Aerobic Exercise Prescription for Optimum Health Benefits

Introduction

In my post, Health Benefits of Exercise: a grossly underutilised therapy, I have discussed the various health benefits associated with physical activity, as a result of which exercise is now aptly being recognised as ‘medicine’. However, to derive optimal health benefits from exercise, an exercise program needs to follow certain parameters, similar to drug prescription, such as frequency, dose (amount), time (duration) and type (of physical activity). In my post Exercise Prescription for Optimal Health Benefits, I have discussed the FITT-VP principles for exercise prescription. In this post, I shall be discussing ‘prescription for aerobic exercise’ based on the FITT-VP principles. Prescription for strength training as per FITT-VP factors will be discussed in the next post. Aerobic exercise is referred to in the literature, both scientific and lay, by various terms. In the scientific literature, it is referred to as cardiorespiratory endurance or aerobic endurance, while colloquially it is referred to as aerobic exercise or just aerobic, or cardio, or cardio exercises, or cardio workout. In this post, these terms will be used interchangeably.

People engaging in various aerobic activities.

In my post on Health Benefits of exercise: a grossly underutilised therapy, referred to above, I have discussed the various components of physical fitness (viz. cardiorespiratory or aerobic endurance, muscular fitness, flexibility and body composition), and the benefits of these on health. The post also gives a detailed explanation of the term cardiorespiratory endurance or aerobic endurance. However, in simpler terms, cardiorespiratory endurance or aerobic endurance can be described as the ability of the cardiovascular and respiratory systems to supply oxygen and other nutrients continuously during sustained physical activity.  As is evident from the health benefits of aerobic exercises, discussed in my post, Health Benefits of exercise: a grossly underutilised therapy, referred to above, aerobic exercises are one of the essential components of physical fitness, which can help you maintain good health.

Aerobic exercise prescription based on FITT-VP factors

Frequency of aerobic exercise

For optimum health and fitness benefits, cardio workouts must be done 3 to 5 days per week. The frequency varies with the intensity; while moderate intensity cardio exercises must be done at least 5 days per week, vigorous-intensity aerobic exercises provide similar health and fitness benefits even when done on 3 days per week. Alternatively, a combination of moderate and vigorous intensity aerobic exercises may be done on 3-5 days per week.

While aerobic exercise frequency less than 3 days per week does not result in optimum benefits, on the other hand vigorous intensity cardio workouts performed on more than 5 days per week might increase the risk of fatigue, overtraining and musculoskeletal injuries. Infrequent training, like weekend-warriors, especially when engaging in unaccustomed vigorous exercise, also increases the risk of musculoskeletal injuries and/or adverse cardiovascular events associated with physical activity.

The intensity of cardio exercise

In simple terms, the intensity of exercise means how hard a person works to do the activity. There is a dose-response relationship between the aerobic exercise intensity and the resulting health and fitness benefits, i.e. the health and fitness benefits increase with increasing aerobic exercise intensity. The higher health and fitness benefits as a result of higher aerobic exercise intensity are probably a result of a greater increase in aerobic capacity resulting from increased aerobic exercise intensity. Also, according to the principle of individual difference, all individuals do not respond similarly to a given training stimulus. The intensity required to produce improvement will also vary depending on factors such as the physical fitness status of an individual at the time of starting a training program. While individuals with a higher baseline fitness will need higher intensity aerobic exercise (maybe even close to maximal), to further improve, habitually sedentary individuals with low levels of fitness will show improvement even with seemingly low-intensity aerobic exercise. Therefore, it may be difficult to precisely lay down universally accepted guidelines for exercise intensity prescription to improve aerobic (cardiorespiratory) fitness.

Methods for measuring the intensity of aerobic exercise

 There are several methods available for prescribing exercise intensity for improvement in aerobic fitness. Methods for monitoring intensity of cardio exercises fall into two basic categories – laboratory methods and field methods. Use of laboratory methods is impractical for application in field settings, for the general population exercising for fitness and health benefits, and hence will not be discussed here.

