心臟·肺和許多血管構成了一個連續循環，從而將血液輸送到全身，最終構成了心肺系統。血液由血漿(55%~65%)·白細胞和血小板 (約1%) 以及紅細胞組成 (38%~45%)。紅細胞負責把全身的氧氣輸送道各個運動組織。人體內有許多紅細胞，每個紅細胞內大約有2.5億個血紅蛋分子。血紅蛋白是輸送氧氣的蛋白質，因此它是向人體組織提供富氧血液的關鍵。通過補充水分和電解質，促進紅細胞和血紅蛋白的生成來增加血液量，是運動員適應耐力訓練的關鍵。
The heart, lungs, and many blood vessels of the body form a continuous circuit that transports blood throughout the body and makes up the cardiorespiratory system. Blood consists of plasma (approximately 55 to 65 percent), leukocytes and platelets (approximately 1 percent), and red blood cells (approximately 38 to 45 percent). Red blood cells (RBCs) are the key component responsible for transporting oxygen throughout the body to working tissues. There are many RBCs in the human body, and within each RBC are approximately 250 million molecules of hemoglobin. Hemoglobin (Hb) is the protein that transports oxygen; thus it is key in providing oxygen-enriched blood to the body’s tissues. An increase in blood volume through increased water, electrolyte, RBC, and hemoglobin production is a key adaptation an athlete receives from endurance training.
The lungs and other organs involved in breathing are responsible for providing oxygen-rich air and removing carbon dioxide through gas exchange. For each breath, a person takes, oxygen is supplied and carbon dioxide is removed. With training, athletes adapt the muscles associated with the lungs so that breathing rate becomes more efficient. A larger amount of air is exchanged with each breath.
The response of the cardiorespiratory system to exercise training is characterized by the measures of cardiac output (CO), maximal oxygen-carrying capacity (O2max), and the fractional percentage of maximal oxygen consumption (% O2max) needed to perform a set workload. Cardiac output is the amount of blood pumped by the heart over a 1-minute period. It can be defined as heart rate (HR) multiplied by stroke volume (SV), where HR refers to the frequency the heart contracts and SV is the volume of blood ejected from the heart with each contraction. O2max is the maximal amount of oxygen the body can consume. It is found by multiplying CO by the (a-)O2 difference, which is the average difference between the oxygen content of the arterial and mixed venous blood. Oxygenated blood is supplied to the working muscles via the arterial blood vessels, and oxygen remaining in the blood once it has passed through the body is returned to the lungs via venous blood vessels.
Effects of Training on the Muscular and Cardiorespiratory Systems
Adaptations to training require a repeated stimulus for approximately 2 to 4 weeks before the body fully takes on the stress that has been applied to the muscular and cardiorespiratory systems. Training adaptations for triathlon involve both local muscular endurance and whole-body cardiorespiratory endurance; however, it is the training performed by the muscular system that stimulates and drives adaptations of the heart and lungs. As a result, it is important that the effects of training be understood first and foremost in regard to the muscular system.
Training and the Muscular System
根據有氧能量系統和無氧能量系統，訓練可以劃分為無氧耐力訓練和有氧耐力訓練。有氧耐力訓練包括強度等於或低於無氧閾 (血乳酸未明顯提高的情況下，運動員在穩定的運動狀態中所能達到的最高強度) 的適應訓練。運動員只有把訓練強度維持在這個水平，才能保證自己可以在與實際比賽時間相當或更長的一段時間裡持續運動。結構性適應可以提高氧氣的使用效率，而有氧耐力訓練的目的就是通過提高和改善結構性適應來提高肌肉的運動效率。通過增加慢氧化肌肉纖維的能力，並把快縮糖酵解型肌纖維轉化成快縮氧化糖酵解型肌纖維，運動員可以提高肌肉使用氧氣的效率。
Training can be classified based on the aerobic and anaerobic energy systems. Aerobic endurance training encompasses those adaptations that result from training at intensities at or below the anaerobic threshold (defined as the highest intensity a steady state of exercise can be maintained without significant rises in blood lactate). The intensity of training is intended to be only so high as to ensure it can be sustained for a period similar to or greater than the actual competition duration. The goal of aerobic endurance training is to help the muscles perform more efficiently through increasing and improving structural adaptations that promote the use of oxygen. This is accomplished by increasing the capacity of SO muscle fibers and potentially by converting FG fibers to FOG fibers, which improves their use of oxygen.
