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Instructor note: A good instructor resource for the material presented in this lecture is provided in the Lesson-1 folder (Brown SP. Exercise Physiology. In: Brown SP, ed. Introduction to Exercise Science. Baltimore: Lippincott Williams & Wilkins; 2001. ).
Exercise physiology is the study of the bodys response and adaptation to exercise.
We will begin by discussing the major body systems that adapt to exercise. There are many more adaptations to exercise than are listed here. These are the most important in terms of physical performance.
The purpose of the respiratory system is gas exchange. Specifically, oxygen diffuses across the alveoli and is loaded on hemoglobin molecules in the red blood cells of the capillaries surrounding the lung. Oxygen moves in a diffusion gradient. The concentration in the lung is greater than the concentration in the capillary circulation. At sea level, blood leaving the lung is 98% (or more) saturated with oxygen. Even in conditions of very hard exercise, blood oxygen saturation does not typically drop (with the exception of a few highly elite athletes, where a very small drop can occur). This means that delivery of oxygen to the bloodstream by the lungs is not a limiting factor for exercise performance. The other gas that is exchanged in the lung is CO2 (carbon dioxide). The body produces carbon dioxide as a by product of producing energy from carbohydrates, fat and protein. Blood that arrives at the lung has a high concentration of CO2 due to these metabolic processes (even more during exercise), so CO2 flows from the bloodstream to the lungs (the opposite of the direction of travel of oxygen). Carbon dioxide is offloaded and oxygen is on loaded at the lung/capillary interface. Strength training has little impact on lung function. Endurance/aerobic training can increase the efficiency and fatigue resistance of the diaphragm (muslce that inflates the lungs) and the intercostals and abdominal muscles that are active during very heavy breathing. Endurance/aerobic training can also increase maximal ventilation rate. Reference: Essentials of Exercise Physiology, Second Edition, McArdle, W.D., Katch, F.L., and Katch, V.L., Chapter 10, Page 229-260, Chapter 14, Page 373-374 Lippincott Williams & Wilkins, 2000.
The cardiovascular system consists of the heart, blood and circulatory system (arteries, arterioles, capillaries, venules and veins). The purpose of the cardiovascular system is to deliver blood to all parts of the body. Blood carries oxygen, nutrients, hormones and other substances necessary for body function. The heart has a right side and a left side with separate pumps. The right side of the heart receives blood from the body and pumps blood to the lungs. The left side of the heart receives blood from the lungs and pumps it out to the rest of the body. Because the left side does more work, it is comprised of a larger chamber and more muscular walls. Strength training can produce some changes in the heart. The heavy straining and increased pressure in the thoracic cavity during heavy lifts can produce a large back pressure for the heart to work against. This can result in a thickening or hypertrophy of the left ventricular muscular wall. Total stroke volume (blood pumped with each beat of the heart) can increase slightly as well. In the muscle tissue, capillary density (how much capillary area per square centimeter) can decrease as the muscle grows larger. This is thought to reduce the aerobic capacity of the muscle by reducing the capacity to deliver oxygen rich blood to all parts of the muscle rapidly. However, the impact of this adaptation is small and probably unimportant. Endurance or aerobic training can produce substantial changes in the cardiovascular system. The left ventrical (main pump) gets larger and stronger, pumping more blood per beat (increased stroke volume). Because maximum heart rate is fixed, this adaptation allows an athlete to pump more blood at the highest work levels. It is also partially responsible for a lower resting heart rate (more blood pumped per beat = fewer beats per minute necessary). A greater stroke volume results in a lower heart rate at any given work rate. For example, after completing a training program a runner will have a lower heart rate at the same running speed, compared to before completing the training program. Blood volume and capillary density, especially in the muscles, is increased, resulting in more muscle blood flow. Together these adaptations result in a greater capacity to deliver blood to working muscles. Blood pressure can also decrease with endurance or aerobic training. There is some evidence to suggest that the decline in maximum heart rate with age can be reduced with regular endurance exercise. The ability to deliver sufficient oxygenated blood to working muscles is a primary determinant of maximum aerobic capacity, often referred to as VO2 max (maximum rate of oxygen consumption). A high VO2 max is associated with a high potential for aerobic/endurance sports. The aforementioned adaptations can increase aerobic capacity and VO2 max. Reference: Essentials of Exercise Physiology, Second Edition, McArdle, W.D., Katch, F.L., and Katch, V.L., Chapter 11, Page 263-294, Chapter 14, Page 370-373, Chapter 15, Page 415, Lippincott Williams & Wilkins, 2000.