The common, more easily measurable field measures of aerobic exercise intensity include heart rate, metabolic equivalent of task (METs), Talk Test and Rating of Perceived Exertion (RPE).

Cardio Exercise prescription based on metabolic equivalent of task (METs)

MET is an absolute measure of intensity, commonly used to assess and prescribe cardio exercise intensity. It is based on multiples of resting metabolism denoted by METs (metabolic equivalents). One MET is the rate of energy expenditure while sitting at rest. Light-intensity aerobic activity is an activity done at 1.1 to 2.9 METs (i.e. energy expenditure is 1.1 to 2.9 times the energy expenditure while sitting at rest), moderate intensity aerobic activity is an activity done at 3 to 5.9 METs while vigorous aerobic activity is an activity done at > 6 METs. Various organisations have drawn out compendiums giving MET equivalents of common aerobic activities. These tables can be used to assess the approximate intensity of exercise. However, aerobic exercise prescription based on MET values has certain limitations and at best they give only an approximate energy cost of a particular physical activity. More importantly, the concept of METs is difficult to understand and few people are familiar with it.

Aerobic Exercise prescription based on heart rate

Monitoring your heart rate is an objective, practical and yet one of the most accurate way to gauge the intensity of your cardio workouts; it is the best indicator of how hard your body is working out during a cardio workout. To use this method, you first need to determine your ‘age-related maximum heart rate’ (HRmax), i.e. the highest number of times your heart can beat per minute when exercising at your maximum effort. In clinical practice, HRmax is commonly estimated using the formula ‘220-Age’. However, though simple to use, this formula is not perfectly accurate for all people; there is a considerable variability (Standard Deviation of + 10 to 12 beats per minute) associated with this estimate. Some studies have shown that this equation overestimates HRmax in young adults while underestimating in the older adults, thus impacting both the assessment of the aerobic exercise intensity as well as prescription of exercise intensity for aerobic exercises, based on heart rate.

Presently, a multitude of HRmax regression equations, also known as HRmax prediction equations exist. Dr Hirofumi Tanaka and colleagues determined one such equation through a meta-analysis utilising healthy adults, who were non-smokers and not on any medications. The equation has been found to better predict HRmax compared to the traditional equation. The equation is: HRmax = 208 – (0.7 X age).

The accuracy of heart rate methods for aerobic exercise prescription can be further modulated and refined by your subjective endpoints of exercise, such as breathlessness and/or fatigue level, i.e. perceived level of exertion and affective valence (intrinsic attractiveness/‘good’-ness [positive valence] or averseness/‘bad’-ness [negative valence] of an event, object or situation). Studies have shown that your perceived exertion compares well with your heart rate. So, if you think you are working hard, your heart rate is probably higher than usual. These methods based on the subjective endpoints of exercise will be discussed in the next section.

Though, for greater accuracy in determining cardio exercise intensity for the exercise prescription, direct measurement of HRmax should be used, when feasible. HRmax can be measured directly using a graded maximal exercise test. The test is preferably done in a laboratory using a treadmill or a cycle ergometer. However, there are several limitations, both in terms of availability of test facility for direct measurement of HRmax as well as in terms of individual’s willingness to participate in the graded maximal exercise test. In view of the above, estimation of HRmax using HRmax prediction equations, and refining it with subjective measures of perceived exertion and breathlessness, is an acceptable and recommended method in individuals exercising for health and fitness.

