There is little doubt that a conscientious program of physical activity, especially aerobic exercise, significantly improves the health and wellness of the exerciser. What is more, evidence is mounting that the benefits of regular and continuing exercise can be enjoyed over an entire life span. It has been proven fairly convincingly that individuals who maintain an active lifestyle may actually add years to their lives while significantly enhancing the quality of life in later years as well. Physical activity reduces the premature, deleterious effects of degenerative diseases, especially cardiovascular disease. Individuals of all ages, from children to the elderly, should be encouraged to exercise. It is never too late to begin a regular exercise program, but the best time to start is usually now. Clearance by a health care provider is always recommended prior to the commencement of an exercise program.
General Principles of Aerobic Exercise
Aerobic exercise is generally recognized as exercise that is rhythmic, uses the major muscle groups, and is maintained at a fairly continuous intensity for a prolonged period of time. Provided the intensity is such that the exercise is maintained without undue fatigue for at least 10-15 minutes, the aerobic system serves as the predominant source of energy. Because of the significant role the cardiovascular and respiratory systems play in aerobic exercise, adaptations occur in the heart, lungs, blood vessels, and skeletal muscles with regular and continued aerobic training. These adaptations can lead to significant health benefits and to an improvement in aerobic exercise performance.
Modes of Aerobic Exercise
Most authorities consider the best aerobic exercises to be those that consistently maintain intensity at a constant level. This is especially true for individuals who are just beginning an exercise program or for those individuals who may experience symptoms of cardiovascular disease, including chest pain, abnormal heart rhythms, or unusually high blood pressure. Many other exercise modes, in which the intensity is more variable, also provide significant aerobic benefits, but may not be recommended for all people.
Constant Intensity Exercise
Some of the best examples of aerobic exercise, in which the intensity can be maintained at a constant level, include walking, hiking, jogging, running, stepping, aerobics, step aerobics, stationary cycling, road cycling, mountain biking, rowing, roller skating, inline skating, and cross-country skiing. All of these exercises, provided they are performed at the appropriate frequency, duration, and intensity, improve the performance of the heart and lungs to an equal degree. However, some might be considered more beneficial than others because they train a larger portion of the body’s muscle mass as well. The important factor is not which of these modes is used, but that any mode practiced consistently will result in beneficial health and performance changes.
Variable Intensity Exercise
Other modes of exercise, in which the intensity is less predictable and more variable, also have the potential for providing aerobic benefits. These include recreational pursuits, such as tennis, racquetball, squash, handball, soccer, basketball, roller hockey, and ice hockey. However, these sports must be played in a continuous manner. In the racquet sports, the time spent serving and receiving a serve must be intentionally reduced, and the players must have levels of skill that enable them to maintain longer rallies. In the team sports, penalties and play stoppages must be eliminated or severely restricted, so that the level of activity is more evenly maintained. For example, if basketball is played recreationally, if no free throws are taken, and the ball is quickly put back into play after each foul or basket, it can provide significant aerobic benefit. As it is played competitively at the collegiate or professional level, however, basketball is very intermittent and highly dependent on anaerobic metabolism.
Before and After Exercise
To maximize safety, certain precautions should be taken before and after exercise. Adequately warming up prior to the aerobic exercise session may prevent damage to skeletal muscle, connective tissue, and the heart. A sufficient cooldown is necessary after exercise to alleviate a potential rapid drop in blood pressure that could cause light headedness, dizziness, or fainting. Proper warm-up and cool-down are increasingly important before and after higher intensity exercise.
Prior to aerobic exercise, low-intensity, dynamic exercise is performed to gradually prepare the body for the exercise and to prevent damage to skeletal muscle, connective tissue, and the heart. The main benefits of the warm-up are to increase blood flow to the previously mentioned tissues and to increase body temperature. Skeletal muscle and connective tissue become more pliable, stretching more easily, and thus become more resistant to tearing. The gradual increase in exercise intensity allows adequate blood flow to the heart. Without proper warm-up, especially in older adults, exercise can result in an inadequate blood flow to the heart which can lead to chest pain, tissue damage, or an irregular heartbeat.
Inappropriate changes in blood pressure can be observed with inadequate warm-up. The large blood vessels in the arms and legs are constricted (narrow) at rest and provide a high resistance to blood flow. If exercise is started at a low intensity, it allows time for the blood vessels to slowly dilate and for the resistance to drop. If however, the exercise intensity is increased too fast, the blood vessels are still constricted and blood pressure rapidly rises to very high levels. Exercise blood pressures of 250/115 mmHg are considered too high, and increase the likelihood of stroke and the rupturing of other blood vessels throughout the body. Proper warm-up makes the working skeletal muscles warm and acidic, which makes it easier for the muscles to extract more oxygen from the circulating blood.
