You probably remember that first run you attempted after a long layoff from exercise. Your breathing rate skyrocketed and your legs felt leaden after just 10 minutes of running. However, after several weeks of consistent running you can maintain this pace for 30 minutes pretty comfortably, and your legs feel strong. Alas, these leg muscles have undergone physiological changes to adapt to this endurance exercise.
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Changes in Muscle Fiber Type
Skeletal muscles are composed of Type I, Type IIa, and Type IIb fibers. The respective classifications refer to the speed with which they can contract and their aerobic endurance capacity -- a Type I fiber contracts slowly and has the greatest endurance, whereas Type IIb fibers contract rapidly and have the lowest endurance capacity. Type IIa fibers contract rapidly as well, but they have a high aerobic endurance capacity. Endurance training increases the aerobic capacity of these Type IIa and IIb fibers in particular, resulting in more fibers with fast-contracting, fatigue-resistant properties and thus enabling you to run longer distances.
Muscle Blood Supply
During endurance exercise, your muscles need a greater supply of oxygen than they do at rest. Therefore, they have a large network of capillaries that supply oxygen-rich blood to the muscle. The oxygen diffuses across the capillary into the muscle fiber, where it supports sustained energy production. Endurance training increases the number of capillaries per area of muscle, thus increasing oxygen supply to the muscle. Oxygen supply to the muscles is critical for maintaining endurance as muscles fatigue very rapidly without sufficient oxygen supply.
Your muscles primarily rely on the breakdown products of carbohydrates -- stored as glycogen, and fats -- stored as triglycerides, for fuel during exercise. Carbohydrates are the most efficient source of energy, and their usage proportionally increases with increased exercise intensity. However, your body has a very limited supply of carbs as compared to fat -- about 2,500 calories worth for carbohydrates versus 74,500 calories worth of stored fat. Therefore, it is advantageous to spare muscle glycogen usage as much as possible in the early stages of endurance exercise. Glycogen depletion is a major factor in the onset of fatigue, particularly in endurance exercise lasting longer than one hour. Endurance training enables your body to use proportionally more fat at a given exercise intensity, sparing the prized muscle glycogen and allowing you to exercise longer.
Whether your muscle uses carbohydrates or fats for energy, it must be able to convert these energy sources into usable cell energy, or ATP. Your mitochondria are energy powerhouses of the muscle cell -- they use oxygen and the activity of several enzymes to produce the majority of ATP that the muscle cell needs to fuel endurance exercise. Endurance exercise increases the amount of mitochondria per area of muscle, increasing the ATP-producing capacity. In addition, endurance training increases the number of enzymes in the mitochondria, which speed up this nutrient breakdown and energy formation.
Myoglobin is a special protein in your muscles that binds the oxygen that enters the muscle fiber. When oxygen becomes limited during exercise, myoglobin releases the oxygen to the mitochondria. Although scientists do not know the degree to which myoglobin content contributes to the muscle’s oxidative capacity, endurance exercise training increases myoglobin content, likely increasing the oxygen reserve in the muscle.