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HEALTH & PE

Volume 5, Issue 4, 2021

HEALTH & PE

Volume 5, Issue 4, July 2021

GETTING PUMPED ABOUT EXERCISE AND THE HEART

Dr Angela Spence, Senior Lecturer in Exercise Physiology, Deakin University

The heart is the primary organ responsible for ensuring that enough oxygen and nutrient-rich blood can be circulated around the body to working muscles during exercise.  But does it really matter how fast the heart beats during exercise? Is there an optimal heart rate to exercise at? How and why does the heart have to beat faster during exercise? Let’s explore these questions in more detail.

Figure 1: The heart pumps oxygen and nutrient–rich blood to muscles during exercise.

Introduction

Exercising regularly and maintaining a physically active lifestyle (sitting less and moving more) is crucial for maintaining physical health and reducing risk of developing chronic diseases like diabetes and cardiovascular disease and can improve mental health. However not all exercise is created equally. 

Aerobic exercise, defined as activities that cause breathing and heart rates to increase like brisk walking, running and swimming, are the recommended forms of activity for health benefit. Current guidelines stipulate that we should be doing at least 150 minutes (up to 300 minutes is better) of aerobic exercise per week to stay healthy. That works out to somewhere between 30 to 60 minutes a day, 5 days a week. 

But these guidelines only refer to the type of exercise, not how ‘hard’ the exercise should be or how intensely we should be exercising. To answer this question, it’s first necessary to appreciate the human biology of the heart, lungs and muscles at rest and during exercise.

Figure 2: The heart pumps nutrients and oxygen-rich blood to muscles during exercise.

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The physiology of exercise: How do the heart, blood vessels, lungs and muscles work together?

The heart is arguably the body’s hardest-working organ: it beats every single minute of every single day of our lives, from weeks after conception until the day we die. Indeed, the heart of a fetus starts to contract as early as 16 days after conception. To truly appreciate the complexities of the heart, blood vessels, lungs and muscles during exercise, it is first important to understand the anatomy and physiology of the heart. 

Figure 3: The heart is one big muscle that contracts and relaxes in response to nerve signals from the brain.

The heart is made of muscle

The heart is about the size of your fist, weighs about 300 grams and is located on the left side of your chest. Essentially, it’s a specialised muscular organ, composed of cardiac muscle cells which, like our skeletal muscle cells, contract in response to electrical signals sent from the brain. The main difference between cardiac and skeletal muscle is that cardiac cells contract involuntarily, meaning we don’t need to consciously think about contracting our heart muscles.

When the cardiac muscle cells receive these electrical signals, they contract and then relax. Each contraction and relaxation phase is together referred to as a cardiac cycle or heartbeat and is typically measured per minute, so heart rate is expressed as the number of heart beats per minute (BPM). Heart rate in humans can range from 40 BPM when asleep or resting quietly, increasing up to 200 BPM during very intense exercise. 

The anatomy and function of the heart 

During every beat, the heart fills with blood and pumps it between the lungs and the rest of the body. Four chambers make up the heart’s anatomy: left atria, right atria, left ventricle and right ventricle. The left side of the heart receives oxygen-rich blood from the lungs and pumps it to the body while the right side of the heart receives oxygen-depleted blood from the body and sends it back to the lungs to pick up more oxygen. When you exercise, your heart and breathing rate increase  to allow your body to inhale more oxygen and ensure more oxygen-rich blood is pumped to the exercising skeletal muscles. 

Of the four chambers the heart, the left ventricle is arguably the ‘hardest working’ chamber, as it is responsible for receiving oxygen-rich blood from the lungs and pumping it to the rest of the body. The more intensely we exercise, the faster the heart needs to beat to get enough oxygen to the muscles. Heart rate can also be influenced by emotions such as excitement, stress or fear, and stimulants such as caffeine, and circulating hormones like adrenaline so taking these effects into account is also important when measuring heart rate. 

