What is exercise physiology?

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The science that studies the effects on the body of various intensities and types of physical activity, including cellular metabolism, cardiovascular responses, respiratory responses, neural and hormonal adaptations, and muscular adaptations to exercise.
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Science and Profession

The primary aim of research in the field of exercise physiology is to gain a better understanding of the quantity and type of exercise needed for health maintenance and rehabilitation. A major goal of professionals in exercise physiology is to find ways to incorporate appropriate levels of physical activity into the lifestyles of all individuals.

Physiology is the science of the physical and chemical factors and processes involved in the functioning of living organisms. Exercise physiology examines these factors and processes as they relate to physical exertion. The physical responses that occur are specific to the intensity, duration, and type of exercise performed.

Exercise of low or moderate intensity relies on oxygen to release energy for work. This process is often referred to as aerobic exercise. In the muscles, carbohydrates and fats are broken down to produce adenosine triphosphate (ATP), the basic molecule used for energy. Aerobic exercise can be sustained for several minutes to several hours.

Higher-intensity exercise is predominantly fueled anaerobically (in the absence of oxygen) and can be sustained for up to two minutes only. Muscle glycogen is broken down without oxygen to produce ATP. Anaerobic metabolism is much less efficient at producing ATP than is aerobic metabolism.

During anaerobic metabolism, a by-product called lactic acid begins to accumulate in the blood as blood lactate. The point at which this accumulation begins is called the anaerobic threshold (AT), or the onset of blood lactate accumulation (OBLA). Blood lactate can cause muscle soreness and stiffness, but it also can be used as fuel during aerobic metabolism.

A third and less often used energy system is the creatine phosphate (ATP-CP) system. Using the very limited supply of ATP that is stored in the muscles, phosphate molecules are exchanged between ATP and CP to provide energy. This system provides only enough fuel for a few seconds of maximum effort.

The type of muscle fiber recruited to perform a specific type of exercise is also dependent on exercise intensity. Skeletal muscle is composed of “slow-twitch” and two types of “fast-twitch” muscle fibers. Slow-twitch fibers are more suited to using oxygen than are fast-twitch fibers, and they are recruited primarily for aerobic exercise. One type of fast-twitch fiber also functions during aerobic activity. The second type of fast-twitch fiber serves to facilitate anaerobic high-intensity exercise.

Exercise mode is another factor in people's physiological responses to exercise. Dynamic exercise (alternating muscular contraction and relaxation through a range of motion) using many large muscles requires more oxygen than does activity using smaller and fewer muscles. The greater the oxygen requirement of the physical activity, the greater the cardiorespiratory benefits.

Many bodily adaptations occur over a training period of six to eight weeks, and other benefits are gradually manifested over several months. The positive adaptations include reduced resting and working heart rates. As the heart becomes stronger, there is a subsequent increase in stroke volume (the volume of blood the heart pumps with each beat), which allows the heart to beat less frequently while maintaining the same cardiac output (the volume of blood pumped from the heart each minute). Another beneficial adaptation is increased metabolic efficiency. This is partially facilitated by an increase in the number of mitochondria (the organelles responsible for ATP production) in the muscle cells.

One of the most recognized representations of aerobic fitness is the maximum volume of oxygen (VO2max) an individual can use during exercise. VO2max is improved through habitual, relatively high-intensity aerobic activity. After three to six months of regular training, levels of high-density lipoproteins (HDLs) in the blood increase. HDL molecules remove cholesterol, a fatty substance, from the tissues to aid in protecting the heart from atherosclerosis.

Various internal and external factors influence the metabolic processes that take place during and after exercise. Internally, nutrition, degree of hydration, body composition, flexibility, sex, and age are some of the variables that play a role in the physiological responses. Other internal variables include medical conditions such as heart disease, diabetes, and hypertension (high blood pressure). Externally, environmental conditions such as temperature, humidity, and altitude alter how the exercising body functions.

Various modes of exercise testing and data collection are used to study the physiological responses of the body to exercise. Treadmills and cycle ergometers (instruments that measure work and power output) are among the most common methods of evaluating maximum oxygen consumption. During these tests, special equipment and computers analyze expired air, heart rate is monitored with an electrocardiograph (ECG), and blood pressure is taken using a sphygmomanometer. Blood and muscle-fiber samples can also be extracted to aid in identifying the fuel system and type of muscle fibers being used. Other data sometimes collected, such as skin temperature and body-core temperature, can provide pertinent information.

Metabolic equivalent units, or METs, are often used to translate a person’s capability into workloads on various pieces of exercise equipment or into everyday tasks. For every 3.5 milliliters of oxygen consumed per kilogram of body weight per minute, the subject is said to be performing at a workload of one MET. One MET is approximately equivalent to 1.5 kilocalories per minute, or the amount of energy expended per kilogram of body weight in one minute when a person is at rest.

Another factor greatly affecting the physical response to exercise is body composition. The three major structural components of the body are muscle, bone, and fat. Body composition can be evaluated using a combination of anthropometric measurements. These measurements include body weight, standard height, measurements of circumferences at various locations using a tape measure, measurements of skeletal diameters using a sliding metric stick, and measurements of skinfold thicknesses using calipers.

