You probably remember that first race you tried to do later of a long period out of training. Your breathing range went up and your legs felt exhausted after the first 10 minutes. However, after several weeks of constant operation you were able to keep this rhythm for 30 minutes very comfortably and your legs feel strong. Unfortunately, the leg muscles have undergone physiological changes to adapt to this resistance exercise.
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 resistance (a Type I fiber contracts slowly and has greater resistance, while Type IIb contracts quickly and has the lowest resistance capacity). Type IIa contracts rapidly as well, but they have a high capacity for aerobic resistance. Resistance training increases the aerobic capacity of these fibers, resulting in more fibers with fast contracting, fatigue-resistant properties and this allows you to run longer distances.
Blood in the muscles
During endurance exercise, the muscles need a greater supply of oxygen. Therefore, they have a wide network of microscopic blood vessels (capillaries) that are the supply of oxygen-rich blood. Oxygen diffuses through the capillaries in the muscle fiber, where it encourages the production of energy. Resistance training increases the number of capillaries per muscle area, which increases the oxygen supply. The supply of oxygen to the muscles is essential for the maintenance of resistance, since the muscles fatigue very quickly without sufficient supply.
Your muscles are based primarily 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 use increases proportionally with the increase in exercise intensity. However, your body has a very limited supply of them, compared to fat (about 2,500 calories per carbohydrate value compared to 74,500 calories from stored fat). Therefore, it is advantageous to save the use of muscle glycogen as much as possible in the early stages of resistance exercise. Glycogen depletion is an important factor in the onset of fatigue, particularly in endurance exercise lasting more than an hour.Resistance training allows your body to use more fat proportionally to the intensity of the given exercise, saving precious muscle glycogen.
Whether your muscle uses carbohydrates or fats for energy, it must be able to convert these energy sources into usable cellular energy, or ATP. Mitochondria generate energy in the muscle cell and use oxygen and the activity of various enzymes to produce most of the ATP that the muscle cell needs to fuel endurance exercise. Resistance exercises increase the amount of mitochondria by muscle area, and increase the capacity of ATP production. In addition, resistance training increases the number of enzymes in the mitochondria, which accelerate the decomposition of nutrients and the formation of energy.
Myoglobin is a special protein in the muscles that binds oxygen that enters the muscle fiber. When oxygen is limited during exercise, myoglobin releases oxygen to the mitochondria. Although scientists do not know the degree of myoglobin content in which it contributes to the muscle's oxidative capacity, resistance training increases the myoglobin content, and probably increases the oxygen reserve in the muscle.