Typically, two different types of exercise are performed during the warm-up session. Mild, dynamic exercise is used to increase blood flow and body temperature. This exercise is usually of the same mode as the exercise used for the aerobic conditioning, but is performed at a very low intensity (30-40% of maximum). Prepartory stretching exercises are a second type of exercise that can be included in the warm-up to prepare muscles and help minimize the risk of soft tissue injury. For instance, cycling should be preceded by preparatory stretches emphasizing the muscles in the legs; swimming is preceded by stretches emphasizing the arms and upper body. While emphasizing particular muscle groups is a good idea, it should not be done at the exclusion of other muscles throughout the body.
The cool-down period following exercise is used primarily to prevent a rapid drop in arterial blood pressure. Many arteries are fully dilated following exercise due to changes that have occurred in the skeletal muscle around them. This significantly reduces the resistance to blood flow. If heart rate, stroke volume, and cardiac output are allowed to drop rapidly after exercise, the reduced blood flow, in combination with the reduced resistance, can result in very low blood pressure. As a result of inadequate blood pressure, an insufficient supply of blood reaches the brain, which can cause lightheadedness, dizziness, or fainting. To avoid rapid drops in blood pressure, it is important to continue some type of dynamic exercise during recovery. Due to the rhythmic muscular contractions, blood flow back to the heart is enhanced maintaining stroke volume and cardiac output. Standing or sitting still after exercise should be avoided because of the pooling of blood in the legs that will occur due to gravity. The blood pooling reduces stroke volume, which reduces cardiac output and thereby leads to a significant drop in arterial blood pressure.
During the cool-down period, heart rate, breathing rate, oxygen consumption, and caloric expenditure remain elevated above resting levels and gradually decline. This period of increased oxygen consumption and energy expenditure after exercise has traditionally been called oxygen debt. This name was given because the increased expenditure during recovery was thought to “pay back” the deficit incurred during the warm-up (the first 3-5 minutes) when the anaerobic pathways predominate. Today, oxygen debt is called excess post-exercise oxygen consumption, or EPOC. The length of the extra oxygen consumption and corresponding increased caloric expenditure depends on the intensity and the duration of the exercise. The harder and longer the exercise, the greater the EPOC, as the body works to restore resting levels and homeostasis.
Before and After Exercise
In this discussion, aerobic training is assumed to consist of continuous aerobic exercise. It is generally performed by healthy adults at 64-94% of maximum heart rate (HR max) or 40-85% of heart rate reserve (HRR). The main purpose of aerobic exercise is to improve the ability of the cardiorespiratory system to deliver oxygen and to improve the aerobic endurance of the skeletal muscle used during the exercise. This type of training is highly recommended for the general public because of the associated health benefits provided by such activity. It is believed to facilitate a normalization of blood pressure, to lower body fat, to improve glucose utilization, to reduce psychological stress, and to reduce the risk of heart attack and stroke.
Three important characteristics of exercise must be considered. The frequency, duration, and intensity of the exercise are all interrelated and must be monitored and adjusted to provide maximal aerobic benefits. As discussed previously, to ensure the safety of the exercise, careful consideration must be given to the activity used in warming up prior to the exercise and in cooling down afterward. Provided all of these principles are followed correctly, they may be applied to improve the aerobic fitness of any apparently healthy adult or athlete.
In order to see improvement in aerobic power, the exercise should be performed at least 3-5 days per week. Exercising only once or twice a week has been shown to maintain fitness for the most part, but does not provide enough stimulus to achieve significant gains in aerobic fitness. On the other hand, performing the same mode of exercise too frequently has a tendency to increase both the possibility of exhaustion and the risk of overuse injuries. It is recommended that 1-2 days per week be rest days with no hard training. Cross-training, using more than one primary mode of exercise (e.g., aerobics, running, cycling, swimming), is also a popular way of exercising more frequently with less risk of injury. It also relieves, for many people and athletes, much of the boredom associated with long months of training. In addition to aerobic training, it is strongly recommended that strength training (or resistance training) be done at least 2 days a week to gain the added benefits it provides, including increased strength, improved glucose utilization, and increased bone mineral content.
The duration of the exercise must be at least 20 minutes to achieve gains in aerobic endurance. More significant gains are observed when the duration is extended to 30-60 minutes per session. There is nothing wrong with exercising continuously for over an hour if it is tolerated well. Some individuals, however, become chronically fatigued and are prone to overuse injury due to exercising for too long with insufficient rest. Long training sessions, of over an hour in duration, should be divided into bouts of differing modes of exercise, or should be limited to two to three times per week. While exercise bouts of less than 20 minutes have not proven useful in improving aerobic endurance, there is recent evidence they may still result in health benefits and in psychological and emotional benefits. In other words, benefits can be gained by taking advantage of brief exercise breaks, such as walking the dog, walking during your lunch break, or riding your bike to work.
The intensity of the exercise is more difficult to monitor than frequency or duration. With less experienced exercisers, intensity is best monitored by using heart rate as an indicator of stress. An added benefit of using heart rate is that it is sensitive to environmental changes the exerciser may encounter. To improve aerobic power (determined by your V02max), the intensity of the exercise should be maintained between 55% and 85% of heart rate reserve (HRR) in order to stimulate structural changes in your heart and peripheral changes in your skeletal muscles, which in turn lead to an increase in V02max. Keep in mind that the benefits of aerobic exercise (e.g., reduction in blood pressure, decrease in body fat) can occur at intensity thresholds as low as 40% HRR.