Body fat can be estimated using several methods, the most accurate of which is based on a calculation of body density. This method, called hydrostatic weighing, involves weighing the subject under water while taking into account the residual volume of air in the lungs. The principle underlying this measurement of body density is based on the fact that fat is less dense than water and thus will float, whereas bone and muscle, which are denser than water, will sink. One biochemical technique often used to determine levels of body fat is based on the relatively constant level of potassium-40 naturally existing in lean body mass. Another method uses ultrasound waves to measure the thickness of fat layers. X-rays and computed tomography (CT) scanning can be used to provide images from which fat and bone can be measured. Bioelectrical impedance (BIA) is a method of estimating body composition based on the resistance imposed on a low-voltage electrical current sent through the body. The most widely used and easily assessable method, however, involves measurement of skinfolds at various sites on the body using calipers. In all cases, mathematical formulas have been devised to interpret the collected data and provide the best estimate of an individual’s body composition.

Other tests have been developed to determine muscular strength, muscular endurance, and flexibility. Muscular strength is often measured by performance of one maximal effort produced by a selected muscle group. Muscular endurance of a muscle or muscle group is often demonstrated by the length of time or number of repetitions a particular submaximal workload or skill can be performed.

Two major types of flexibility have been identified. One type consists of the ability to move a muscle group or joint through its full range of motion at low speeds or hold a part of the body still at the extent of its range of motion. This is called static flexibility, and it can be measured using a metric stick or a protractor-type instrument called a goniometer. Dynamic flexibility, the other major identified type of flexibility, is the flexibility through the full range of motion of a muscle group or joint at normal or high speeds. Measuring dynamic flexibility is much more difficult.

Overlapping the science of exercise physiology are the studies of biomechanics or kinesiology (sciences dealing with human movement) and nutrition. Only through an understanding of efficient body mechanics and proper nutrition can the physiological responses of the body to exercise be identified correctly.

Diagnostic and Treatment Techniques

Exercise prescription is the primary focus in the application of exercise physiology. General health maintenance, cardiac rehabilitation, and competitive athletics are three major areas of exercise prescription.

Before making recommendations for an exercise program, an exercise physiologist must evaluate the physical limitations of the exerciser. In a normal health-maintenance setting, often called a “wellness” program, a health-related questionnaire can reveal relevant information. Such a questionnaire should include questions about family medical history and the subject’s history of heart trouble or chest pain, bone or joint problems, and high blood pressure. The presence of any of these problems suggests the need for a physician’s consent prior to exercising. After the individual has been deemed eligible to participate, an assessment of the level of physical fitness should be performed. Determining or estimating VO2max, muscular strength, muscular endurance, flexibility, and body composition is usually part of this assessment. It is then possible to design a program best suited to the needs of the individual.

For the healthy adult participant, the American College of Sports Medicine (ACSM), a widely recognized authoritative body on exercise prescription, recommends three to five sessions of aerobic exercise weekly. Each session should include a five- to ten-minute warm-up period, twenty to sixty minutes of aerobic exercise at a predetermined exercise intensity, and a five- to ten-minute cool-down period.

To recommend an appropriate aerobic exercise intensity, the exercise physiologist must determine an individual’s maximum heart rate. The best way to obtain this maximum heart rate is to administer a maximal exercise test. Such a test can be supervised by an exercise physiologist or an exercise-test technician; it is advisable, especially for the older participant, that a cardiologist also be in attendance. An ECG is monitored for irregularities as the subject walks, runs, cycles, or performs some dynamic exercise to exhaustion or until the onset of irregular symptoms or discomfort.

Exercise prescription using heart rate as a measure can be achieved by various methods. A direct correlation exists between exercise intensity, in terms of oxygen consumption, and heart rate. From data collected during a maximal exercise test, a target heart-rate range of 40 to 85 percent of functional capacity can be calculated. Another method used to determine an appropriate heart-rate range is based on the difference between an individual’s resting heart rate and his or her maximum heart rate, called the heart-rate reserve (HRR). Values representing 60 percent and 80 percent of the HRR are calculated and added to the resting heart rate, yielding the individual’s target heart-rate range. A third method involves calculating 70 percent and 85 percent of the maximum heart rate. Although this method is less accurate than the other two methods, it is the simplest way to estimate a target heart-rate range.

Intensity of exercise can also be prescribed using METs. This method relies on the predetermined metabolic equivalents required to perform activities at various intensities. Activity levels reflecting 40 to 85 percent of functional capacity can be calculated.

The rating of perceived exertion (RPE) is another method of prescribing exercise intensity. Verbal responses by the participant describing how an exercise feels at various intensities are assigned to a numerical scale, which is then correlated to heart rate. Through practice, the participant learns to associate heart rate with the RPE, reducing the necessity of frequent pulse monitoring in the healthy individual.

Adequate physical fitness can be defined as the ability to perform daily tasks with enough reserve for emergency situations. All aspects of health-related fitness direct attention toward this goal. Aerobic exercise often provides some conditioning for muscular endurance, but muscular strength and flexibility need to be addressed separately.

