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Exercise Prescription

Ms. Ashe is Doctoral Candidate in Medicine, University of British Columbia, Vancouver, BC, Canada. Dr. Khan is Assistant Professor, Department of Family Practice, University of British Columbia.

Reprint requests: Ms. Ashe, University of British Columbia, Suite 211, 2150 Western Parkway, Vancouver, BC V6T 1V6, Canada.

	Abstract

Top Abstract Exercise Physiology Exercise Prescription Osteoarthritis and Osteoporosis Summary References

 
Although many studies suggest that exercise provides various health benefits, the optimal dimensions of exercise (ie, frequency, intensity, duration, type) remain to be carefully defined. Research encompasses many areas, from activities of daily living to elite athletic performance. To properly prescribe a beneficial exercise program, a physician must have a basic understanding of the elements of exercise physiology and energy metabolism, as well as of the cardiovascular, respiratory, and musculoskeletal responses to exercise and training. Also, integration of specific programs for muscle strengthening with aerobic conditioning is important to achieve optimal results in patients with musculoskeletal conditions such as osteoarthritis and osteoporosis.

Determining appropriate exercise prescriptions for postsurgical patients or individuals diagnosed with osteoporosis or osteoarthritis can be confusing; although recent studies have suggested that exercise is helpful, optimal parameters have not been carefully defined. Exercise physiology and prescription encompass a range of intensity, from activities of daily living to elite athletic performance. Understanding the principles of exercise physiology, such as how the body responds and adapts to the workload imposed on it, is an important component of effective prescription. Without underestimating the importance of cardiovascular fitness, it is critical to emphasize specific elements of exercise that directly affect the musculoskeletal system.

	Exercise Physiology

Top Abstract Exercise Physiology Exercise Prescription Osteoarthritis and Osteoporosis Summary References

 
Energy Pathways
Performance during exercise depends on energy. Adenosine 5'-triphosphate (ATP) is the substrate that most cells use as their energy source because it can both receive and provide energy; the supply is constantly replenished.¹ The ATP molecule has three components. The center is ribose, with a nitrogenous base (adenine) attached to one side and, on the other side, a string of phosphate groups that provides the site of energy transfer. With enzymatic cleavage, the terminal phosphate is released, energy is given off, and adenosine 5'-diphosphate is formed.¹

Energy metabolism is a complex series of steps that depends on the available type of fuel (ie, carbohydrate, fat, protein). The fuel source and the type of exercise determine the physiologic pathway for energy production. The three pathways of carbohydrate metabolism are the ATP-phosphocreatinine system (anaerobic), glycolysis (aerobic and anaerobic), and oxidative phosphorylation and the electron transport chain (aerobic). Which pathway responds depends on the individual and the challenge that is presented. The three basic areas of exercise are power, speed, and endurance. Depending on the sport and the type of energy required, the athlete trying to maximize performance needs to develop the anaerobic energy pathway (eg, sprinter, basketball center), the aerobic pathway (eg, endurance runner), or a combination of both (eg, soccer player).

Cardiovascular System
The cardiovascular system is the principal determinant of oxygen consumption. VO₂ is the maximum volume of oxygen consumed per minute. V_max is the maximum volume of oxygen that can be consumed. V_max can be expressed as an absolute value (liters per minute) or a relative value (with reference made to body weight and so measured in milliliters per minute per kilogram of body weight).²

During exercise, the cardiovascular system must meet the increased demand for fuel and oxygen while removing metabolic by-products. Consequently, blood flow is redistributed to the exercising muscle while cardiac and cerebral homeostasis is maintained. As a result, cardiac output, the total volume of blood pumped by the ventricles per minute, can increase in a highly trained athlete from an average resting value of 5 L/min to 30 L/min. The initial increase in cardiac output is the result of increases in both stroke volume (amount of blood ejected from the ventricles during systole) and heart rate. However, as exercise progresses above approximately 40% V_max, the stroke volume remains stable and the accelerating heart rate increases cardiac output.³

Blood pressure is controlled by cardiac output and total peripheral resistance. Baroreceptors in blood vessel walls respond to both average pressure and the rate of change; these receptors exert their influence through the autonomic nervous system. The autonomic nervous system, in turn, influences heart rate, stroke volume, and peripheral vascular resistance.