While using heart rate for aerobic exercise prescription, the intensity of aerobic exercise can be expressed as a percentage of a person’s maximum heart rate (%HRmax) or heart rate reserve (%HRR), which is the difference between the maximum heart rate and the resting heart rate (RHR). Of the two approaches, the latter method is more accurate as the resting heart rate varies from person to person (can vary from 60 to 100 beats per min), and thus the HRR gives individual specific heart rate targets for training taking into consideration each individual’s resting heart rate. Various factors, such as the individual’s fitness levels and certain drugs such as beta blockers significantly lower resting heart rate. A lower resting heart rate is common for people who engage in a lot of physical activity or are very athletic. Intensive, prolonged, and repetitive endurance and strength training leads to a constellation of normal physiologic structural and functional changes in the heart which facilitate the heart to work more efficiently. These changes are collectively known as athlete’s heart. Though the changes will depend on the type of training (aerobic or strength training), in endurance trained (the term endurance training generally refers to training the aerobic system as opposed to anaerobic) individuals, the main changes include increased muscle mass and chamber size of the left ventricle (Left lower chamber of the heart which receives oxygen rich blood from the lungs and pumps it to the whole body). As a result, a larger and stronger heart now pumps out larger volume of blood in a single beat, which allows the heart to circulate same amount of blood with fewer contractions per minute. Thus, low resting heart rate is a prominent feature of well-trained athletes, especially endurance trained athletes.  A low or moderate amount of physical activity doesn’t usually change the resting pulse much. To get the most accurate reading, count the number of beats in 60 seconds.

Target Heart Rate (THR) for prescribing intensity of exercise training is usually determined as a range, laying down a lower limit and an upper limit of desired intensity range. Though physical activity intensity is classified into 5 zones (very light, light, moderate, vigorous/hard, very vigorous/very hard), from a public health perspective, to simplify physical activity recommendations, it suffices to classify them more broadly into light, moderate and vigorous.

Intensity %HRR %HRmax
Light 20 – 39 50 – 63
Moderate 40 – 59 64 – 76
Vigorous 60 – 84 77 – 93

The relative training intensity (i.e. light, moderate or vigorous) for an individual is determined based on age, fitness and health status. For older and less fit people, exercise training begins at light levels and follows the principle of ‘start low and go slow’. When people commit to starting an exercise program they are often so pumped up and excited that they do ‘too much too soon’, which may lead to various health risks, such as musculoskeletal injuries.

Calculation of THR based on HRmax method

 In this text, for all calculations of THR, the equation derived by Tanaka et al will be used. However, you may choose to use the traditional equation (HRmax = 220 – Age).

Step – 1

             Determine your age-related maximum heart rate using the equation:

             HRmax = 208 – (0.7 × age)

Step – 2

             Determine the ‘Target Heart Rate (THR):

             THR = HRmax × % training intensity desired

(For calculation of the lower and upper limit of THR, the calculation using the above formula needs to be repeated twice – once for the lower limit of desired intensity range and once for the upper limit of the desired intensity range). 

Example

For the purpose of illustration, we take the example of a 40 years old person, who wants to figure out his/her target heart rate zone for moderate intensity training based on HRmax method (i.e. 64 – 76%).

Step – 1

 Determine age-related maximum heart rate, by substituting the age in the equation:

             HRmax = 208 – (0.7 × 40)

                         = 208 – 28

                         = 180

Step – 2

Determine the THR by substituting the appropriate values in the equation; determine THR for both the lower and upper limit of the desired training intensity.

             Lower limit – HRmax × 64%  

                                    180 × 64 ÷ 100

                                 =  180 × 0.64     

                                 =  115.2

             Upper limit – HRmax × 76%

                                   180 × 76 ÷ 100

                                 = 180 × 0.76

                                 = 136.8

So, the THR for this 40 years old person, wanting to train at moderate intensity would be 115 to 137 beats per minute, according to the HRmax method.

Calculation of THR based on HRR method

Step – 1

             Determine age-related maximum heart rate, using the equation:

             HRmax = 208 – (0.7 × age)

Step – 2

             Determine your resting heart rate (HRrest) by counting your pulse rate for one minute, preferably first thing in the morning.

Step – 3

             Calculate your heart rate reserve (HRR)

             HRR = HRmax – HRrest

Step – 4

             Determine your target heart rate.

             THR = HRR × % of training intensity desired + HRrest

(As in the HRmax method, for calculation of the lower and upper limit of THR, the calculation using the above formula needs to be repeated twice – once for the lower limit of desired intensity range and once for the upper limit of the desired intensity range). 