The ACSM recommends resistance training using the “overload principle,” which involves placing habitual stress on a system, causing it to adapt and respond. For this training, it is suggested that eight to twelve repetitions of eight to ten strengthening exercises of the major muscle groups be performed a minimum of two days per week.

Flexibility of connective tissue and muscle tissue is essential to maximize physical performance and limit musculoskeletal injuries. At least one stretching exercise for each major muscle group should be executed three to four times per week while the muscles are warm. Three methods of stretching that have been designed to improve flexibility are ballistic stretching, static stretching, and proprioceptive neuromuscular facilitation (PNF). Ballistic stretching incorporates a bouncing motion and is generally prescribed only in sports that replicate this type of movement. During a static stretch, the muscles and connective tissue are passively stretched to their maximum lengths. PNF involves a contract-relax sequence of the muscle.

In addition to exercise prescription for cardiorespiratory fitness, muscular fitness, and flexibility, it is appropriate for the exercise physiologist to make recommendations concerning body composition. Exercise is an effective tool in fat loss. Dietary caloric restriction without exercise results in a greater loss of muscle mass along with fat than if exercise is part of a weight-loss program.

For persons with special health concerns, such as diabetes mellitus or high blood pressure, the exercise physiologist works with the participant’s physician. The physician prescribes necessary medications and often decides which modes of exercise are contraindicated (that is, should be avoided).

A second application, cardiac rehabilitation, takes exercise prescription a step further. Participation of a heart patient in cardiac rehabilitation is more individualized than in wellness programs. The conditions of the circulatory system, pulmonary system, and joints are only a few of the special concerns. Secondary conditions such as obesity, diabetes, and hypertension must also be considered. The responsibilities of cardiac-rehabilitation specialists include monitoring blood sugar in diabetic patients and blood pressure in all patients, especially those with hypertension. Many drugs affect heart rate or blood pressure, and most of these participants are taking more than one type of medication. Patients with heart damage caused by a heart attack may display atypical heart rhythms, which can be seen on an ECG monitor. Furthermore, the stage of recovery of the postsurgical patient is a major factor in recommending the type, frequency, intensity, and duration of exercise.

Patient education is also important. Lifestyle is usually the main factor in the development of heart disease. Cardiac patients often have never participated in a regular exercise program. They may smoke, be overweight, or have poor eating habits. Helping them to identify and correct destructive health-related behaviors is the focus of education for the heart patient.

A third application of the study of exercise physiology involves dealing with the competitive athlete. In this case, findings from the most recent research are constantly applied to yield the best athletic performance possible. A delicate balance of aerobic training, anaerobic training, strength training, endurance training, and flexibility exercises are combined with the optimum percentage of body fat, proper nutrition, and adequate sleep. The program that is designed must enhance the athletic qualities that are most beneficial to the sport in which the athlete participates.

The competitive athlete usually pushes beyond the boundaries of general exercise prescription in terms of intensity, duration, and frequency of exercise performance. As a result, the athlete risks suffering more injuries than the individual who exercises for health benefits. If the athlete sustains an injury, the exercise physiologist may work in conjunction with an athletic trainer or sports physician to return the athlete to competition as soon as possible.

Perspective and Prospects

The modern study of exercise physiology developed out of an interest in physical fitness. In the United States, the concern for development and maintenance of physical fitness was well established by the end of the twentieth century. As early as 1819, Stanford and Harvard Universities offered professional physical-education programs. At least one textbook on the physiology of exercise was published by that time.

Much of the pioneer work in this field, however, was done in Europe. Nobel Prize–winning European research on muscular exercise, oxygen utilization as it relates to the upper limits of physical performance, and production of lactic acid during glucose metabolism dates back to the 1920s.

In the early 1950s, poor performance by children in the United States on a minimal muscular fitness test helped lead to the formation of what became known as the President’s Council on Physical Fitness and Sport. Concurrently, a significant number of deaths of middle-aged American males were found to be caused by poor health habits associated with coronary artery disease. A need for more research in the areas of health and physical activity was recognized by the mid-1960s. The subsequent research was facilitated by the existence of fifty-eight exercise-physiology research laboratories in colleges and universities throughout the country. Organizations such as the American Physiological Society (APS), the American Alliance of Health, Physical Education, Recreation and Dance (AAHPERD), and the American College of Sports Medicine (ACSM) were established by the mid-1950s. In an effort to ensure that well-trained professionals were involved in cardiac-rehabilitation programs, the ACSM developed a certification program in 1975. Certifications for fitness personnel were added later.

Increasingly sophisticated testing equipment should lead to a better understanding of fundamental physiological mechanisms, allowing practitioners to be more effective in measuring physical fitness and prescribing exercise programs. Health maintenance has become a priority as the number of adults over the age of fifty continues to increase. Advances in medical techniques also increase the survival rate of victims of heart attacks, creating a need for more cardiac-rehabilitation programs and practitioners. Health-care professionals and the general population need to be made more aware of the benefits of exercise for the maintenance of good health and the rehabilitation of individuals with medical problems.


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