Heart rate at rest and during maximal exercise

We can measure every time the heart contracts and describe this as heart rate measured in beats per minute (BPM). Normal heart rate at rest varies between 60 – 80 BPM for adults. In highly-trained athletes, resting heart rate can be as low as 40 BPM as the heart becomes more efficient with each beat.

Evidence suggests that long-term exercise training increases the size of the heart, specifically the left ventricle, a phenomenon known as the “Athlete’s Heart”. This adaptation is a normal physiological response to exercise and allows to greater efficiency; the bigger the heart, the more blood that can be pumped with each beat, therefore fewer beats per minute are required to maintain blood flow to the body.

The amount of blood that is pumped out of the left ventricle every beat is called stroke volume. Measured in mL, an average estimate of stroke volume during rest is ~ 70 mL, just less than a standard glass of wine. The volume of blood that circulates the body every minute is termed cardiac output and is determined by multiplying stroke volume by heart rate. As an example, 70 beats multiplied by 70 mL gives 4900 mL or 4.9 litres per minute. The total volume of blood in an average sized adult is approximately 5 litres – this means all the body’s blood circulates every minute!

Figure 4: Use the index and middle fingers to measure your heart rate in beats per minute.

Maximal Heart Rate: What it is and how to calculate it

It’s vital that the heart can beat faster or slower as this allows for close regulation of the blood being pumped to the rest of the body and brain. Maximal heart rate, abbreviated to HRmax, is defined as the fastest rate that the heart is capable of contracting. Again, there is substantial variation in maximal heart rate between individuals and even within the same individual – and genetics may also play a role. 

The only true method of determining HRmax is to conduct a maximal exercise test or VO2max test which is done in an exercise physiology laboratory under the direction of an exercise scientist. This test involves measure the heart rate and breathing response to exercise of increasing intensity until the individual cannot continue. 

As most people will likely never perform these exercises tests, HRmax can also be estimated based on age. As the relationship between maximal heart rate and age is negatively correlated (in both males and females), our ability to generate high maximal heart rates decreases as we get older. Subtracting your age from 220 will indicate your estimated HRmax (HRMax = 220 – Age). Using this method, predicted HRmax for a 45-year-old individual is 175 BPM. 

However, a study done in 2001 revisited this equation by measuring maximal heart rates achieved during maximal exercise tests and found that the 220 – age equation may not be particularly accurate, particularly for people over the age of 40 years. The researchers proposed a revised equation: 208 – (0.7 x Age). Using this equation, that same 45-year-old has a predicted HRmax of 177 BPM. 

However, HRmax is not a major determinant of athletic performance. What is far more important is the body’s efficiency when exercising: being able to run faster at a lower % HRmax is considered efficient. 

Figure 5: Your body is most efficient when you can run faster at a lower maximal heart rate (HRmax).

Exercise intensity: what happens when we go ‘all out’

The primary role of skeletal muscles is to contract and allow joints to move, which essentially enables us to walk, run, swim, or kick a ball. To do this, skeletal muscles need two key ingredients: fuel and oxygen.

The type of fuel muscles need comes from carbohydrates in the foods we eat, which get broken down in the body to their simplest form, which is glucose (C6H12O6). Each muscle cell relies heavily on the blood vessels to carry, transport and deliver the necessary nutrients and oxygen and removing by-products such as carbon dioxide. The harder and more quickly the muscles contract, for example during very intense exercise, the more blood is distributed towards metabolically active tissues and away from competing organs like the digestive system. 

When the intensity of the exercise is very high, the muscles start to produce another type of by-product called lactic acid which quickly converts to another molecule called lactate. The body is extremely efficient at managing these products, and so they can be sent to the liver and reconverted into glucose to be reused as fuel. 

However, if the rate of production exceeds the rate of removal and these by-products start to accumulate in the muscle, this can interfere with muscle contraction, meaning that we either must reduce the intensity of exercise or stop exercising completely. In physiology, the point at which this by-product starts to accumulate is termed the ‘lactate threshold’. Any exercise intensity that we can comfortably sustain is usually below this threshold and will have an accompanying heart rate. Since it is much easier to measure heart rate than lactate production, we can use heart rate as a surrogate measure of exercise intensity. 

