General Fitness>>Aerobic Training - 1/6/2007

Aerobic Training

Aerobic exercise is an essential component of fitness and is important in achieving a healthier, more active lifestyle.  In this chapter, we will discuss how your body responds to regular aerobic activity.  We will give you the tools and knowledge to select the appropriate m odes of exercise and show you how to monitor your exercise intensity.

   Regular aerobic exercise has powerful implications for positive changes physically and mentally.  Regular aerobic exercise is even more important if you are deconditioned and want to avoid health risks that result from a sedentary lifestyle, such as cardiovascular disease.  Studies have shown that people who participate in wheelchair sports and fitness programs show decreased medical complications and hospital admissions.

   Aerobic exercise and activities are commonly called endurance training because they involve elevating the heart rate over a prolonged period of time.  In order to do this, aerobic training requires a continuous, long-duration activity that works large muscle groups, typically the large muscles of the torso and legs that are used in jogging.  Aerobic training is also referred to as cardiorespiratory endurance because it relies on oxygen to provide energy.  When the cardiorespiratory system is stressed, positive central changes occur, resulting in a more efficient heart and lungs.

Your Body’s Response

Cardiorespiratory endurance relies on the strength of the heart muscle and performance of the lungs to withstand the increased load that aerobic exercise requires.  As with other muscles, the heart and lungs need to be stressed to produce positive training changes.  For the cardiorespiratory system, exercise must be appropriately intense to force the aerobic energy system to use oxygen as a source for its energy metabolism.  Aerobic exercise allows no rest periods during the activity, because with rest the energy system would not be forced to use oxygen and would turn to anaerobic (without oxygen) energy sources.  Anaerobic energy sources are what you use for strength training.

   The primary stimulus for a more efficient heart is an increased load on the heart that is proportional to the size of the muscle mass active during training.  As mentioned earlier, typical activities that train the heart muscle usually involved the use of large muscle groups.  Hence jogging stresses the cardiovascular system more than wheelchair rolling because jogging uses larger muscle groups.  Cardiovascular training produces an increased ability to circulate blood from the heart and to obtain a lower resting heart rate.  Basically the heart muscle becomes stronger and more efficient by being able to beat less and pump out more blood for circulation to the lungs and all other body systems.  People with some conditions, such as quadriplegia, may not be able to recruit, or may not have, a sufficient active muscle mass to create an adequate volume load on the heart during aerobic exercise.  Often muscular fatigue of the arms or legs sets in before target heart rates can be achieved.  As a result, central cardiovascular training changes may not occur, but other significant and beneficial peripheral changes do, including increased exercise tolerance, improved muscle endurance due to muscle fiber growth in size (hypertrophy) and their improved ability to extract oxygen from the blood, increased good cholesterol (HDL-C) with a decreased risk of cardiovascular disease, increased peripheral circulation, and strength gains that may allow increased cardiorespiratory endurance.  These peripheral changes help people perform the activities of daily living, such as wheelchair propulsion, that require muscle endurance.  Table 3.1 summarizes the physiological effects of aerobic training and how these effects improve the body’s performance.

Blood Pressure and Heart Rate Changes

Before moving on to a discussion of training modes and exercise intensity, it is important to have a basic understanding of how your heart and circulation system actually work.  This is especially important because you will monitor exercise intensity and progress through blood pressure and heart rate.

   The heart consists of two pumps, the right and left ventricles.  The right ventricle pumps blood to the lungs, creating pulmonary circulation.  The left ventricle pumps blood to all other body systems, creating systemic circulation.  Systemic circulation is accomplished through arteries that carry blood away from the heart and veins that return blood to the heart.

   Blood pressure, or peripheral arterial pressure, measures two separate components, systolic, the lateral pressure of the blood against the walls of the arteries associated with the pumping of blood from the heart, and the diastolic, the arterial pressure during the relaxation phase of the heart cycle as the heart refills with blood that holds the arteries open.  Diastolic pressure is normally lower than the systolic pressure.  Systolic blood pressure increased during aerobic (endurance) exercise, whereas diastolic blood pressure should remain stable or increase only slightly.  The average normal blood pressure for a young adult is 120/80.  The top number represents the systolic blood pressure and the bottom number represents the diastolic blood pressure.

   The heart is able to vary how much blood is actually pumped out to allow it to adjust to different stresses or demands such as exercise.  The amount of blood that the heart pumps out to your circulation is referred to as cardiac output.  Cardiac output is defined as the products of stroke volume and heart rate: Cardiac output = stroke volume (SV) x heart rate (HR).

