USING HEART RATE AS A TOOL TO GAUGE EXERCISE INTENSITY
Jeremy Bamford
Edmonton, Canada
Abstract
Using heart rate as a tool to gauge exercise intensity, J. Perf. Enhan. 1999 1(1):21-30. To achieve optimal results, athletes must train at an intensity appropriate to their goals. Intensity is important because too low an intensity will not produce optimal results while too high an intensity can lead to overtraining and injury. In order to maximize training benefits an athlete needs a practical and accurate system to gauge exercise intensity. Heart rate measurement can be used for this purpose. In order to be accurate the athlete must choose a system which directly measures the electrical activity of the heart at the chest. Other methods of monitoring heart rate lack validity. While heart rate is affected by exercise intensity, it is not a direct measurement of it. Heart rate can be affected by other factors such as drug use, hydration status and body position. In order to maximize accuracy, these factors must be controlled.
Keywords: heart rate; exercise intensity; monitoring; accuracy
Physiology Lesson
The human heart acts as a pump to drive blood through the circulatory system. Though it is convenient to divide the heart and circulatory system into two distinct functional groups, the actions of the cardiovascular system as a whole are coordinated and interdependent. Blood is pumped by the right heart into the pulmonary circulation and through the lungs where it is oxygenated. After returning to the left heart, blood is then pumped through the systemic circulation and finishes the loop by returning to the right heart (see Figure 1.0).
When talking about heart rate (HR) it is important to understand the interplay between HR and stroke volume (SV). This interrelationship is defined by the equation:
HR (beats/minute) H SV (L/beat) = Q (L/minute)
where HR is heart rate (the number of times the heart beats in a minute), SV is stroke volume (the volume of blood ejected per beat), and Q is cardiac output (the volume of blood ejected per minute). This equation demonstrates the relationship between HR and SV. For example, if SV drops, HR must increase to maintain a steady Q and vice versa.
The autonomic nervous system (ANS) plays a major role in regulating the rate of heart contractions as well as the force of these contractions. The ANS is composed of the sympathetic and parasympathetic systems. The sympathetic nervous system (SNS) acts to increase HR and contraction force through the release of norepinephrine. In contrast, the parasympathetic nervous system (PNS) acts to decrease HR and the force of contraction through the release of acetylcholine. These neurotransmitters are released at the sinoatrial node (SA), also referred to as the "pacemaker" of the heart, located in the right atrium. The sinoatrial node creates an action potential, which is a rapid change in voltage potential of a cell. In a healthy human heart, this action potential is relayed through the heart in a consistent and predictable manner. This electrical activity can be measured via a device called an electrocardiograph (ECG); a typical ECG waveform can be seen in Figure 2.0. It should be noted that the ECG is considered the "Gold Standard", or most valid measurement of HR.
Review of the Literature
Moderate intensity exercise is associated with many improvements in health-related variables (5). The improvement of endurance performance requires increases in qualities such as aerobic power and anaerobic threshold (10). In order to elicit positive changes in these qualities a minimum intensity must be reached in training (16). Therefore, too low an intensity will lead to positive health benefits, but is unlikely to result in a change in endurance performance. On the other hand, a chronically high intensity with lack of sufficient recovery can lead to decreases in performance; this phenomena is known as overtraining (see Figure 3.0). A practical and accurate method for monitoring training intensity is required.
Methods for monitoring intensity include:
Of these methods, the most accurate is to directly measure the O2 uptake during training. However, this requires some sort of metabolic measurement system and an operator trained in its use. All of these systems are prohibitively expensive for the individual athlete and many are too large to use outside of a laboratory situation. Lactate analysis also requires the purchase of an expensive hand-held analyzer. Interpretation of lactate data is difficult for the average athlete. Subjective ratings of exertion have been used to monitor intensity but Gilman (10) and Hopkins (11) have shown that athletes judge intensity poorly with this method. Heart rate, however, can be considered both an accurate and practical measurement of exercise intensity (10,15). It should be noted, though, that HR is not a direct measurement of exercise intensity. Heart rate is often used as a tool to estimate O2 consumed at a certain workload. There are factors such as caffeine ingestion and dehydration which can alter HR (3,7) and affect its validity as an indicator of intensity.
Monitoring Heart Rate
Methods of monitoring HR are commonly used by endurance athletes as a
measure of their training intensity, or as a pacing mechanism during competition as
reported by Costill (8). Costill also reported that some athletes may use heart rate
monitoring as a warning sign of overtraining. Heart rate has been prescribed as a method
for monitoring pre-competition emotional anxiety in Weightlifters (19). Heart rate can be
used to estimate energy 
expenditure in exercises lasting more than 3-4 minutes (11). According to Arts
and Kuipers (1), this is due to the generally linear relationship between heart rate,
power output, and oxygen consumption. This relationship predicts that an increase in power
output should be reflected in a proportional increase in HR. Regression equations have
been used to create charts like the one shown here (see Figure 4.0). This chart is
a compilation of the data from the 53 male cyclists used in the Arts and Kuipers study.