有氧耐力訓練可以產生4種關鍵的結構性適應: (1) 向肌纖維供應氧氣的毛細血管數目增加; (2) 肌血球速含量增加; (3) 骨骼肌線粒體數目和大小增加; (4) 氧化酶濃度增加。嵌在骨骼肌深處的毛細血管是非常小的血管。它們能直接輸送氧氣和營養物質 (如碳水化合物·電解質)，也能排出二氧化碳和代謝副產物 (如乳酸和氫離子)。肌肉周圍毛細血管數量的增加提高了氧氣的輸送效率。肌血球素是肌肉的血紅蛋白。有氧訓練可以提高肌血球素，從而提高肌肉使用氧氣的能力。肌血球素會把血紅蛋白中氧氣輸送道最需要氧氣的肌肉區域。這個過程主要涉及線粒體中的氧化方式。線粒體被認為是肌肉細胞的發電廠，線粒體通過氧化代謝合成ATP。氧化酶進一步增加了氧化代謝的進程，最終，身體可以在運動中額外使用脂肪作為能量來源。這個過程增加了有氧代謝所產生的能量，也省下了肌肉糖原，這兩點對於運動員在耐力賽事中保持運動表現至關重要。
Four key structural adaptations can occur as a result of aerobic endurance training: (1) an increase in the number of capillaries supplying the muscle fibers, (2) an increase in muscle myoglobin content, (3) an increase in the number and size of the mitochondria in skeletal muscle, and (4) an increase in concentrations of oxidative enzymes. Capillaries are very small blood vessels that are embedded deep within the skeletal muscle. They serve as the direct transporter of oxygen and nutrients (e.g., carbohydrate, electrolytes) and also remove carbon dioxide and metabolic by-products such as lactate and hydrogen ions. An increase in the number of capillaries that surround the muscle promotes oxygen delivery. Myoglobin is the muscle’s equivalent of hemoglobin. The increase in myoglobin that occurs as a result of aerobic training improves the ability of the muscle to utilize oxygen. Myoglobin accepts the oxygen from hemoglobin and transports it to areas of the muscle that need it most. This primarily entails the oxidative pathways that exist in the mitochondria. Mitochondria are considered the powerhouse of the muscle cell. They utilize the oxygen that is delivered to create ATP through the oxidative metabolic pathways. These pathways are further enhanced by the increase in oxidative enzymes, and as a result, the body is able to increase the utilization of fat as a fuel source during exercise. This boosts the amount of energy derived through aerobic metabolism and also spares muscle glycogen, both of which are critical for sustaining performance in endurance events.
Anaerobic interval training for endurance events increases the amount of energy that can be efficiently produced through anaerobic glycolysis and the ATP–CP energy systems. Interval training improves the buffering capacity of skeletal muscle and, if designed properly, improves maximal power, strength, and anaerobic capacity. This type of training frequently referred to as high-intensity interval (HIT) training, involves repeated exercise bouts of short to moderate duration (30 seconds to 5 minutes). The training intensities associated with HIT are above the anaerobic threshold and are predominantly based on critical power outputs and paces at and above what is sustained during competition. After a period of HIT, athletes can perform the same workload with lower levels of lactate and a decreased rating of perceived exertion. In addition, higher work intensities can be sustained for longer, and the athlete also tolerates greater levels of lactate while removing lactate from the muscles at a faster rate.