The lactate threshold is used to estimate fitness level. Lactic acid is produced when oxygen is absent during glycolysis, and its accumulation in the body can lead to early fatigue. Once the lactic acid level begins to rise, it continues to do so rapidly. Reaching the lactate threshold means that the body has enough lactic acid to affect performance and can cause performance to cease. Training increases the lactate threshold; a highly trained athlete will reach the threshold much later than an average person exercising at the same level of intensity.⁴ Training also causes changes in the cardiovascular system, increasing resting stroke volume, cardiac output, blood volume, and hemoglobin, and decreasing resting heart rate and blood pressure.² Some elite athletes have resting heart rates as low as 40 to 50 beats per minute.

Respiratory System
Alveolar ventilation, which allows simple diffusion of oxygen and carbon dioxide by a pressure gradient across the alveolar-capillary membranes, is tightly controlled by peripheral sensors; the brainstem control center, which detects balance between frequency and depth of breathing; and the respiratory muscles.⁵ Although the average intake of air is 6 L/min, during exercise it can increase to 192 L/min and 48 breaths/min.⁵

The normal respiratory response to steady-state exercise occurs in three stages. Initially (within 30 seconds), there is a rapid increase in ventilation (breaths/min), which may be followed by a plateau. As steady-state exercise progresses, there is a gradual increase in ventilation that continues until either maximum voluntary ventilation or fatigue is reached.^5,6 Training can increase ventilatory efficiency by increasing diffusion capacity, thus improving the match of ventilatory efficiency with ventilation/perfusion.²

Neuromuscular System
Skeletal muscle fibers are either slow twitch or fast twitch, with the type of muscle fiber determined by the nerve supply⁷ (Table 1). Muscle fibers are recruited in a specific pattern, according to the size of the motor unit, to produce the necessary force and speed. This is the so-called size principle. For example, motor units with smaller diameter innervations (slow-twitch muscles) are recruited before fast-twitch muscles.⁷

Two types of muscle soreness can occur with exercise. Acute soreness is caused by fatigue or lactic acid buildup within the muscle. Exercise should not continue if such discomfort persists.⁸ The other type of muscle soreness, delayed-onset muscle soreness (DOMS), can occur 12 hours or more after activity. DOMS is thought to be the result of microtears within the working muscles, with subsequent swelling. Eccentric contractions, in which the muscle is slowly elongated during the contraction, are a major cause of DOMS.^8,9 Adequate warm-up, cooldown, and moderate progression of intensity can help prevent DOMS.

	Exercise Prescription

Top Abstract Exercise Physiology Exercise Prescription Osteoarthritis and Osteoporosis Summary References

 
Applying the principles of exercise physiology can help improve the health of special populations. When considering exercise, the following parameters should be considered to achieve a training effect (eg, increased cardiac output, increased lactate threshold): frequency, intensity, duration, and type of exercise. With these elements in mind, the physician can determine how often the patient should exercise, which exercise is best, the optimal amount of exercise, and how challenging the exercise should be. Asound program includes three key types of exercise: aerobic (ie, cardiovascular), resistance training (ie, strengthening), and stretching. Done in moderation, exercise can help provide a balanced approach to improving and maintaining health and fitness^10–12 (Table 2).

Recently there has been a shift from prescribing vigorous exercise several times a week to prescribing moderate daily activity.¹³ The American College of Sports Medicine (ACSM) recommends moderate-intensity exercise with longer duration range, that is, "20 to 60 minutes of continuous or intermittent (minimum 10-minute bouts accumulated throughout the day)" aerobic activity.¹⁴ The US Surgeon General’s Report¹⁵ recommends a total of 30 minutes of moderate activity (eg, walking, doing housework), preferably daily. For older adults with limitations or who are extremely sedentary, the first goal of exercise prescription is to reduce time spent sitting. Exercise can progress to parking farther away from entrances at malls and shopping centers, using the stairs instead of the elevator, or taking several brief walks a day (Table 3).

Aerobic exercise intensity is measured in several ways. For healthy adults, the ACSM recommends a training rate between 55% and 90% of maximum heart rate, which can be measured in a maximal test or estimated by subtracting the individual’s age from 220.¹⁴ It is also recommended that the lower intensity value (55% to 64% of maximum heart rate) be used for unfit or deconditioned individuals if appropriate. Aerobic exercise intensity also can be measured by the rate of perceived exertion (Table 4).

