of all the factors, exercise intensity matters the most in determining which fuel is used for muscle contraction. As the table below shows, carbohydrate is the preferred fuel for high intensity exercise (performed at over 70% of aerobic capacity or VO2max). Link to Exercise Physiology: VO2max.
In this graph, the bread slices represent carbohydrate stores (glucose and glycogen) and the butter pats represent fat stores (fatty acids and triglycerides). At rest and at low exercise intensities (expressed as %VO2max), fat is the predominant fuel. During moderate intensity exercise (you feel the effort but can carry on a conversation while you are moving) fat and carbohydrate contribute equally to fuel muscle contraction. However, as exercise intensity increases to moderately hard or hard (talking while moving is difficult or impossible), a point is reached (ranging from 65-75% VO2max) where carbohydrate becomes the predominant, and then the exclusive, fuel for working muscles. Why? Look below for the answers.
Remember the four major fuels for exercise: muscle glycogen, plasma glucose, muscle triglyceride, and plasma fatty acids. This graph shows how the use of these fuels changes as energy intensity increases. Subjects for this study (Romijn, 1993) were endurance-trained men who had fasted overnight. These data were derived after the individuals exercised for thirty minutes at the given intensities. Note the following changes in substrate contribution to the total energy supply as energy intensity increases from 25% to 65% to 85% of VO2max: