Exercise physiology and its role in clinical sports medicine.

Annotation: Increasingly, the greater sports medicine world has been able to turn to exercise physiology for information that targets the capabilities of our recreational and competitive athletes. Functional physiologic deficits can be quantitated. Erasure of those deficits can be documented. Sports-specific physiologic qualities can help direct coaches in their training program, and in choosing athletes for their short- and long-term event participation.

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Over the past three decades exercise physiology has assumed an increasingly prominent role in clinical sports medicine. A field that was once pursued almost exclusively in research laboratories and cardiac rehabilitation programs is now commonly applied in comprehensive sports medicine clinics. The body of knowledge in exercise physiology has developed the breadth and depth needed to warrant its extensive application with athletes of both genders and all ages and competitive levels. This article is intended to overview the role of exercise physiology in contemporary clinical sports medicine. Specifically, this paper will address the applications of exercise physiology in precompetition health screening of athletes, assessment of performance characteristics, and reduction of special training problems. In addition, considerations that apply to selected special populations of athletes will be discussed.

Definitions and Historical Perspectives

Exercise physiology is a scientific discipline that is dedicated to elucidating the effects of exercise (in its many forms) on the physiologic systems and tissues of the body. Exercise physiologists are often interested in the body's physiologic adaptations to regular participation in exercise, as well as the influence of environmental stressors on physiologic responses to exercise and the physiologic factors that underlie the health effects of exercise. The discipline of exercise physiology dates back to the early 20th century. In 1927 the Harvard Fatigue Laboratory was founded, and for the succeeding two decades researchers at that facility conducted numerous studies that advanced our knowledge of the physiologic and metabolic responses to exercise. "Alumni" of the Harvard Fatigue Laboratory were subsequently responsible for developing exercise physiology research laboratories in several prominent US universities. From those rather humble origins exercise physiology has grown dramatically, so that it is now a focus of academic training and research in most major universities in the United States and many around the world. The American College of Sports Medicine (ACSM), a multi-disciplinary professional organization of over 20,000 members, is an important scientific society for exercise physiologists, particularly those with an interest in applications of the field to athletes.

In recent decades "clinical exercise physiology" has emerged as an important direction for this field. Research has generated important guidelines for treatment and secondary prevention of a broad range of clinical conditions, including cardiovascular disease, pulmonary diseases, cancer, diabetes mellitus (types 1 and 2), and certain psychological disorders. In addition, "performance-related exercise physiology" has developed as a direction for both research and clinical application. Researchers have elucidated physiologic phenomena that are evident during sports training and competition, and that knowledge is now being systematically applied in evaluating, guiding, and treating athletes. Accordingly, state-of-the-art clinical sports medicine centers are now equipped and staffed to administer and interpret sophisticated physiologic data in athletes ranging from youth sport participants to Olympic competitors.

Athlete Safety and Clearance for Participation

The precise number of sudden deaths occurring in athletes under the age of 35 years is not known. Cardiac abnormalities known to cause sudden death in young athletes occur infrequently in the general population, with the most common being hypertrophic cardiomyopathy, occurring at a rate of 1:500. (1) The prevalence of sudden death during athletic training or competition is in the range of 1:100,000 to 1:300,000 in high school athletes, with a higher prevalence in males. (2) Athletes older than 30 years have coronary artery disease as the most common cause of sudden death, with a prevalence of 1:15,000 in joggers (3) and 1:50,000 in marathon runners. (4) Thus, the overall prevalence of sudden cardiac death in athletes is relatively low. This fact does not reduce the dramatic and devastating effect of sudden death on families, communities, and society.

Because of the increasing number of people participating in competitive athletics and the desire to prevent sudden deaths among athletes, cardiovascular preparticipation screening of athletes has become an important issue. Screening benefits are based on the likelihood that athletic exercise training will increase the risk of sudden death in athletes who have significant underlying heart disease. Quantification of this risk is not possible, but most young athletes who die suddenly do so during the extreme exertion of training or competition. Early detection of significant cardiovascular conditions would allow for appropriate therapeutic management or lifestyle change and avoidance of high-risk activity.

Currently, the American Heart Association recommends preparticipation cardiovascular screening for all high school and college athletes. (1) This screening process should include a complete family and personal history and a physical examination designed to identify cardiovascular conditions associated with sudden death. Because of the potential cost associated with expensive screening procedures, some thought regarding the practicality of screening large populations of athletes is necessary. In older athletes, a personal history of cardiovascular disease with emphasis on coronary risk factors and a family history of premature coronary artery disease, as well as other cardiovascular disorders (eg, Marfan syndrome), should be obtained before starting competitive exercise. In addition, an exercise test with an electrocardiogram should be performed on men older than 40 and women older than 50 who wish to participate in regular athletic exercise training, if they are at high risk for coronary disease based on the medical history and physical examination.