Example

 For the purpose of illustration, we will again take the example of a 40 years old person, who has a resting heart rate of 70 beats per minute and who wants to figure out his/her target heart rate zone for moderate intensity training based on HRR method (i.e. 40 – 59%)

Step – 1

Determine age-related maximum heart rate, by substituting the age in the equation:

             HRmax = 208 – (0.7 × 40)

                         = 208 – 28

                         = 180

Step – 2

The given resting heart rate is 70 beats per minute.

Step – 3

Determine your HRR by substituting the appropriate values in the equation.

             HRR = 180 – 70

                      = 110

Step – 4

Determine the THR by substituting the appropriate values in the equation; determine THR for both the lower and upper limit of the desired training intensity.

             Lower limit – 110 × 40% + 70

                                  = 110 × 40 ÷ 100 + 70

                                  = 110 × 0.4 + 70

                                  = 44 + 70

                                  = 114

             Upper limit – 110 × 59% + 70

                                  = 110 × 59 ÷ 100 + 70

                                  = 110 × 0.59 + 70

                                  = 64.9 + 70

                                  = 134.9

 So, the THR for this 40 years old person, wanting to train at moderate intensity would be 114 to 135 beats per minute, according to the HRR method.

Though the THR ranges in the above examples for HRmax and HRR methods are similar, however, as the normal resting heart rate can vary from 60 to 100 beats per minute (in some elite sportsman, resting heart rates can be much lower than 60; elite champion cyclist Miguel Indurain of Spain, in his prime, had a resting heart rate of just 28 beats per minute and Bjorn Borg, former world No. 1 tennis player, in his prime had a resting heart rate of 36 beats per minute), for resting heart rate lower or higher than 70, the THR values will change significantly in the HRR method.

How to find out if you are training in your target heart rate zone?

To check if you are in your target heart rate zone, follow the steps to check your heart rate during the exercise:

  • Stop momentarily to check your heart rate, 5 minutes after the start of your aerobic exercise program session and check it again before you go into cool-down. Your heart rate will drop when you stop exercising, so it is important to check your pulse quickly if using the manual method, counting for only about 15 seconds. You will need a timing device that shows time in seconds.
  • To measure your heart rate, simply check your pulse. The best places to find your pulse are wrist, inside of your elbow, and side of the neck. To check your pulse at your wrist, place two fingers between the bone and the tendon over the radial artery — which is located on the thumb side of your wrist. To check your pulse at the inside of the elbow, place two fingers on the inside of your elbow while keeping the arm straight. To check your pulse at the side of the neck, place your index and third fingers on your neck to the side of your windpipe, over the carotid artery, just below the jaw angle (angle of the mandible or the gonial angle). Though you may need to stop to locate your pulse, however, once you are able to locate it, try to keep walking slowly or marching in place to keep it from slowing at a fast rate.
  • Multiply this number by 4 to calculate your heart beats per minute. For e.g., you stop exercising once you are about 5 minutes into your aerobic exercise session and count your pulse for 15 seconds and your count is 26. This number multiplied by 4 gives you your heart rate per minute, which is 104 in this example. So for the person in the above two examples of calculation of THR, this heart rate is below the calculated THR. Therefore this person needs to increase his exercise intensity to get into the appropriate THR zone and then maintain that pace for the required duration of the aerobic exercise.

However, the use of technology has made monitoring of THR easy. Heart Rate Monitors and smartwatches allow you to monitor your heart rate without interrupting your cardio workout. Advanced models even allow you to set a target heart rate zone and provide a visual and/or auditory alert when you are outside your THR zone.

Talk Test

Research has shown talk test is a valid, reliable, practical and inexpensive tool for prescribing and monitoring aerobic exercise intensity across a range of populations including competitive athletes, healthy active adults and patients with cardiovascular disease.