Patients with chronic diseases (eg, arthritis) may have to limit range of motion for some exercises and therefore use lighter or easier weights with additional repetitions. Proper breathing and equipment techniques are important to minimize injury. Educating patients about the normal response to exercise and the possibility of DOMS may assist with compliance.

Strengthening can be grouped into three categories: isotonic, isometric, and isokinetic. Isotonic exercise involves the development of muscular force through range of motion or movement. Isokinetic exercise involves the force being generated through a constant speed. Isometric exercise involves the development of force without movement, as in tensing and holding a muscle at a certain part of the range.

Core stability (ie, abdominal) strengthening exercises are still possible when some limb exercises are contraindicated. For example, if the lower leg cannot be exercised because of recent injury or surgery, the abdominal muscles still can be worked. Pilates exercises are ideal upper body and abdominal workouts for those whose lower leg resistance training is limited. The Pilates method is a series of approximately 500 exercises done on specialized equipment or mats. The principles behind the Pilates method include proper breathing technique, positioning of the body, balance, coordination, strength, and flexibility. Instructors must be certified and can provide either one-on-one or group training.¹⁹

Patients often are interested in using their noninjured extremities for functional activities that allow them to remain in condition and retain some skill practice. For instance, Thomas Muster, the Austrian professional tennis player, underwent significant lower extremity surgery after a motor vehicle accident in the spring of 1989, but within weeks he returned to the tennis court, playing in a wheelchair. By doing so, he was able to maintain some aerobic and anaerobic fitness, practice his motor skills, and use his time productively. That training was combined with clinic-based physical therapy, and Muster returned to competition much more rapidly than had originally been anticipated.

Stretching
The scientific evidence supporting stretching as part of an overall health-and-fitness program is somewhat controversial. A recent meta-analysis reviewed the evidence for stretching, both immediately before activity and independent of activity.²⁰ There was little evidence of favorable effects for stretching before exercise, but stretching independently of activity periods may have some benefits.²¹ Despite the lack of solid evidence in favor of stretching, most health professionals regularly prescribe it as a preventive measure.

The three main types of stretching or flexibility exercise are static and ballistic stretching and proprioceptive neuromuscular facilitation. Static stretching is the type most often recommended by health professionals. It involves slowly stretching the muscle and holding the pain-free position for 10 to 30 seconds without bouncing or other movement. Ballistic stretching involves bouncing, and the final position is not held. Proprioceptive neuromuscular facilitation has its origins in physical therapy. Muscular contractions are elicited to produce relaxation within a muscle. Properly done stretching appropriate for a specific orthopaedic problem also can incorporate breathing exercises to promote overall relaxation and a sense of well-being. Yoga or tai chi may be indicated, but patient limitations must be considered.

Precautions and Contraindications
When prescribing exercise, the physician should consider the patient’s age, medical history, and current level of health and fitness. In patients with a history of a cardiac event and pulmonary and/or metabolic disease, a medically supervised program is recommended. The American College of Sports Medicine Position Stand states:

The contraindications to exercise testing and exercise training for older men and women are the same for young adults. The major absolute contraindications precluding exercise testing are recent ECG changes or myocardial infarction, unstable angina, uncontrolled arrhythmias, third degree heart block and acute congestive failure. The major relative contraindications for exercise testing include elevated blood pressure, cardiomyopathies, valvular heart disease, complex ventricular ectopy and uncontrolled metastatic diseases. It is of paramount importance to remember that symptomatic and asymptomatic CV disease and the absolute and relative contraindications precluding exercise testing are more prevalent in older adults. In addition, there is an increased prevalence of comorbidities in older adults that affect CV function, including diabetes, hypertension, obesity and left ventricular dysfunction. This adherence to the general ACSM testing guidelines with respect to the necessity for exercise testing and for medical supervision of such testing is imperative.¹³

Although isometric exercise is prescribed to maintain muscle mass and strength when range of motion is restricted, it may be contraindicated in a patient with a history of cardiac problems. Resistance training can cause a dramatic increase in blood pressure, especially if the patient holds her or his breath and evokes a Valsalva maneuver. Therefore, a team approach to the treatment of the patient is ideal. The orthopaedic surgeon, family physician, cardiac specialist, and treating therapist can work together to prescribe the right exercise and then monitor and assist the patient to maintain benefits.