The addition of echocardiography to the screening process clearly improves detection of some cardiovascular abnormalities, including hypertrophic cardiomyopathy, valvular heart disease, aortic root dilation, and left ventricular dysfunction. At the same time, echocardiography cannot distinguish all abnormalities, and the question remains whether the echocardiographic screening of large groups of athletes is cost-effective. A different approach is to obtain a detailed family history, completed with the aid of a qualified person (a nurse or physician's assistant), and a physical examination to identify persons with potential cardiovascular problems (eg, Marfan syndrome). In this way, echocardiography is reserved for individuals identified as being at risk for cardiovascular problems identifiable by echocardiography. Finally, in making the final determination of eligibility for athletic competition and training, any athlete identified as having a cardiovascular abnormality should be considered in the context of the 26th Bethesda Conference consensus panel guidelines. (5)

Evaluation of Performance Characteristics in Athletes

An important application of exercise physiology in clinical sports medicine involves measurement and evaluation of physiologic factors that relate to athletic performance. Research conducted over the past several decades has generated a substantial body of knowledge regarding the physiologic factors that determine performance in many types of sports. Often this research has involved developing physiologic profiles of successful athletes in a particular sport, comparing those profiles with the profiles of normal persons or less successful athletes, and ultimately conducting intervention studies to determine if enhancement of selected physiologic factors enhances performance. An example of such research is the long line of investigations which have demonstrated that performance in endurance sports (eg, distance running, cross-country skiing) is determined largely by mode-specific expressions of three physiologic variables: maximal aerobic power (eg, maximal oxygen uptake [V[O.sub.2]max]), lactate threshold, and economy.

In sports for which the physiologic determinants of performance have been identified, it has become commonplace for physiologic testing to be used in multiple ways. Often testing is used to advise athletes regarding the sport-specialties in which they might have the greatest success. A person's status on most physiologic variables is a function of genetically determined predisposition and environmental factors such as training history. Physiologic testing can characterize an athlete's current status on a key variable and thereby provide information about the genetically determined range on that performance characteristic. However, physiologic testing at a single time point cannot accurately predict how a person will respond to training over an extended period. For example, it is known that highly competitive male distance runners typically manifest maximal aerobic power values in excess of 70 mL/kg/min. A novice runner who scores in the high 50s or higher might be assumed to have inherited a high range on this variable and hence may possess the capacity to increase his V[O.sub.2]max to a value in the 70s as a result of training. On the other hand, a novice runner whose V[O.sub.2]max is in the low 40s would be unlikely to attain a value in the 70s as a result of training, because V[O.sub.2]max rarely increases more than 20 to 25% with training. However, it is not certain that an athlete with a relatively high baseline value will achieve a substantially higher value with training, because adaptations to training vary markedly across individuals. So, physiologic testing can indicate whether or not an athlete has the potential to achieve a highly competitive status, but such testing cannot accurately predict which specific athletes will actually achieve such outcomes.

Physiologic testing can also be useful in monitoring the effectiveness of a training program. For example, since endurance performance is determined by V[O.sub.2]max, lactate threshold, and economy, effective training should produce increases in these variables. Periodic administration of standardized physiologic testing protocols allows objective tracking of these key performance characteristics over time. This information can be useful in modifying training programs to optimize performance improvements. As an example, if a distance runner's lactate threshold is lower than desired or has not increased as anticipated, the training program can be modified to emphasize activities that are targeted specifically at increasing this variable.

Performance Evaluation 

If an athlete's physiologic profile has been established at some baseline (eg, beginning of a collegiate career), the profile can be used as the basis for evaluating progress during the subsequent training program. Likewise, that profile may be helpful in evaluating problems that are encountered during the training process. In general, a training program should be targeted at improving the key physiologic factors that relate to performance in the athlete's sport. Each training technique produces differential effects on different physiologic factors. For example, in endurance activities, high intensity interval training tends to increase maximal aerobic power (V[O.sub.2]max), whereas longer duration intervals and sustained high intensity exercise are directed toward improvements in lactate threshold. Sustained, moderate intensity, steady distance training is most likely to improve economy and factors related to fat metabolism. An athlete whose physiologic profile suggests that improvement in one of the key determinants of endurance performance would be particularly helpful might be exposed to a program emphasizing related training activities. Of course, it would be anticipated that future physiologic profiles on that athlete would reflect the desired improvements in key variables.