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Measuring your heart rate: which device is best?

With the ever-increasing technological advancements and popularity of smart-watches, devices and apps, measuring heart rate during exercise as never been easier. Continuously measuring heart rate using wrist-based heart rate monitors, like those made by Apple, Garmin and FitBit, provides fitness-enthusiasts with data such as average, target and peak heart rates. But these devices can be very expensive, and using  the easy palpation method can be just as effective for providing an indication of your heart rate before, during or after exercise.

Figure 6: There are lots of devices now to measure heart rate during exercise but measuring it yourself can be just as effective.

New versus older technologies

The gold-standard for determining heart rate is by measuring the cardiac electrical signals and conductivity using an ECG (electrocardiogram). Heart rate monitoring devices that use a chest-strap also use this technology and measure the hearts electrical activity at the chest. More recently, the surge in popularity of wearable technologies like smart watches makes heart rate measurement simple, but not always that accurate. These devices often use green LED light sensors that reflect against blood flowing through capillaries near the surface of the skin on the wrist. Using algorithms and equations, an estimation of heart rate is given based on peripheral blood flow. This technology, called photoplethsmography (PPG) has been around for some time, and is useful for those individuals wanting to get an estimation of heart. However, there is still some way to go in terms of the accuracy of these devices compared to traditional chest-strap monitoring.

The biggest issues with getting an accurate result occur when the device is not fitted well and so cannot get an optimal signal. Also, these devices are often placed on the wrist, which move during exercise like running and walking, and the movement can result in errors. People with darker skin colour, tattoos or darker skin markings may also experience errors when using these devices as the LED light sensor cannot detect blood flow very well.

What heart rate should I be exercising at?

Determining an optimal heart rate for exercise depends on the goal of the exercise, your age and how fit you already are. Exercising at a maximal heart rate for every single exercise session will not produce efficient fitness results. Often, you will have to stop exercising before you have a chance of gaining any benefit. While interval-style exercise training is a popular choice for time-poor individuals, the intermittent nature of the exercise means that heart rate will fluctuate. This type of exercise involves alternating intense exercise efforts with recovery efforts, usually lasting 45-60 seconds for each. Overall, the average sessional heart rate will be elevated to transfer the health benefits. 

Training to improve sport performance

From an athlete training perspective, longer exercise efforts, like running or cycling, can benefit from a more scientific approach. Exercising at specified intensities that are known to elicit adaptive responses, for example exercising at intensities at or below the lactate threshold, are necessary when prescribing training. These intensities are associated with heart rates and are referred to as training zones, expressed relative to HRmax. A light aerobic training session would be prescribed at or below 75% HRmax, while training at or just below the lactate threshold will induce physiological change. 

Is exercising at maximal heart rates unsafe?

The cardioprotective benefits of regular exercise are well established, yet evidence is emerging that more exercise is not necessarily better when it comes to heart health. Likewise, the likelihood of experiencing a heart attack during exercise is heightened for sedentary individuals who are unaccustomed to high-intensity exercise and those that have an underlying cardiac condition. With a third of Australians not meeting the WHO recommended guidelines of accumulating 150 minutes of exercise per week, encouraging regular physical activity continues to be a pervasive public health message. For most adults, the risk from not doing enough exercise is far greater than doing too much exercise. Undergoing an exercise pre-screening assessment with a university-qualified exercise-specialist will be able to assess and mitigate the risk of doing exercise. 

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How Fit are You? Estimating Maximal Oxygen Consumption (VO2Max)

What is the test?

Fitness is important to measure as this can provide information can help to create an exercise program to improve your fitness or sporting performance and give an idea about how healthy you are. 

Step tests are a submaximal exercise test which involves stepping on and off a raised step. Submaximal intensity means that you may feel your heart rate and breathing rate increase, and you could feel hot and maybe a bit sweaty. All of these experiences are normal when exercising, but the test should not feel exhausting.