Figures like these are used to establish training zones for athletes and recreational
fitness enthusiasts. However, Arts and Kuipers (1) suggest that an athlete should
determine his or her own relationship between power output and heart rate.
There are many methods used to measure HR. Commonly used methods include:
The popularity of automated methods of recording HR has led to the inclusion of heart rate monitors on many pieces of aerobic fitness equipment. Leger and Thivierge (12) have identified that the most valid and reliable system measures the electrical impulses of the heart at the chest. These systems commonly consist of a strap which is positioned just below the breast. Two electrodes lie on the inside of the strap, one on either side of the chest; they directly measure the interval between the 'R' sections of the heart contraction (see Figure 2.0). This method of monitoring HR is extremely accurate for men, women and children as compared to an ECG reading (4,12,18). Other methods of recording HR are unreliable. Leger and Thivierge (12) reported that assorted monitors using other methods may underestimate HR by up to 20-54 beats per minute.
Manual methods of counting have been used for a long time. According to the PFLC manual (4) the most accurate method is to place a stethoscope over the left chest and count for a 10 second interval. Boone (2) has shown that more practical options such as counting beats by palpating the neck (carotid artery) or wrist (radial artery) are less accurate. Boone identified a number of reasons for this inaccuracy. Firstly, during the time taken to find the pulse and count for 10 seconds there is a significant drop in HR, resulting in an inaccurate measurement of exercise HR. As well, palpation of the carotid artery can directly cause a decrease in HR due to pressure on the carotid baroreceptors. Other errors occur due to human mistakes in counting the beats per minute. Norton et al, (14) reported that males and females underestimated their HRs by an average of 13 bpm after a 1 mile walk and by an average of 17 bpm after a 1 mile jog.
Factors affecting the variability and validity of heart rate
As mentioned before, HR itself is not a direct indicator of exercise intensity. It is used to indicate intensity because it varies with exercise intensity. However, it can also be affected by a number of other factors. These can include:
Anything that can affect HR and does not reflect exercise intensity can reduce the validity of HR as a monitoring tool.
Stimulants such as amphetamines (benzedrine, dexedrine), caffeine, and ephedrine act on the central nervous system and mimic sympathetic neural activity (6). They act to increase HR and blood pressure by binding to alpha and beta adrenergic receptors(7). beta blockers, on the other hand, are used by some athletes to reduce tremors or nervousness before a competition. They work by blocking the actions of norepinephrine, thereby decreasing the effects of the sympathetic nervous system(6). Beta blockers have been shown to reduce not only HR but also VO2max. As such, they are typically only used by pistol shooters or other marksmen.
In certain exercise modalities body position can affect HR. This effect is most often noticed by cyclists when changing positions from a bent over (aerodynamic) position to an upright riding position. In the upright position the rider experiences a diminished return of blood volume to the heart which causes a decrease in SV. According to the equation given in the Physiology Lesson, a decrease in SV is accompanied by an increase in HR to maintain a steady cardiac output.
Dehydration can also cause inaccuracies in monitoring HR. During dehydration, blood volume decreases. This effect is exacerbated when exercising in warm conditions. To cool the body, water is removed from the blood and diverted to working muscles and throughout the skin (13). This causes a decrease in SV due to the diminished return of blood to the heart. Therefore, HR increases to maintain a stable cardiac output. Bothorel et al (3) showed that ingestion of fluids during intermittent submaximal exercise significantly reduced HRs as compared to exercise in dehydrated states. Even if an athlete is properly hydrated before beginning exercise, HR can increase during exercise prolonged for 30-60 minutes, despite no increase in workload (9). This is due to a progressive decrease in blood volume, leading to a decreased SV and an increased HR. This phenomena, known as cardiovascular drift, can affect the accuracy of HR as a measurement of exercise intensity.
Recommendations and Conclusions
To gain optimal benefit from training, an athlete requires a practical and accurate method of measuring intensity. Of the various methods available, monitoring HR is the most practical and accurate. The method of choice is a monitor which straps around the chest and measures electrical activity of the heart. Heart rate can also be used to monitor overtraining and to determine psychological stress before competitions.