Troubleshooting Training Problems

Athletes frequently encounter unanticipated problems and challenges in training. These include unexplained drop-offs in performance and failure to improve in performance despite conscientious and consistent training. Finding solutions to these frustrating developments may be aided by physiologic profiling. An athlete who encounters a training problem should be screened first for clinical conditions that require medical treatment. If no such clinical condition is identified, comparison of the athlete's current physiologic profile with earlier profiles may reveal the basis for the difficulty. If a physiologic deficiency is identified, the training program can be modified to address the deficiency. For example, if an endurance athlete's economy of movement has decreased, emphasis on training techniques that are expected to improve this variable would be appropriate. Also, physiologic profiling may identify clinical conditions that are not revealed by a standard medical evaluation. An example would be a decrease in lactate threshold. Such an outcome could be explained by a medical condition that reduces the athlete's ability to buffer the lactic acid produced during high intensity endurance exercise. Clearly, in such cases, a team approach involving a collaboration between the sports medicine physician and the exercise physiologist can work to the athlete's best advantage.

Special Populations

Older athletes

Significant muscle atrophy occurs by the fifth decade of life despite training intensity. (6) Associated with this decline in muscle mass over time is a reduction in V[O.sub.2]max. However, for older endurance athletes who have maintained their training intensity over many years, the rate of functional capacity decline is attenuated. For endurance athletes who continue to exercise for many decades, but at reduced exercise intensity and exercise volumes, the attenuation in V[O.sub.2]max is not as great. Most importantly, both of these groups have greater V[O.sub.2]max capacities than do sedentary individuals. It is also important to note that older individuals who begin an exercise program are able to increase their V[O.sub.2] max capacity by 10 to 20%, depending on their exercise intensity, duration, and frequency. The bottom line is, exercise training throughout the lifespan can reduce the loss of functional capacity, and an exercise program begun in later life can increase functional capacity.

Female athletes

The participation of girls and women in organized athletics has increased dramatically in the past 30 years. Unfortunately, increased participation by women in athletics has not been accompanied by an increased understanding of the specific needs of the female athlete. Although the benefits of exercise and sports participation far outweigh the risks of permanent damage, there is an increased concern regarding some potential health problems and injuries among women, including stress fractures and anterior cruciate ligament tears. Concern about these health problems and injuries has recently gained recognition as the female athletic triad, which describes the coexistence of three distinct medical conditions that may occur in girls and women involved in vigorous competitive athletic training. Initially, eating disorders, amenorrhea, and osteoporosis composed the triad. Currently, the triad is considered to include disordered eating, amenorrhea, and decreased bone mineral density. (7) These disorders are defined as:

Disordered eating. Abnormal eating habits (eg, crash diets, binge eating) or excessive exercise which prevents the body from getting proper nutrition.

Amenorrhea. Improper nutrition, low calorie intake, high energy demands, physical and emotional stress, and low body fat percentage can lead to hormonal changes that stop the menstrual periods (amenorrhea).

Osteoporosis. Lack of menstrual periods disrupts the body's bone-building processes and weakens the skeleton, making bones more likely to break.

When coupled with inadequate nutrition, the high caloric expenditure of exercise training can result in a sustained negative caloric balance or low energy availability. This negative energy balance is sensed by the hypothalamus, which initiates an elaborate neuroendocrine sequence of events leading to decreased blood estrogen concentrations that can result in menstrual and reproductive dysfunction. This triad of health problems is not always related to sports or physical activity participation; however, exercise can add to the disturbance of caloric balance that can trigger the female athletic triad.

Sports and regular exercise are healthy activities for girls and women of all ages. Regular physical activity helps girls and women maintain a healthy weight, build strong bones, and reduce the risk of many chronic diseases. However, female athletes who place excessive emphasis on being thin or lightweight may eat too little and/or exercise too much. These behaviors can cause long-term damage to their health or even result in death. They can also hurt athletic performance and/or make it necessary for the participant to limit or even stop exercise.