As a rule of thumb, a person doing moderate-intensity aerobic exercise, can talk, but not sing, during the activity. However, a person doing a vigorous-intensity aerobic exercise cannot say more than a few words without pausing for a breath. The goal of the talk test is to select intensity where ‘comfortable speech is just possible’. Keeping in view its close correlation with heart rate, the talk test can be used in a manner similar to heart rate monitoring. After a 5- to 10-minute warm-up gradually increase your walking or jogging speed, depending on your fitness level. After about 5 minutes into the aerobic exercise session, after the warm-up, recite something you know really well (for e.g. a popular rhyme) while continuing to exercise, and subjectively assess the level of speech difficulty associated with that intensity of exercise. When comfortable speech is just barely possible, the aerobic exercise intensity is within the suggested guidelines for aerobic exercise prescription. However, when comfortable speech is no longer possible, the aerobic exercise intensity is greater than what is normally recommended, and if you are able to sing, aerobic exercise intensity is below than what is normally recommended. Adjust your aerobic exercise intensity accordingly, so as to select an intensity where comfortable speech is just possible and maintain that pace for the duration of the exercise. The talk test can be recommended as an effective primary method for prescribing and monitoring aerobic of exercise intensity.

Rating of Perceived Exertion

Rating of Perceived Exertion (RPE) is an index of how hard the person feels he or she is exercising. When you are exercising, your heart beats faster, your breathing becomes faster and deeper, you work up a sweat and your muscles begin to tire and complain. These feelings are not as objective as when you actually measure your heart rate, but they can give you an estimate of your aerobic exercise intensity zone.

To rate your perception of your exertion when you are exercising, pay close attention to how hard you feel the exercise work rate is. This feeling should reflect your total amount of exertion and fatigue, combining all sensations and feelings of physical stress, effort and fatigue. Don’t concern yourself with any one factor such as leg pain, shortness of breath, or aerobic exercise intensity, but try to concentrate on your total inner feeling of exertion. Try not to underestimate or overestimate your feelings of exertion; be as accurate as you can. The original Borg Scale or Category scale (6 to 20 scale) developed by Swedish researcher Gunnar Borg, is commonly used for assessment of perceived exertion. This scale matches how hard you are working with numbers from 6 to 20, where 6 means ‘no exercise at all’ and 20 means ‘maximal exertion’. When a measurement is taken, a number is chosen from the scale by an exercising individual that best describes his/her perceived level of exertion during the aerobic exercise.

Rating of Perceived Exertion (RPE) Original Borg Scale
Numerical Borg rating of your exertion How you might describe your exertion
6 No exertion at all
7 Very, very light
8  
9 Very light
10  
11 Fairly light
12  
13 Somewhat hard
14  
15 Hard
16  
17 Very hard
18  
19 Very, very hard
20  

The scale unusually starts at 6, because it is designed to give you a fairly good estimate of your heart rate during aerobic exercise. For a particular level of activity, heart rate can be estimated by multiplying the numerical rating for that particular level of exertion, by 10. For e.g. the numerical rating of 15, heart rate will be approximately 150 (15 × 10 = 150) beats per minute.

 RPE scale is used in a manner similar to the Talk Test. Here, instead of assessing the level of speech difficulty associated with a particular level of exercise, you assess your RPE from the scale.  The major drawback of the RPE method is that the scale ideally needs to be visible (or memorised) to the individual while exercising, for the rating to be accurate.

Time (i.e. Aerobic exercise duration)

Aerobic exercise time (or duration) prescribes the length of time in which aerobic physical activity or exercise is performed. Aerobic exercise duration will vary according to the intensity – longer for low-intensity and shorter for high-intensity aerobic activity. As per guidelines recommended by WHO in its Global Recommendations on Physical Activity for Health, for children and adolescents (5-17 years old), 60 minutes of moderate to vigorous intensity aerobic physical activity is recommended to be done daily. Cardio workouts longer than this will provide additional health benefits. Keeping in view the rate at which childhood obesity is increasing worldwide, promotion of physical activity among children is very important. The cardio workout need not be completed in a single bout, instead 60 minutes can be ‘accumulated ‘ in several bouts of at least 10 minutes duration each, throughout the day. For children and adolescents aerobic physical activity includes play, games, sports, active transportation (walking, cycling etc), recreation, physical education or planned exercise.