   Stroke volume is the amount of blood that is ejected with each heart contraction.  The more full the heart is before a contraction the more blood is pumped to the circulatory system (stroke volume).  Of course, the more blood that is pumped out with each heart contraction, the more blood can reach your exercising muscles.  The amount of blood that is returned to the heart can be affected by pooling of blood in inactive extremities, as in people with paraplegia or quadriplegia and some people with hemiparesis following a stroke.  This decrease in stroke volume decreases cardiac output as well as the overall effectiveness of your heart during exercise.

   During exercise, heart rate is also an important factor for increasing cardiac output.  Regulation of heart rate is under the control of the autonomic nervous system, a regulatory control system that governs involuntary organs (visceral) or internal function.  It consists of two branches, the parasympathetic and sympathetic, which normally act in a balanced reciprocal fashion.  Stimulation of the parasympathetic system promotes relaxing and vegetative (resting) functions of the body, maintaining a lower heart rate.  Stimulation of the sympathetic system prepares the body for movement and emergency situations by alerting the body’s organs as needed, elevating the heart rate.  At rest, the parasympathetic branch dominates, and during exercise, the sympathetic branch dominates.  Disruption of the sympathetic system, possibly seen following a spinal cord injury, shifts control of involuntary organs and glands to the parasympathetic system, which modifies the organs’ physiological responses and adaptations to exercise by restricting the maximum heart rate.  This limited, or lower, heart rate also decreases the overall cardiac output (see Table 3.2).

Modes of Aerobic Training

Your mode of aerobic training depends on your available muscles.  If you can walk, you have the same options as the nondisabled population, with modifications for balance and bracing as needed. If you are a wheelchair user, you can participate in modified wheelchair aerobics; upper body ergometry, which is bicycle pedaling with the upper extremity (arm cranking); or wheelchair ergometry, which is pushing a wheelchair on a treadmill or stationary rollers.

   The main advantage of wheelchair ergometry as compared to forearm cycle ergometry is its training specificity for wheelchair users and wheelchair sports.  The disadvantage of wheelchair ergometry is that its net mechanical efficiency is lower than that of forearm cycle ergometry.  Wheelchair ergometry may require more energy than forearm cycle ergometry because neural pathways tend to favor asynchronous over synchronous arm movements, greater isometric muscle activity may be required to stabilize the trunk during application of force to the hand rims, and transmission of forces by a handle is more efficient than by rim pressure.  Consider these differences when starting an aerobic exercise program.  If you have quadriplegia and limited trunk balance or if you have been diagnosed with multiple sclerosis and mechanical efficiency and ease of movement is more important than task-specific wheelchair propulsion, upper body ergometry is an appropriate choice; the proper mode is based on whether you are training for general fitness (upper body ergometry) or sport (wheelchair ergometry).

Monitoring your Exercise Intensity

Target heart rate range and rating of perceived exertion (RPE) are two ways to monitor your aerobic exercise intensity.  They appear to be the most accurate and easy to use measures of exercise intensity for strength and conditioning at home or in a group exercise setting.  Heart rate and RPE are individually specific, and after instruction and guidance in their use you can monitor either indicator independently.  Based on your monitoring, you can control the aerobic intensity of an activity by changing the speed of movement and the amount of resistance.  Table 3.3 relates exercise levels to common physical activities, starting at resting levels and progressing to athletic levels of aerobic conditioning.

Target Heart Rate Range

An easy way to monitor aerobic exercise intensity is to take your heart rate during or immediately after exercise.  As you become more fit, the same speed of movement will elicit a lower heart rate.  This is why it is important to know your target training heart rate range.  This range indicates a progressive adjustment of activity level as you get stronger so you can train your heart safely.  Three methods can be used to monitor heart rate: age-adjusted maximal heart rate, actual tested exercise heart rate, and the Karvonen formula.

Age-Adjusted Maximal Heart Rate.  Your target heart rate is found by taking a percentage of your maximum heart rate according to your fitness level.  Maximum heart rate (max HR) decreases with age and is difference for each individual, but it can be approximated by subtracting your age from 220.  However, if you are a beginner in an exercise program doing only arm work from a wheelchair, one source recommends using 200 as a starting point instead of 220.  Maximum heart rate for individuals with high-level spinal cord injuries (Class 1A, B, C) is an average 20 to 40 beats lower, so people with there injuries should subtract their age from 190.