	Osteoarthritis and Osteoporosis

Top Abstract Exercise Physiology Exercise Prescription Osteoarthritis and Osteoporosis Summary References

 
Osteoarthritis
For patients with osteoarthritis of the knee or hip, exercise can provide modest improvements in measures of disability and physical performance.^22,23 Although both resistance and aerobic exercise programs are helpful, as is a mixture of several types of therapy, data are insufficient to suggest that one protocol is better than another.

One successful exercise protocol is a 3-month facility-based program of walking 3 times a week with an instructor, followed by a 15-month, home-based, 3-times-a-week walking program closely monitored by an instructor.²⁴ Each aerobic session consists of 10 minutes of warm-up and cooldown bracketing a 40-minute walk at between 50% and 70% of heart rate reserve, as determined by a screening exercise test. Time commitments for resistance training are similar to those for the aerobic exercise program. The 40-minute program depends on individual findings. It includes exercises for strength, mobility, and coordination. Weight is begun at the lowest possible level (1 kg) and increased in a stepwise fashion as long as the participant can complete two sets of 10 repetitions of each exercise. Once a plateau is reached, weight is increased when the patient successfully completes two sets of 12 repetitions for 3 consecutive days. Although more research is needed to test the various modalities of exercise for osteoarthritis of the knee or hip,²⁴ this protocol provides a basis for prescribing exercise.

Osteoporosis
Part of managing osteoporosis itself, not just its complications, is an exercise program.²⁵ Most types of activity are preferable to a sedentary life-style and should be encouraged. The patient may choose to exercise alone, or, if one is available, join a group program for individuals with osteoporosis.

Resistance training should be considered for persons with osteoporosis because such training can cause mild improvement in, or at least help maintain, bone mineral density.^26,27 In a 1-year prospective randomized controlled trial, Kerr et al²⁸ studied the impact of resistance training (8 to 12 repetitions) on bone mass in the forearm and hip in postmenopausal women. There was a statistically significant (P < 0.05) improvement in bone mass, which was site-specific and dependent on the load applied.²⁸ In their meta-analysis of resistance training and bone mineral density in premenopausal and postmenopausal women, Kelley et al²⁹ concluded that resistance training had a positive impact on the lumbar spine in all women, but only postmenopausal women gained bone mineral at the femur and radius. Resistance training also improves lower limb muscle strength, which is a key factor in preventing falls and fractures.³⁰

Exercise sessions should begin with an 8- to 15-minute warm-up of gentle stretching and joint motion, followed by cardiovascular exercises for 5 to 10 minutes. Targeted heart rate should be 60% of 220 less the patient’s age for a beginner or a deconditioned person, and 70% to 75% of 220 less the patient’s age for those in better shape. Range-of-motion exercises for the shoulder and hip joints should follow. Gradual weight training with light free weights and rubber tubing then can be incorporated. Effective upper arm exercises include pushing against a wall or pulling and twisting against a partner.³¹

Quadriceps muscle strength can be improved with a wall slide exercise or by practicing standing from a seated position. Back posture correction can be done standing, sitting in a chair, or lying prone. Abdominal strengthening and leg strength exercises also are important. The exercise program should conclude with a 7- to 8-minute cooldown period; postexercise stretches can be done while sitting on the floor. Although no guidelines have been scientifically validated, these steps represent current best practice guidelines that are safe and appear to be appropriate from clinical experience. The physician should discover activities the patient values; many popular activities require some degree of strength, flexibility, endurance, balance, and coordination.

	Summary

Top Abstract Exercise Physiology Exercise Prescription Osteoarthritis and Osteoporosis Summary References

 
Exercise provides major health benefits, but specific prescription is necessary. The basis for prescription is the exercise physiology of the energy metabolism pathways and the response of various systems to different types of training. Despite inconsistencies in exercise prescription, a commonsense approach based on the principles of exercise physiology provides a starting point. Specifically, patients with osteoarthritis or osteoporosis may benefit from participation in exercise programs.

	Footnotes

 
None of the following authors or the departments with which they are affiliated has received anything of value from or owns stock in a commercial company or institution related directly or indirectly to the subject of this article: Ms. Ashe and Dr. Khan.

	References

Top Abstract Exercise Physiology Exercise Prescription Osteoarthritis and Osteoporosis Summary References

 

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