Aerobic training places extreme demands on muscle glycogen and intra-muscular fat. Training adaptations to these demands make energy production more efficient and reduce our risk of fatigue. Adaptations to repeated mechanical loading on muscle fibers stimulates changes in fiber structure and function. Adaptations include changes in muscle fiber type, capillary supply, myoglobin content, mitochondrial function and oxidative enzymes. Through these adaptations, we will discuss how a trained body metabolizes both carbohydrate and fat for energy.
Muscle glycogen is used extensively during training sessions. With adequate rest and dietary carbohydrate consumption, trained muscles will store more glycogen than untrained muscles. In addition to its greater glycogen content, endurance trained muscles contains substantially more fat than untrained fibers do. The vacuoles that contain triglyceride are distributed throughout the muscle fiber and are generally close to the mitochondria, making access to them for fuel during exercise easy.
In addition, activities of many muscle enzymes responsible for the beta-oxidation of lipids increase with endurance training. This adaptation is beneficial because it allows endurance-trained muscle to burn lipids more efficiently, in turn reducing the demand placed on a muscle's glycogen supply. Aerobic training also increases the rate at which free fatty acids are released from storage during prolonged exercise (since muscles can only use fatty acids for energy, triglyceride must be broken down into a glycerol group and three fatty acids).
This elevation of FFA enables muscle to burn more fat and fewer carbohydrates. Endurance trained muscles capacity to use fat is caused by the enhanced ability to mobilize FFA and the improved capacity to oxidize fat.
Since carbohydrates are the preferred energy source for high intensity work, and aerobic training results in a shift toward greater fat at a given sub-maximal exercise intensity, it is logical to deduct that the intensity (%VO2max) of exercise is the determinant in the balancing of each substrates use. At an exercise level below 45% of maximal oxygen uptake, lipid is the main substrate used for energy. For more intense levels of exercise, greater than 70% of maximal oxygen uptake carbohydrate is the predominant substrate of choice. While training time increases above 90 minutes and the stimulation from the sympathetic nervous system declines, the need for carbohydrates as the primary energy source decreases while the use of fat increases.
Why Do I Need Intense Aerobic Training?
These improvements in muscles' aerobic energy system result in a greater capacity to produce energy, with a shift toward greater reliance on fat for ATP production. Since intense aerobic training places extreme demands on muscle glycogen and intra-muscular fat, energy production becomes more efficient.
Through adaptations in muscle fiber type, capillary supply, myoglobin content, mitochondrial function and oxidative enzymes, individuals can enhance the metabolic conditions in their bodies at rest. Through these adaptations, the average body, which uses 40% of energy at rest from fat, can be aerobically conditioned to utilize 70% of its energy at rest from fat and only 30% from carbohydrates. These adaptations lead to a more permanent and efficient fat loss.
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