高強度間歇訓練之所以能夠提高運動能力是因為運動員達到了3個關鍵的適應。訓練提高了無氧酵解供能系統和ATP-CP供能系統的功能效率增加，進而促使身體分泌出更多與合成ATP有關的酶。這大大提高了人體利用和氧化 (通過有氧代謝以產生能量) 碳水化合物以作為能量來源的效率。高強度的間歇訓練使運動員更多地使用有氧代謝的方式來產生能量，從而減少在一定強度下血液中的乳酸含量; 這是碳水化合物通過線粒體的三羧酸循環近一步合成ATP的結果。
The improvements in work capacity reported with HIT are a result of three key adaptations. The first key adaptation is an increase in the enzymes associated with the production of ATP through anaerobic glycolysis and the ATP–CP energy system. This allows for an improved utilization and oxidation (generating energy through oxygen-based metabolism) of carbohydrates as a fuel source. High-intensity interval training increases the use of the oxidative energy pathways and decreases the amount of lactate that is spilled into the blood at a specific workload; this is a result of carbohydrates continuing through what is known as the Krebs cycle of the mitochondria to further generate ATP.
In addition to improving the oxidation of carbohydrates, the body is also capable of utilizing more carbohydrates as a result of HIT. High-intensity interval training increases the number of muscle fibers that can be recruited to do work and thus increases work capacity. As a result, higher levels of carbohydrate are utilized, and a greater amount of lactate is produced at the end of a maximal effort, thus indicating an increase in the capacity of the anaerobic energy system. The ability to tolerate high lactate levels also stimulates the development of bicarbonate. As discussed previously, bicarbonate soaks up hydrogen ions that are produced during the breakdown of carbohydrates through nonoxidative energy production. When bicarbonate levels are higher, more hydrogen ions can be removed, thereby allowing for the continuous formation of a greater number of muscle cross bridges and sustainment of more forceful muscle contractions, resulting in improved performance.
Another significant benefit that comes from HIT is a decrease in the core body temperature reached during exercise. As core body temperature—the amount of heat stored by the body—increases over time, work output will begin to decrease. At a set workload, the energy cost of submaximal exercise is substantially decreased after HIT where maximal power has been improved. As a result of the improved exercise economy, the body does not accumulate heat as rapidly and muscle function is not impaired as quickly, thus resulting in higher sustainable work output.
The development of neuromuscular patterns has also been suggested as one of the benefits of HIT. High-intensity interval training facilitates adaptations in the neuromuscular patterns that are recruited during race-pace activity. As was mentioned earlier, the brain has a region known as the motor cortex. Within this region of the brain, muscular patterns are stored along with the number of motor units required to perform them. During the competition, these patterns are called upon to facilitate performance, and those muscular patterns that have been utilized the most will predominate during this time of physical stress.
Resistance training is another means of improving performance through adaptations in the muscular system. There are three key goals with this type of training. The first is to improve muscular strength as defined by the maximum force that can be generated by a muscle or group of muscles. A second goal is to improve muscular power, which is the explosive aspect of strength, by performing a specific movement at a given speed. The third goal of resistance training for endurance athletes is to improve muscular endurance. This is defined as the ability to sustain repeated muscular contractions at a fixed workload for an extended period of time. Increasing the amount of force that can be produced at a given speed, and improving the ability to sustain that force over a distance, will enhance performance because the muscles will not be as susceptible to fatigue.
