Physiology of Exercise

The physiology of exercise is a broad concept that addresses the central issue as to how the body adapts itself to the demands of physical activity.

Physiology is the academic study of the various processes, systems, and functions of the human body as influenced by the performance of physical activity. Exercise is a term that has a variety of possible meanings, each dictated by circumstances. In a sports context, exercise is the performance, conditioning or training undertaken in respect to a particular athletic or sporting purpose. Exercise may also be directed to improvement of a person's general health, physical fitness, or as physical therapy, to augment an existing treatment to remedy or to ameliorate the effects of a disease or illness upon the body.

The term exercise physiology is used to identify the corresponding course of academic study offered at universities around the world.

The human body undergoes adaptations on a continuous basis. Sport tends to heighten the power of the body to adapt to training, competition, or other circumstances, as sport is often the most profound stress experienced by the body. As an example, when the body is subjected to a fever, where the subject's temperature becomes significantly elevated above its usual range centered at 98.6°F (37.7°C), the body's metabolism (the overall rate of activity in the body's processes) increases; the running of a marathon may increase metabolic rates to many time their normal level.

Virtually every process and organ within the body is affected by exercise. As an example, the skin, the largest human organ, undergoes physical changes when exposed to the environmental factors encountered in sport, such as increases and decreases in external temperatures. The physiology of exercise is tends to center upon the most important physical systems to athletic performance: the cardiovascular system, the cardiorespiratory system, the thermoregulatory system, body composition and the musculoskeletal system. It is these aspects of human function that tend to have the greatest impact upon the ability of an athlete to maintain or improve their level of performance in any sport.

The cardiovascular system is the physical network composed of the heart and its connected arteries, veins, and capillaries. The cardiovascular system is the vehicle through which the oxygen and fuels required by the cells within the body are supplied; the cardiovascular system removes all waste products from the cells and organs for disposal. When the body is subjected to exercise, and its increased physical demands, the cardiovascular system is forced to work more quickly and more efficiently to fulfill bodily needs. A number of physiological changes occur over time to this system through exercise.

The first and the most fundamental change to the cardiovascular system is with respect to the function of the heart. The cardiac muscle of the heart will grow stronger over time, as the heart becomes adapted to working harder during exercise. A stronger and more efficient heart reduces the resting pulse of the subject; as the heart strengthens, it does not have to beat as frequently as when at rest to achieve the same effect in the pumping of blood through the cardiovascular system. The greater flow of blood available to a person who regularly exercises tends to reduce the amount of low-density lipoproteins within the blood vessels that can form a harmful blockage known as plaque, a condition that tends to narrow the passage within each artery. Exercise does not make the arteries larger, but these vessels become more elastic through exercise permitting a greater and more beneficial blood flow through out the body.

The second important physiological change experienced by the cardiovascular system due to exercise is the reduction of blood pressure. Blood pressure is defined as the force of blood being pushed against the walls of the arteries of the cardiovascular system. High blood pressure has two components; systolic pressure is that measured during a heartbeat, and diastolic pressure is that present between heartbeats. Blood pressure is measured as the relationship of systolic to diastolic levels. High blood pressure, expressed as a measurement greater than 140/90 mmHg (millimeters of mercury, a unit of atmospheric pressure), is a condition where the heart is forced to work harder than it was designed in order to direct blood through the entire system. High blood pressure raises the risk for heart attack and stroke. Subject to other genetic factors or environmental impacts such as smoking, exercise will tend to reduce blood pressure. Athletes almost always possess a blood pressure reading significantly lower than that normally found in the regular population.

The most profound impact of exercise upon the cardiorespiratory system also affects the function of the cardiovascular system. The maximum volume of oxygen that an athlete can consume during exercise is known by the expression VO2max. Particularly in the endurance sports, where the athlete is fueling their body by way of the aerobic energy system, endurance training will increase the athlete's VO2. The ability of athletes to increase their maximum oxygen capacity is universal; female athletes will generally possess a VO2max ranging between 60% and 75% of that of a similarly conditioned male, due to the greater muscle mass present in a male athlete which must be serviced through the delivery of oxygen to the energy producing cell. Female athletes are as strong as a male counterpart when muscle strength is measured per unit, as per cubic inch of muscle (cm3).