For adults (18 years and above) and older people (65 years and above), 150 minutes of moderate-intensity aerobic physical activity throughout the week or 75 minutes of vigorous-intensity aerobic physical activity throughout the week or an equivalent combination of moderate- and vigorous-intensity cardio workout is recommended. Higher amounts (up to 300 minutes of moderate-intensity aerobic physical activity or up to 150 minutes of vigorous-intensity aerobic physical activity) of aerobic exercise will provide additional health benefits. For adults aerobic exercises include recreational or leisure time physical activity, active transportation, occupational (i.e. work related), household chores, play, games, sports or planned exercise. If the fitness status or time constraint doesn’t allow continuous cardio workout of 30 minutes or more, aerobic exercise can be accumulated in bouts of at least 10 minutes duration. For the older people, when they cannot do the above recommended amount of cardio exercises due to health conditions, they should be physically active as their abilities and conditions allow.

Types of Aerobic Exercise

An aerobic exercise may be defined as any activity that uses large muscle groups, can be maintained continuously, and is rhythmical in nature. In general, aerobic exercises requiring little skill to perform and which can be easily modified to accommodate individual fitness levels are recommended for all adults to improve cardio fitness. Examples include brisk walking, leisure cycling, swimming, aqua aerobics, and slow dancing. Adults who are habitually physically active and have at least average physical fitness, can perform higher intensity aerobic exercises, including jogging, running, aerobics, spinning (indoor cycling), stepping exercise, elliptical exercise and fast dancing. Recreational sports such as racquet sports, basketball, soccer, handball and hiking etc can also improve physical fitness; however, because of their intermittent nature, these are generally recommended as ancillary physical activities, performed in addition to the above recommended physical activities.

Exercise Volume

Exercise volume (also known as quantity or dosage of aerobic exercise) is the product of the frequency (F), intensity (I), and Time (T), of the exercise performed or FIT factors of the core principles of exercise prescription i.e. FITT principles discussed above. Calculation of aerobic exercise volume involves some complicated calculations and for the general population engaging in cardio exercises for health and fitness benefits, it suffices to express the amount of aerobic exercise in simpler terms of minutes of moderate- and vigorous-intensity activity as discussed above.

Rate of progression

According to the principle of progression, the workload should be increased progressively or gradually for improvements to continue while taking care to minimise the risk of injuries and overtraining. However, the recommended rate of progression for aerobic exercise program depends on the individual’s health and fitness status, goals and objectives of the exercise program and individuals’ adaptation and response to an exercise program. Progression may be achieved by manipulating any of the components of the FITT principles of exercise prescription, as tolerated by the individual. When the aerobic exercise program is begun, it is prudent to follow the principle of “start low and go slow.” It not only safeguards you against the risk of musculoskeletal injuries and adverse cardiovascular events but also enhances adoption and adherence to exercise program.

A recommended sequence of progression of an aerobic exercise program is as under:

  1. Increase duration – In currently physically inactive individuals, begin by exercising at a light intensity, on alternate days, and then gradually start increasing time/duration of the exercise session. In an average adult, increasing exercise session by 5 to 10 minutes every 1 to 2 weeks over the first 4 to 6 weeks is reasonable.
  2. Increasing frequency and intensity – After the individual has been exercising regularly for about 4 to 6 weeks, start increasing the frequency and intensity of the aerobic exercise session, as tolerated, over the next 4 to 8 months to achieve the recommended guidelines.

Following an increase in any of the FITT components, individual should be monitored for any adverse effects of the increased volume such as muscle soreness and fatigue; downward adjustment should be made if the exercise is not well tolerated. Always listen to your body; it is a good guide which enables you to progress at the right pace.

By Dr Yatendra Kumar Yadav

Lifestyle Medicine Consultant and Sports Medicine Specialist who actually lifts.

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