   Ideally, you should reach 60% of maximum heart rate when you begin aerobic exercise, the lowest point that will adequately stimulate the heart and lungs.  But remember, if you are deconditioned or unable to achieve an elevated heart rate level with starting aerobic exercise (e.g., secondary to medication or muscle fatigue), you will still benefit from the peripheral changes of aerobic activities.  Achieving 75% of the maximum heart rate may require too strenuous aerobic exercise unless you train regularly and are close to your ultimate fitness level.  According to the American Heart Association then, your target heart rate range should be 60% to 75% of your maximum heart rate, and after 6 months or more of regular exercise training you can work up to 85% of your maximum heart rate if you have no heart rate restrictions.

Actual Tested Maximal Heart Rate.  The age-adjusted estimate of maximum heart rate above varies by plus or minus 10 to 15 beats per minute according to two different sources.  A second method for determining target heart rate range takes a fixed percent (60%-85%) of the actual tested maximal heart rate.  This fixed percent depends on your desires intensity and training level.  It is measured by a maximal heart rate test running or pushing on a treadmill or riding or pushing a bicycle ergometer.  Most fitness centers and gyms do not have the equipment to perform maximal exercise testing.  Although it is the most accurate method to base aerobic training on, it is costly and is typically only available in exercise physiology laboratories or research centers.

Karvonen Formula.  A more aggressive method, which is 15% more intense than the age-adjusted heart rate or the actual maximal heart rate calculation, is the Karvonen formula.  This formula takes into account your maximum heart rate and resting heart rate, thus incorporating your aerobic fitness level as you improve with training.  First your heart rate range or reserve (HRR) must be calculated by subtracting the resting heart rate (rest HR), which is taken from an observed exercise test or estimated from the age-adjusted equation.  The heart rate range is then used to calculate the desired percentage or target heart rate (THR):

                                                HRR = max HR – rest HR

                                           % THR = (% x HRR) + rest HR

   This aggressive method of heart rate training can only be used if you have no heart rate restrictions and are training at a consistent level.

   The heart rate ranges in Table 3.4 clearly show the importance of adjusting the conditioning intensity to your fitness level and health status.  This individual should follow the method where the maximal heart rate is actually tested because the age-adjusted and Karvonen formula produce ranges that exceed the actual tested maximal heart rate.  But, realistically, you are not always able to be safely tested and monitored for max HR in the gym or group setting, and the age-adjusted method is often used.  The Karvonen formula may be best if you are an advanced athlete who would like to be more aggressive and progressive as your resting heart rate decreases.  However, the Karvonen formula will be too aggressive if you have sympathetic nervous system involvement and are not able to achieve the maximum heart rates predicted for your age, for example, if you have a complete spinal cord injury about T6 or if you have had a stroke.  If you have a chronic disability such as postpolio syndrome, MS, or a progressive neuromuscular disorder, you may no be able to achieve maximal heart rates because of muscular fatigue.  In this case, an interval approach is recommended with 2 to 3 minutes of aerobic activity followed by a 1-min rest.  You can monitor your exercise by using RPE.

RPE—Rating of Perceived Exertion

If you are not able to meet the target heart rate ranges because of muscle fatigue, if your pulse is difficult to monitor, or if cardiac medication restricts your true heart rate reading (e.g., beta-blockers), then you can modify your exercise intensity by the perceived exertion scale.  Please note that the American College of Sports Medicine (ACSM) recommends blood pressure monitoring for those with fixed heart rates secondary to medication or use of a pacemaker.

   Rating of perceived exertion (RPE) is a technique developed by Borg (1982) to quantify subjective exercise intensity as it relates to the degree of physical strain.  To use the Borg or RPE scale, you select numbers that correspond with your perception of how intense the exercise feels (Table 3.5).  On a scale from 6 to 20, the number 6 is the baseline, which is no exertion.  The top of the scale, number 20, is maximal exertion, the most exhausted you can feel for the exercise being performed.  The scale values correlate to heart rates ranging from 60 to 200 beats per minute for subjects 30 to 50 years old.

   Using RPE, heart rate increases parallel to aerobic exercise (VO₂).  On the original scale by Borg, the numbers 12 to 13 correspond to approximately 60% of the max HR, whereas a number rating of 16 corresponds to approximately 85% of the max HR.  On an updated ratio scale, 4 to 6 corresponds to approximately 60% to 85% of the heart rate range.  If your RPE is in this range, exercise intensity should not exceed the intensity at which you are working.