By measuring a person’s heart rate before and especially after the exercise test, this allows us to easily determine someone’s fitness level. It also means we can do it without needing expensive equipment; a stopwatch, a step, and a metronome* are all that you need.

How does this test work?

Muscle cells need oxygen to exercise, and oxygen gets to the cells by travelling via red blood cells (erythrocytes) in arteries that is pumped by the heart. We know that when we exercise more intensity, our cells need more oxygen, so our heart must beat faster to make sure enough blood (and the oxygen it is carrying) is pumped to those muscles. We can therefore say that there is a linear relationship between heart rate and oxygen consumption. As the need for oxygen increases, so does the heart rate. The results of this test are based on heart rate measured after exercise and fitter people will generally have a lower heart rate compared to less-fit people.

*a metronome is a device that produces a sound at a regular interval that can be set by the user and is usually measured in beats per minute (BPM). There are free smartphone metronome apps that you can download or using Google [hyperlink https://g.co/kgs/zYMHb9

ACTIVITY 1: Draw a line graph

Using the graph paper below, create a line graph using the data from Table 1, where oxygen consumption is the x-axis variable and heart rate is the y-axis variable. The unit of measure for oxygen consumption is litres per minute (L/min) and heart rate is beats per minute (BPM). Oxygen consumption can also be described relative to a person’s body mass (mL per kg per min). 

Table 1: Oxygen consumption and heart rate during exercise
Oxygen Consumption (L/min)Heart Rate (BPM)
0.589
0.7112
1.0121
1.3129
1.4132
1.6148
1.8154
1.9161
2.0169
2.3178

ACTIVITY 2: Complete the Test

Test Equipment

  • Find a step or bench that is 41.25 cm high
  • Stopwatch
  • Metronome
 
 

Test Method

You may want to ask someone to help you with this test. You will need to take measures before and after doing it. Make sure that you read theses instructions the whole way through before starting the test.

  1. Measure your heart rate by using the palpation method at your wrist (radial pulse) and record this result in beats per minute (BPM) in the table below. (Find your pulse and count the number of beats in 15 seconds and multiply this number by 4.)
 
 
  1. Practice the stepping action for the test which requires you to step up and down on the step using a four-step count, i.e. ‘up left-up right-down left-down right’. For safety, make sure you step into the centre of the step every time.
 
 
  1. The test is 3 minutes long
 
  1. Set the correct metronome pace:
    1. Males: 96 BPM on the metronome (24 steps per minute)
    2. Females: 88 BPM on the metronome (22 steps per minute).
 
 
  1. Get ready, make sure the stopwatch and metronome are set and go! If you’re working with a partner, some motivation and encouragement is helpful. 
 
 
  1. Be sure to maintains the pace with the metronome throughout the whole test. Sometimes calling with the rhythm of “up-two-down-four”, “up-up-down-down” or “up-together-down-together” in time to the metronome can help to keep the correct pace and rhythm.
 
 
  1. As soon as the test is finished, stop stepping immediately. You need to get ready to measure your HR 5 seconds after the test has finished. This is the recovery heart rate. Start counting the number of beats 5 seconds post-test and continue for 15 seconds (i.e. between the 5th and 20th second of recovery).  
 
 
  1. Measure heart rate by gently feeling for the radial pulse and counting the beats for 15 seconds. Multiply the number by 4 to get the number of beats per minute and round up to the nearest whole number. Record your results in the table.

ACTIVITY 3: Record your data

Pre-Test Heart RatePost-Test Heart RatePredicted VO2max
(mL∙kg∙min-1)
Rating
BPMBPMCalculated using the equation From normative data table below




ACTIVITY 4: Solve an equation

Predicting VO2max using an equation

An estimation of VO2max (maximal oxygen consumption) can be calculated from your test results using the equation provided below. Substitute the measure you recorded for the Post-Test Heart Rate. You can compare your results to normative data of other individuals of your same age and sex. Record the rating in the table.