Women in any sport can develop one or more aspects of the triad. At greatest risk are women in sports that reward being thin for appearances' sake (eg, figure skating, gymnastics) or where being thin is associated with improved performance (eg, distance running, rowing). A number of social as well as athletic competitive reasons exist for the movement of women toward one or more aspects of the triad. Fashion and the dominant culture are leading offenders, because they "reward" thinness and often encourage women to try to reach unhealthy weight levels. Some female athletes can suffer from low self-esteem and/or depression, and as a result focus on weight-loss, thinking they are heavier than they really are. Others feel pressure from coaches or parents to lose weight to enhance their performance. Female athletes who feel some of these pressures should consider the following questions:

I. Are you dissatisfied with your body?

II. Do you strive to be thin?

III. Do you continuously focus on your weight?

If the answers to these questions are yes, the athlete may be at risk for developing abnormal patterns of disordered eating that can lead to menstrual dysfunction and early osteoporosis. Most important is the recognition that a problem exists; the next step is treating it. Once problems like disordered eating, missing several menstrual periods, or stress fractures are identified, the athlete needs to see a doctor immediately. In preparing for the physician visit, the athlete should be able to give the doctor a complete medical history that includes:

I. What you do for physical activity and what you eat.

II. How old you were when you began to menstruate and whether you usually have regular periods.

III. If you are sexually active, do you use birth control pills, and have you ever been pregnant?

IV. Have you ever had stress fractures or other injuries?

V. Have you experienced any changes (up or down) in your weight?

VI. List any medications that you are taking or symptoms of other medical problems.

VII. What is your family history of diseases (eg, thyroid disease, osteoporosis)?

VIII. What are the factors that cause stress in your life?

With the answers to the above questions, the doctor will complete a physical and decide whether a pelvic examination is necessary. He or she may order laboratory tests to check for pregnancy, thyroid disease, and other medical conditions. The physician may also request a bone density test. Finally, treatment for the female athletic triad often requires help from a team of medical professionals including a doctor, nutritionist, and psychological counselor.

Children and youth

Structured youth sports programs, once largely limited to baseball and basketball programs for preadolescent boys, have grown rapidly in recent decades. In most communities programs in numerous sports are now available to both boys and girls. The age groups served by these programs often extend down to 6 years or even younger. As youth sports programs have expanded in scope they have also become increasingly competitive, and for the first time in history some young children are now formally training for competitive sports with the hope of ultimately performing at a high level as young adults. These trends, while seemingly positive in some ways, raise concerns about the short- and long-term well being of young athletes.

Pediatric exercise physiologists have generated knowledge that can be helpful in insuring that sports participation is healthful for children and youth. Studies examining acute responses to vigorous exercise have shown that young persons tend to manifest maximal aerobic power (V[O.sub.2]max, mL/kg/min) values that are comparable to those of moderately active young adults. (8) However, endurance performance is generally lower in children than adults, and this distinction is probably explained largely by the child's lower economy (ie, greater rate of energy expenditure at a given work rate). Also, children tend to have a lower anaerobic capacity than adults. (9) This distinction limits the child's ability to sustain very high intensity exercise. Children are not as strong as adults, but this difference is explained primarily by the child's smaller muscle mass. Per unit of muscle cross-sectional area, children and adults are capable of generating roughly comparable amounts of force.

Investigations of systematic exercise training in young persons have shown that children are physiologically adaptive to most forms of training. (10) However, young children appear to be less adaptive to training than postpubescent adolescents, and this difference is seen for both aerobic and high-resistance strength training. Consequently, pediatric exercise physiologists have often recommended that prepubescent children take part in training primarily for the purpose of learning about appropriate training methods and not for the expectation that profound improvements in performance will result from the training process. This guideline applies to both aerobic and strength training.

Sports performance depends on a complex set of determinants, most of which are heavily influenced by growth and developmental factors. Each sport has a unique set of determinants, and in some sports early competitive experience appears to be a prerequisite to later high level performance. Examples include gymnastics and tennis, both of which are sports in which performance depends heavily on mastery of unique motor skills. But in most sports, both professional experience and available scientific evidence indicate that it is impossible to predict adult performance from either performance itself or physiologic correlates of performance in children. This suggests that the emphasis in youth sports programs should be on mastery of the fundamental and specific motor skills associated with performance in the particular sport. Perhaps most importantly, sports programs for children should emphasize enjoyable participation in the sport so that participation continues into the age range at which high levels of performance can be attained.

Summary and Conclusions

Exercise physiology plays an important role in the practice of clinical sports medicine. Exercise physiology research has identified important effects of exercise on the body's systems, tissues, and cells. Ongoing research is investigating the role of exercise in subcellular, molecular, and chemical processes. Increasingly, sports medicine physicians and other practitioners are using the findings of this research to help athletes achieve peak performance, and nonathletes achieve better health through exercise. Many areas of sports medicine practice, including exercise testing, safety, performance evaluation, correction of training problems, and prevention of problems that affect specific populations (eg, older athletes, women, children), benefit from the application of exercise physiology theory and research. The continued demand for athletes at all levels to be better, faster, and stronger, combined with the national focus on getting all Americans involved in some form of physical activity, will require that sports medicine practitioners and exercise physiologists increasingly work together to optimize sports and exercise performance, health, and safety.