   According to Borg, the original RPE scale is the best one for exercise testing and for prescribing exercise intensity in sports and medical rehabilitation.  The Borg scale is considered in exercise prescription because it is related to heart rate and integrates other important strain variables.  The new category scale with ratio properties is most often used for determining other subjective symptoms, such as breathing difficulties, aches, and pain.

   Studies have shown that practice with the Borg scale can help you learn the relationship between heart rate and RPE, allowing less frequent monitoring of heart rate and independent use of RPE.

   RPE is a valid and reliable indicator of exercise intensity for the general population, but caution should be used when applying these scales to specific disability populations because they have not been validated with all these groups.  However, the RPE scale can be used as a guide to safely adjust exercise intensity to your own tolerance.

Aerobic Exercise Program Design

Intensity is just one of the variables to control when starting an aerobic exercise program.  The other variables are duration and frequency.

Duration

The recommended aerobic exercise duration is 15 to 60 minutes of continuous activity or a series of work/rest period intervals in which the work time equals 15 to 60 minutes of exercise.

   To significantly increase your aerobic fitness you should exercise a minimum of 20 min at your target heart rate.  However, inactive and untrained individuals should not maintain exercise target heart rates for the entire 20 min when starting an aerobic program.  In the presence of chronic diseases, 20 min at the target heart rate may be even more difficult or unsafe to achieve.  Your initial aerobic exercise duration should be within your tolerance, whether it be 5 min or 50 min.  If you have difficulty maintaining continuous activities, decreased strength may be a major factor, and you should start out with low intervals, for example, 1 min of exercise and 1 or 2 min of rest.

Frequency

The recommended exercise frequency also depends on your starting fitness level, including muscular strength, cardiorespiratory endurance, and how easily you become fatigued.  Three to five exercise periods a week is a common recommendation.  If you are significantly deconditioned, you can benefit from short daily sessions until you can tolerate longer exercise periods.  For example, you could perform three 10-min exercise bouts throughout the day and gradually increase your exercise time to 30 min three times a week if tolerated.  Once again, it is important to work within your limits.  It is desirable to alternate a day of exercise with a day of rest, especially if you are easily fatigued.

Rate of Progression

Each of you will progress at a different rate depending on your starting fitness level and how your disabling condition affects you.  The ACSM breaks the exercise rate of progression into three stages of varying intensity or duration that you can easily modify to fit your needs: the initial conditioning stage, the improvement conditioning stage, and the maintenance conditioning stage.

Initial Conditioning Stage

This is your beginning exercise program.  This stage includes selecting an appropriate exercise mode, whether it is swimming, walking, or wheelchair rolling.  Your program should include light calisthenics and low-level aerobic activities within your exercise tolerance.  Your aerobic activities can last up to 10 to 15 min; however, you may use shorter durations if you have a low tolerance to exercise.  If you are easily fatigued, you may benefit from performing a series of work/rest intervals initially and gradually building yourself up by increasing your exercise time period and keeping your rest period the same.  If you have multiple sclerosis or if you are working with a very limited muscle mass, as can be seen in the muscular dystrophies, the interval approach may allow you to exercise while avoiding fatigue and overtraining.  Your base program should last 4 to 7 weeks before you progress to the next stage.

Improvement Conditioning Stage

The goal of this stage is to progress from short-duration exercise bouts and discontinuous exercise to continuous aerobic exercise lasting 20 to 30 min.  Once again, you need to work within your tolerance.  By this stage, you will have found the exercise mode that best fits your lifestyle and needs.  If overheating or early onset of fatigue is still a factor, you should continue the interval approach but advance by increased the total duration of your work/rest exercise bouts.  For example, you may perform 5-min walks with a 1-min rest period for a total of 30 min and gradually increase your total exercise duration time.

Maintenance Conditioning Stage

This is your long-term training stage.  This stage begins after the first 6 months of training.  By this time, you will have made significant gains in your cardiorespiratory endurance.  Unlike muscular strength, you can lose cardiorespiratory endurance rather rapidly—a few weeks of inactivity can set you back to your starting stages.  So it is important to remain active by performing some type of aerobic activity two or three times a week.  Cross-training may allow you to keep active without boredom from performing the same activity.  Remember, it is much easier to maintain your fitness level than to start over to regain it.

Table 3.1

Physiological Effects of Aerobic Training

Table 3.2

Autonomic Nervous System Responses

Table 3.3

Exercise Levels Related to Common Activities

Table 3.4

Target Heart Rate Range

Table 3.5

Rating of Perceived Exertion (RPE) Scales