SexPredicted VO2Max (mL per kg per min)
Males111.33 – (0.42 x Post-Test HR)
Females65.81 – (0.1847 x Post-Test HR)

ACTIVITY 5: Compare your data

Compare your results to others using the normative data provided in the table below. Don’t worry if your result is below average or poor. All this means is there is room to improve your fitness levels. Well done if your results are excellent or above average – keep up what you’re doing as it’s helping to keep you fit!

Maximal Oxygen Uptake Norms for Men (mL per kg per min)

 17-25 yr26-35 yr36-45 yr46-55 yr56-65 yr65+ yr
Excellent> 60> 56> 51> 45> 41> 37
Good52-6049-5643-5139-4536-4133-37
Above Average47-5143-4839-4235-3832-3529-32
Average42-4640-4235-3832-3530-3126-28
Below Average
37-4135-3931-3429-3126-2922-25
Poor30-3630-3426-3025-2822-2520-21
Very Poor< 30< 30< 26< 25< 22< 20

Maximal Oxygen Uptake Norms for Women (mL per kg per min)

 17-25 yr26-35 yr36-45 yr46-55 yr56-65 yr65+ yr
Excellent> 56> 52> 45> 40> 37> 32
Good47-5645-5238-4534-4032-3728-32
Above Average42-4639-4434-3731-3328-3125-27
Average36-4135-3831-3328-3025-2722-24
Below Average
33-3731-3427-3025-2722-2419-22
Poor28-3226-3022-2620-2418-2117-18
Very Poor< 28< 26< 22< 20> 37< 18< 17

References

Spence, A. L., Naylor, L. H., Carter, H. H., Buck, C. L., Dembo, L., Murray, C. P., Watson, P., Oxborough, D., George, K. P., Green, D. J. (2011). A prospective randomised longitudinal MRI study of left ventricular adaptation to endurance and resistance exercise training in humans. The Journal of physiology, 589(Pt 22), 5443-52.

Fox SM, III, Naughton JP, Haskell WL. Physical activity and the prevention of coronary heart disease. Ann Clin Res. 1971;3:404–432.

Tanaka H, Monahan KD, Seals DR. Age-predicted maximal heart rate revisited. J Am Coll Cardiol. 2001;37:153–156.

Sarzynski, M. A., Rankinen, T., Earnest, C. P., Leon, A. S., Rao, D. C., Skinner, J. S., & Bouchard, C. (2013). Measured maximal heart rates compared to commonly used age-based prediction equations in the Heritage Family Study. American journal of human biology: the official journal of the Human Biology Council, 25(5), 695-701.

Calcium handling precedes cardiac differentiation to initiate the first heartbeat (2016) Richard CV Tyser, Antonio MA Miranda, Chiann-mun Chen, Sean M Davidson, Shankar Srinivas, Paul R Riley  eLife 2016;5:e17113

Wallen MP, Gomersall SR, Keating SE, Wisløff U, Coombes JS (2016) Accuracy of Heart Rate Watches: Implications for Weight Management. PLOS ONE 11(5): e0154420. https://doi.org/10.1371/journal.pone.0154420

Foster, C., Farland, C. V., Guidotti, F., Harbin, M., Roberts, B., Schuette, J., Tuuri, A., Doberstein, S. T., Porcari, J. P. (2015). The Effects of High Intensity Interval Training vs Steady State Training on Aerobic and Anaerobic Capacity. Journal of Sports Science & Medicine, 14(4), 747-55.

Selland CA, Kelly J, Gums K, Meendering JR, Vukovich M (2021). A Generalized Equation for Prediction of VO2peak from a Step Test. Int J Sports Med. 42(9):833-839. 

Rapp D, Scharhag J, Wagenpfeil S, Scholl J. Reference values for peak oxygen uptake: cross-sectional analysis of cycle ergometry-based cardiopulmonary exercise tests of 10090 adult German volunteers from the Prevention First Registry. (2018) BMJ Open. 5;8(3):e018697. 

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