Training and the Cardiorespiratory System
心肺系統對運動的適應是骨骼肌適應運動的直接結果，而骨骼肌的適應反過來又刺激了心肺的適應。從最大攝氧能力和心輸出量的提高，我們可以看出訓練對心肺系統的影響。由於遺傳因素的影響，兩者最終都會趨於平穩; 然而，適應訓練的能力和心肺系統的經濟性仍可得到顯著改善。最大攝氧能力和心輸出量提高源自人體的3種關鍵性適應: (1) 總血量增加; (2) 心臟因為正在進行的運動，而變得更加強壯; (3) 人體向肌肉輸送氧氣的能力得以加強。由於這些訓練，每一次收縮時，心臟能更有效地把更多的血液泵入運動的肌肉中，氧氣才得以輸送，而二氧化碳和其他代謝副產物也得以高效排出。以下各段將對這些適應反應作進一步說明。
Adaptations in the cardiorespiratory system are a direct result of adaptations of the skeletal muscles to the work they are performing, and in turn, this stimulates the heart and lungs to adapt. The effects of training on the cardiorespiratory system are seen as increases in O2max and cardiac output. Because of genetic factors, both will eventually plateau; however, training adaptations and the economy of the cardiorespiratory system can still be significantly improved. Improvements in O2max and cardiac output are a function of three key adaptations occurring in the body: (1) Total blood volume is increased; (2) the heart becomes stronger as a result of the work it is doing; and (3) oxygen delivery to the muscles of the body is enhanced. As a result of these adaptations, the heart is capable of more efficiently pumping larger amounts of blood to the working muscles with every contraction, and in turn, oxygen is delivered and carbon dioxide, along with other by-products of metabolism, is removed more efficiently. These adaptations are described further in the following paragraphs.
在耐力訓練中，有兩個階段會導致總血量的增加。第一種情況是，受賀爾蒙刺激，人體在10天內的保水量增加; 第二種情況是，在4周左右的時間內，紅細胞的產量增加。總血量的提高有利於改善以下3種不同的運動機制: (1) 提高了調節體溫的能力，因為含水量的增加可以改善散熱的狀況，從而提高出汗的概率; (2) 心肌的工作效率和功能得到改善; (3) 紅細胞數量的增加加強了細胞的攜氧能力。
The increase in total blood volume that occurs with endurance training is the result of a two-phase process. In the first, hormones stimulate an increase in total body water retention over a 10-day period, and the second is an increased production of red blood cells over an approximately 4-week period. The improvement in total blood volume benefits athletes through three different mechanisms: (1) an improved ability to regulate body temperature, as the increased water content allows for improved heat dissipation and thus an increase in sweat rate; (2) an improved efficiency and functionality of the heart muscle; and (3) an increased oxygen-carrying capacity as a result of the increased number of red blood cells.
The heart is a muscle that responds to training much like skeletal muscle. A load can be imposed on the heart by increasing the number of times it must contract or the strength with which it contracts. With repeated contraction, the heart muscle becomes stronger and more efficient, and as a result, the heart does not have to contract as frequently to perform the same amount of work. In addition, prolonged engagement in aerobic long-distance training enhances stroke volume, with a resultant reduction in resting and exercising heart rate at a specific workload. Using the formula for cardiac output (stroke volume × heart rate), it can be understood how heart rate is reduced at submaximal exercise intensities as a result of an increase in stroke volume. During maximal exercise, the increase in blood volume causes an increase in maximal cardiac output, and as a result, an increase in O2max is seen. Another benefit of a stronger heart and a larger stroke volume is faster recovery after a hard or near-maximal bout of exercise.
The improvement in total blood volume and heart efficiency also results in improved delivery of oxygen to the working muscles. In addition, because there is an increase in red blood cell volume, the concentration of hemoglobin is increased. This raises the oxygen-carrying capacity of the blood, and the increase in blood volume improves the transit time for supplying oxygen to working muscles. Together, these lead to improved endurance performance.
Adaptations in the lungs also facilitate the improvements in O2max and cardiac output. With training, the lungs become more efficient and can increase the amount of oxygen supplied with each breath; as a result, ventilation is decreased. Ventilation is a function of tidal volume and the frequency of breathing (tidal volume × frequency). Tidal volume is the volume of air that is inspired or expired with each breath. The primary means for a decrease in ventilation is the increase in tidal volume, which allows for a lower frequency of breathing. The improved capacity of the lungs during exercise is an important factor in improving endurance performance.
From USA Triathlon.