It should be remembered that HR itself is not a direct method of monitoring exercise intensity. If used properly it can reflect the intensity of a training bout. Factors that can influence HR and decrease its validity as a monitoring tool should be minimized. These can include the use of stimulants such as caffeine, ephedrine, or amphetamines. Both caffeine and ephedrine are readily available to most athletes without prescription. It is the author's recommendation that foods or products containing these drugs should not be consumed within 2 hours of exercise. For most accurate results the athlete should extend this period to 6 hours. This time period can also vary depending upon the amount of stimulant ingested and the athlete's familiarity with that drug. Spriet (17) suggests that an athlete's response to caffeine may be dampened if that athlete is a habitual user.
Hydration status can also affect HR. Dehydration can elevate HR despite no increase in workload. During exercise that lasts beyond 30 minutes it is normal to see the athlete's HR increase without an increase in power output. This is due to cardiovascular drift. Recommendations regarding fluid intake are difficult to make in light of the variability between events and individual athletes (13). Suffice it to say that the athlete should approach each training or competition setting in a fully hydrated state. During competition the athlete should drink as much cool water as possible. Sports drinks can also be beneficial if an athlete is engaged in endurance exercise lasting more than one hour. These drinks should contain 2-8% solutions of carbohydrate and should also contain small amounts of sodium in the range of 10-60 mmol/L (that is equivalent to .23-1.4 g/L of sodium) (13). Most commercially available sports drinks meet these criteria.
Glossary
Action potential - The mechanism by which sensory impulses are transmitted through the body. It is a rapid change in the voltage potential of the cell membrane.
Aerobic power - A rate measurement of the maximum amount of O2 that a person can use during exercise. Expressed as either an absolute (Liters per minute) or relative (milliliters per kilogram per minute) score on a VO2max test.
Anaerobic threshold - A point, during incremental exercise, where the energy used for activity becomes predominantly derived from anaerobic sources.
Autonomic nervous system - Comprised of the sympathetic and parasympathetic nervous systems. The ANS is responsible for visceral functions in the body. Most functions of the ANS are automatic and require no conscious control.
Beta blockers - Substances which act to block the binding of norepinephrine to a and b adrenergic receptors, thereby decreasing the effectiveness of the sympathetic nervous System and lowering heart rate.
Cardiac output (Q) - The volume of blood ejected from the heart per unit of time. Usually expressed in Liters per minute.
Cardiovascular drift - An increase in heart rate after prolonged submaximal exercise which is not a result of increased workload. Caused by a decreased blood volume due to dehydration. This in turn leads to a decreased venous return and stroke volume. According to the equation given in the Physiology Lesson this will lead to an increased heart rate.
Central nervous system - The brain and spinal cord.
Electrocardiograph (ECG) - A printed record of the electrical activity of the heart (see Figure 2.0).
Heart rate (HR) - The number of contractions of the heart per unit of time. Usually expressed as beats per minute.
Oxygenated blood - Blood which has passed through the pulmonary circulation system and has been saturated with O2. Each deciliter of whole blood will carry about 20 mL of O2.
Parasympathetic nervous system - One portion of the autonomic nervous system. The PNS is generally responsible for decreasing the activity of visceral organs such as the heart.
Pulmonary circulation system - The portion of the circulatory system responsible for carrying blood to and from the lungs in order for it to be oxygenated (see Figure 1.0).
Sinoatrial node - Sometimes referred to as the Apacemaker@ of the heart. The SA node is located in the right atrium of the heart and is the first area of the heart to depolarize. In this way it sets the pace for the contractions of the heart.
Stimulants - Substances such as ephedrine or caffeine that act on the central nervous system to increase arousal and are often associated with increases in heart rate and metabolism.
Stroke volume (SV) - The volume of blood ejected by the heart with each contraction.
Sympathetic nervous system - One portion of the autonomic nervous system. The SNS is generally responsible for increasing the activity of visceral organs such as the heart.
Systemic circulation system - The portion of the circulatory system responsible for carrying blood to the majority of the body including the brain, digestive tract and muscle (see Figure 1.0).
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Questions
1) A major factor in the relationship between heart rate and stroke volume is defined by the equation _____ X ____ = ____?
2) The electrical activity of the heart can by measured by a device called an _________ which is considered to be the most valid measurement of heart rate available.
3) TF Heart rate is a direct measurement of exercise intensity.
4) Hear rate can be used to estimate exercise intensity due to the ________________ relationship between heart rate, oxygen consumption, and power output.
5) The most accurate method of measuring hear rate involves:
a) measurement of electrical activity of the heart at the chest
b) measurement of electrical activity of the heart at the fingers or wrist
c) measurement of pulse pressure at the neck (carotid artery) or wrist (radial artery)
d) measurement of opacity (how much light passes through) at the earlobe