The heart rate reserve (HRR) method is based on the number of beats between resting heart rate (HRrest) and a measured or estimated maximal heart rate (HRmax). HRrest is simply the lowest palpable heart rate achieved while resting. It is usually suggested that HRrest be taken in the morning, but it can be measured at any time during the day provided the subject is well rested and free from stress. Although HRmax is best obtained through the use of a graded exercise test, it can also be estimated based on the subject’s age. Table 5-1 demonstrates the method for calculating training heart rates at 55%, 65%, 75%, and 85% of HRR using a HRmax estimated from age. A directly measured HRmax is preferred, but because it is infrequently measured, a HRmax estimated from age will suffice. (This calculation of intensity is one way to determine an appropriate training HRR.
The minimum intensity believed necessary to produce moderate fitness gains in a healthy adult is about 40-60% HRR. Increasing the intensity of the exercise to 65-85% HRR causes adaptations to occur more quickly, resulting in faster increases in aerobic fitness and aerobic exercise performance. By increasing the intensity still further, however, to over 85% HRR, the exercise now requires anaerobic energy production. Consequently, the exercise leads to fatigue too rapidly and, as such, is not suggested for inclusion in an aerobic program. More experienced exercisers usually do not need to monitor heart rate each time they exercise. Instead, they can eventually rely on their experience to perceive and maintain a level of exertion that produces the desirable heart rate within a minimal range of error.
The most appropriate exercise intensity is determined based on the participant characteristics, fitness level, and exercise program goals. Fitness level may be assumed based on the exercise history of the subject or can be measured using a graded exercise test. Intensity should be kept low ( 40-55% HRR) for apparently healthy individuals who are in poor to fair condition. In certain populations, such as previously sedentary, elderly, or symptomatic participants, even this level of intensity may be too high. In these populations it may be recommended that very low-intensity exercise (30-40% HRR) be performed until the subject reaches a sufficient level of fitness. The main advantage of very low- and low-intensity exercise is found in the health benefits achieved. It can result in weight loss, an improvement in body composition, a modest reduction in blood pressure, improved glucose utilization, and possible improvement in blood lipids. Because of the low intensity, however, it needs to be practiced more frequently and for a longer duration to obtain the desired benefit.
For those in fair to average condition, low- to moderate-intensity exercise (55- 65% HRR) is recommended, but this intensity should not be attempted until some initial level of fitness is attained. Exercise of this type results in health benefits as well as some adaptations that may lead to moderate gains in aerobic fitness. For participants who are in average to good shape, with a previous history of regular aerobic exercise, moderate- to high-intensity exercise ( 65-75% HRR) or high-intensity exercise (75-85% HRR) is required to ensure sufficient stress for continued improvement in aerobic fitness and performance. Exercise of this intensity can lead to significant training effects in the heart, lungs, and skeletal muscles that can improve aerobic exercise performance.
A summary of determining training heart rates based on participant characteristics, fitness level, and exercise program goals is presented in Table 5-2.
Two other means of measuring intensity include the talk test and rating of perceived exertion.
Summary of Exercise Principles
To appreciate the relationship between frequency, duration, and intensity of exercise, a brief summary is necessary. When beginning an exercise program, it is best to maintain a conservative approach and start at the suggested minimums of three times per week, 20 minutes per session, at no more than 55-65% HRR. (This corresponds to an exercise heart rate of 131-142 bpm when considering the example in Table 3-1 for a 40-year-old exerciser.) Provided this level of activity is tolerated well, the next step consists of gradually increasing the duration to 30 minutes, while maintaining the same frequency and intensity. The intensity should not be increased until the subject can exercise for 30-45 minutes without becoming overly tired. At this point, the intensity can be increased to 65-75% HRR (e.g., 142-153 bpm).
The frequency may now also be increased to four to five times per week if so desired. Once the subject feels comfortable exercising 30-45 minutes, three to five times per week, at 65-75% intensity, the intensity may be raised toward the recommended maximum of 75-85% HRR (e.g., 153-164 bpm). If the purpose of the exercise is to maintain aerobic fitness in a healthy non-athlete, the previous recommendations of frequency (three to five times per week), duration (20-60 minutes), and intensity (55-85% HRR) apply.
It may be beneficial to consider a combination of exercise frequency, duration, and intensity almost as a “volume” of exercise per week. Table 5-3 includes key guidelines for physical activity that incorporate all three of these exercise principles. The minimal volume of exercise for substantial health benefits is 150 min of moderate intensity exercise (55-70% HRR) per week. This could be done as 30 min/day for 5 days, as 50 min/day for 3 days, or as any combination of duration and frequency. Or it could be 75 min of vigorous exercise (70-85% HRR). The goal eventually for more extensive benefits would be to increase the volume of aerobic exercise to 300 min of moderate intensity exercise (55-70% HRR) per week, or 150 min of vigorous exercise (70-85% HRR) spread throughout the week.