- The signal to initiate muscle contraction comes from a motor nerve. Typically, a motor nerve controls several muscle cells (from just a few, to over 100). A motor nerve and the muscle cells that it controls are together called a motor unit. All of the muscle cells within a motor unit share similar characteristics. We classify muscle fiber types according to these characteristics. There are 3 basic muscle fiber types: fast twitch large, high force, powerful fibers that fatigue rapidly, store lots of glycogen and respond well to strength training; slow twitch smaller fatigue resistant fibers that have a high aerobic capacity, are very fatigue resistant, respond well to aerobic/endurance training but do not respond well to strength training. Intermediate fibers have characteristics that are in between fast and slow twitch. When the motor nerve sends a signal, all of the muscle cells in that motor unit will contract. Muscles contain many motor units. The amount of force produced by a muscle is controlled primarily by turning on more or fewer motor units. This is done in a specific order and follows Hennimans Size Principle of Muscle Fiber recruitment. At low force levels, small, slow twitch motor units are recruited. As the force level increases, intermediate motor units are recruited. At high force levels, large, fast twitch motor units are recruited. Large initial forces are required to produce explosive/fast movements, so large, fast twitch fibers are recruited to produce very rapid movements, even with lighter loads. Novice weight lifters are unable to recruit all available motor units. Advanced lifters can recruit 95% or more of the available motor units. Strength training results in an increase in the size of muscle cells, especially fast twitch muscle. Through repeated training, fast twitch muscle cells begin to take on the characteristics of intermediate muscle fibers. This improves their fatigue resistance, while retaining the high force and hypertrophy potential of fast twitch fibers. The ability to recruit a higher % of available motor units is one of the first adaptations to a strength training program. This results in increased strength, even before any hypertrophy occurs. Endurance training can cause the aforementioned fast twitch to intermediate muscle fiber conversions. This type of training can also cause intermediate fibers to take on some of the characteristics of slow twitch fibers, particularly their ability for high levels of aerobic metabolism and fatigue resistance. High volume endurance training may result in decreased muscle size. Long endurance sessions (
The purpose of the metabolic systems is to provide energy for all body processes, including movement. There are 3 basic metabolic pathways or systems. The phosphagen system is a short duration system. It can produce a very large amount of energy, but only for a short duration. The phosphagen system can fuel activities up to about 10 seconds in duration. After approximately 10 seconds, phosphagen energy stores start to decline and sustainable power output declines as well. If you did an all out sprint, you would begin to slow significantly after 10 seconds due to depletion of the phosphagen system energy stores. For longer duration activities that do not exceed 2 minutes, the primary energy system used is the glycolytic system. This is an anaerobic (without oxygen) system that is also high capacity (but not as high as the phosphagen system), but glycolytic system energy production starts to decline rapidly after one minute. The glycolytic system produces lactate (lactic acid) as a by product. For events that last longer than 2 minutes, the aerobic energy system provides most of the energy. This system can provide energy indefinitely and is the primary system providing energy to your body at rest and during lower intensity exercise. The aerobic/oxidative system has a large capacity but a much lower energy production rate than the other two systems (lasts a long time but does not provide as much short term energy). A 40 yard sprint would rely mainly on the phosphagen system. A 440 yard run would rely mainly on the glycolytic system. A 10K run would rely mainly on the aerobic/oxidative system. Reference: Essentials of Exercise Physiology, Second Edition, McArdle, W.D., Katch, F.L., and Katch, V.L., Chapter 5, Page 125-140, Lippinc