Exercise improves the ability of the cardiorespiratory system to take oxygen from air inhaled into the lungs, and then load and transport it more efficiently. Greater efficiency in the movement of blood through the cardiovascular system permits greater amounts of oxygen to be transferred from the respiratory system; lung size does not increase due to exercise by any appreciable degree.

Thermoregulation is the ability of the body to maintain the optimal internal temperature levels for the function of all organs in different external environment conditions. Where the athlete is unaccustomed to warm weather exercise, the body will adapt through the process of acclimatization to the new conditions. Within a period of approximately 14 days, the positive physiological changes typically noted through heat acclimatization include expanded blood volume (corresponding greater blood capacity), reduced heart rate (making the heart more efficient), increased direction of blood to the skin surface and the capillaries (greater cooling effect on blood through directing the blood to the cooler skin surface), and an increased conservation of sodium to promote more effective hydration (to preserve the optimal proportion of sodium to water, a part of the body's osmoregulatory system).

Body composition is the most visible of the physiological changes often observed to have occurred through exercise. The body is constructed from body fat, lean muscle mass, and the organs and skeletal bone, the dimensions of which are not altered through exercise. Body composition is affected by two distinct exercise mechanisms—through a reduction in the percentage of body fat in a subject, and through the increase of lean muscle mass developed through specialized exercise. Body fat is the storage form of the triglycerides that are processed by the body from the fats consumed through diet. These fats may be stored for indefinite periods in the adipose tissues located in the region of the abdomen, pelvis, buttocks, and chest. Exercise, when combined with proper attention to diet, will result in a weight loss in any subject where the amount of caloric energy required to fulfill the body's needs, including exercise, exceeds the amount of caloric energy sources ingested as food. One pound of body fat (0.4 kg) represents approximately 3,500 available calories of energy.

Muscle development through exercise programs often occurs in conjunction with the reduction of body fat that results from the difference between energy intake and output. In the early stages of weight reduction where the subject is participating in muscle building resistance exercise, it is common for the subject to experience frustration in terms of their desired weight loss, as the body fat that is available to the body as fuel is countered by the gain of denser muscle tissue.

The musculoskeletal system undergoes a multitude of physiological changes, in addition to the additional muscle produced through particular types of training. Stretching and flexibility exercises tend to create a greater range of motion in all of the joints that are subjected to these stresses. Where the joints of the body are able to move more dynamically, the related structure will generally be capable of both faster, more powerful and more stable movement. A joint with an improved range of motion is less likely to become overstressed and injured.

The bones of the musculoskeletal system also undergo structural changes that result from exercise. Resistance, either through weight training, or in activities that require running or other forces to be directed into the body, generally tends to increase bone density.

In addition to creating greater muscle mass, exercise will have an effect upon existing muscle structures. All humans possess specific kinds of muscle fibers, each of which is distributed relatively evenly throughout the muscles of the body according to the genetic makeup of the individual. The two general muscle fiber types are fast twitch and slow twitch fibers. The designation between fast and slow is determined by the frequency with which the neuron that governs the impulses that control the contraction of the particular fiber. Fast twitch neurons fire at a rate of approximately 10 times greater frequency than does a slow twitch neuron. The effective function of fast twitch fibers is essential to anaerobic sports such as sprinting and jumping. Specialized exercise, such as plyometric programs, can enhance the performance of fast twitch fiber. The proportion of slow twitch muscle fibers, the backbone to the muscle function in endurance sports such as marathon running and cycling, will increase in proportion to fast twitch fibers when the athlete undergoes vigorous endurance training.

SEE ALSO Endurance exercise; Fitness; Metabolic response; Sport performance.