CME Credit--September 2004

Featured CME Topic: Sports Medicine

To receive a CME certificate for 1 category 1 credit toward the AMA Physician's Recognition Award, complete all sections of this form and mail it with a check (payable to SMA) for $15 ($30 for nonmembers) to: Publications Department, SMA, PO Box 190088, Birmingham, AL 35219-0088. The completed form and your check must be received by September 1, 2005. If you wish to preserve your Journal, it is permissible to submit a photocopy of this page. You may also submit your form online at www.sma.org.

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Accepted May 21, 2004.

Recommended Readings

* Eliakim A, Beyth Y. Exercise training, menstrual irregularities and bone development in children and adolescents. J Pediatr Adolesc Gynecol 2003;16:201-206.

* Janse de Jonge X. Effects of the menstrual cycle on exercise performance. Sports Med 2003;33:833-851.

* Manore MM. Dietary recommendations and athletic menstrual dysfunction. Sports Med 2002;32:887-901.

* Maron BJ. Cardiovascular risks to young persons on the athletic field. Ann Intern Med 1998;129:379-386.

* Maron BJ. Sudden death in young athletes. N Engl J Med 2003;349:1064-1075.

* Mees PD. The ripple effect of Title IX on women's health issues: Treating an increasingly active population. Physician and Sportsmedicine 2003;31.

* Miller C, Major N, Toth A. Pelvic stress injuries in the athlete: management and prevention. Sports Med 2003;33:1003-1012.

* Papanek PE. The female athlete triad: an emerging role for physical therapy. J Orthop Sports Phys Ther 2003;33:594-614.

* Pelliccia A, Maron BJ, Culasso F, et al. Clinical significance of abnormal electrocardiographic patterns in trained athletes. Circulation 2000;102:278-284.

* Pfister GC, Puffer JC, Maron BJ. Preparticipation cardiovascular screening for US collegiate student-athletes. JAMA 2000;283:1597-1599.

* Slauterbeck JR, Fuzie SF, Smith MP, et al. The menstrual cycle, sex hormones, and anterior cruciate ligament injury. J Athl Train 2002;37:275-278.

* Zawila LG, Steib CS, Hoogenboom B. The female collegiate cross-country runner: nutritional knowledge and attitudes. J Athl Train 2003;38:67-74.

References

1. Maron BJ, Thompson PD, Puffer JC, et al. Cardiovascular preparticipation screening of competitive athletes. A statement for health professionals from the Sudden Death Committee (clinical cardiology) and Congenital Cardiac Defects Committee (cardiovascular disease in the young), American Heart Association. Circulation 1996;94:850-856.

2. Maron BJ, Shirani J, Poliac LC, et al. Sudden death in young competitive athletes. Clinical, demographic, and pathological profiles. JAMA 1996;276:199-204.

3. Thompson PD, Funk EJ, Carleton RA, et al. Incidence of death during jogging in Rhode Island from 1975 through 1980. JAMA 1982;247:2535-2538.

4. Maron BJ, Poliac LC, Roberts WO. Risk for sudden cardiac death associated with marathon running. J Am Coll Cardiol 1996;28:428-431.

5. 26th Bethesda Conference: Recommendations for determining eligibility for competition in athletes with cardiovascular abnormalities. January 6-7, 1994. Med Sci Sports Exerc 1994;26(10 Suppl):S223-S283.

6. Trappe SW, Costill DL, Vukovich MD, et al. Aging among elite distance runners: a 22-yr longitudinal study. J Appl Physiol 1996;80:285-290.

7. Female athletic triad: an American Academy of Orthopedic Surgeon's fact sheet. 2004. Available at: http://orthoinfo.aaos.org. Accessed March 7, 2004.

8. Zauner CW, Maksud MG, Melichna J. Physiological considerations in training young athletes. Sports Med 1989;8:15-31.

9. Van Praagh E, Dore E. Short-term muscle power during growth and maturation. Sports Med 2002;32:701-728.

10. Naughton G, Farpour-Lambert NJ, Carlson J, et al. Physiological issues surrounding the performance of adolescent athletes. Sports Med 2000;30:309-325.

Russell R. Pate, PHD, and J. Larry Durstine, PHD

From the Department of Exercise Science, Arnold School of Public Health, University of South Carolina, Columbia, SC.

Reprint requests to Russell R. Pate, PhD, Department of Exercise Science, Arnold School of Public Health, University of South Carolina, 730 Devine Street, Columbia, SC 29208.