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Monitoring exercise heart rate during training is not worth the bother!

Proposition for Debate - by Nick Caputi

Contents

Statement of the Topic

Monitoring exercise heart rate during exercise is not worth the bother!

Introduction

Monitoring of heart rate is commonly used in training to evaluate an athlete's training intensity, especially in endurance sports. When looking at whether is it worthwhile there are many factors that must be taken into account. Firstly what does heart rate represent, in terms of energy expenditure and how does it relate to intensity? How are heart rate thresholds calculated? What are we trying to establish when monitoring heart rates? Is the heart rate monitor a valid and reliable source of data? And finally what other factors affect heart rate?

In this debate I will discuss all the above questions and suggest arguments both for and against the use of heart rate monitoring in exercise.

Heart Rate in relation to Intensity and Energy Expenditure

Heart rate is the relatively accurate indicator of cardiovascular work intensity, most exercise prescriptions use heart rate as a preferred measure of intensity (King and Senn 1996).

Exercise intensity has been expressed in terms of speed, heart rate, the percentage of VO2 max, the percentage of lactate threshold and power output (Jeukendrup and van Diemen 1998). Although these are all valid objective measures they are not all true indicators of intensity nor are they all true indicators of energy expenditure. For example if you ride a bike downhill your speed will be greater than riding uphill, but will your energy expenditure and intensity be greater, I expect not. Jeukendrup and van Diemen, 1998 define exercise intensity as the amount of energy expended per minute to perform a task. Using this definition exercise intensity should be measured using a variable that relates to energy expenditure. Both VO2 max and power output are good measures of energy, however they are both very difficult to measure outside a laboratory setting and like speed, power output may vary within a certain exercise, eg cycling up or down hill.

There are a number of advantages to the use of heart rate as a measure of intensity. Heart rate can be related to energy expenditure by its relationship to: percentage of VO2 max, many studies have shown heart rate and oxygen consumption to be linearly related and onset of blood lactic acid accumulation or percentage of lactate threshold. Heart rate is the most easily measured of all the variables, especially with the advent of heart rate monitors. Thus use of heart rate as a measure of exercise intensity and energy expenditure is widespread.

How are Heart Rate Thresholds Calculated

In a study by Gilman and Wells (1993), heart rate’s were calculated for different exercise intensities by relating heart rate to onset of blood lactic acid accumulation (OBLA) and ventilatory threshold (VT). VT is the point at which there is a non-linear rise in minute ventilation. It has been suggested that energy production is primarily aerobic when exercise intensity was below VT. By plotting minute ventilation, blood lactate concentration, heart rate and oxygen consumption against each other, three different exercise intensities were established. Easy intensity, was where heart rates corresponded to below VT, moderate for heart rate between VT and OBLA and hard for heart rates corresponding to OBLA and greater.

Gilman (1996), again graded heart rates in the terms of easy, moderate and hard according to OBLA. He described a lower reference point of 150 bpm that was closely associated with 2 mmol/l of blood lactate. It has been proposed that this intensity of exercise is primarily aerobic energy metabolism, this was described as the upper limit of the easy training zone. The lower limit of the hard training zone was determined by a heart rate of 177 bpm, which was closely associated with 4 mmol/L of blood lactate, this ha been termed the anaerobic threshold. Heart rates between the two limits were moderate in intensity.

Commonly used formulas have been established to create guidelines for exercise intensities and specific heart rates. The Karvonen formula is one of the most widely used methods of determining heart rate range for appropriate exercise intensities. This formula uses heart rate reserve (HRR) in its calculation of target heart rate (THR).

Max Heart Rate (MHR) = 220 - age

HRR = MHR - resting heart rate (RHR)

THR = HRR x (50 to 85%) + RHR

The 50 to 85%, corresponds to the intensity of exercise that is required for the individual.

Stannard and Thompson (1998), define different training zones according to the percentage of maximum heart rate. For example, the basic aerobic endurance is defined within the range of 65% to 75% of MHR; general aerobic endurance is defined within the range between 75% and 85% of MHR. The anaerobic threshold is defined as the range between 85% and 92% of MHR.

Why Monitor Heart Rate

Exercise intensities that are too low may not result in the desired training effect, whereas a too high training intensity may cause overtraining (Kuipers and Keizer 1988). Thus there is a real need for monitoring of exercise intensity during training.

Gilman and Wells (1993) performed a study on six female runners and compared their training heart rates to the laboratory threshold-heart rates (as described earlier) and found that most of their training was completed in the easy to moderate intensity zone, with little training (8.5%) was completed in the hard training zone. However when competing the heart rates for the six runners (70%) was predominantly in the hard intensity zone. Thus with use of heart rate monitoring they would be able to reproduce their competition setting more accurately and train more specifically. It was also noted that the subjects overestimated how hard they felt that they were working during their training sessions, this could be measured more accurately with the use of a heart rate monitor.

Gilman (1996), described a laboratory test on a 21 year old collegiate ski racer in a graded ski walk test. This test was used to create the heart rate markers mentioned earlier. Following the test his average heart rate was taken in several ski races of varying lengths 10 to 20 km, his average heart rate was between 179 and 181 bpm. These heart rates correspond well to the measure of 177 bpm which was described earlier has the lower limit for hard intensity.

In a more recent study Boulay et al (1997), 15 male subjects were asked to perform a continuous 90 minute cycling test. The aim of the experiment was for the subjects to maintain a specific heart rate that had been specified as 5 bpm lower than the heart rate at ventilatory threshold (this was measured previously). Workload was adjusted to maintain the heart rate at the specified level, with pedaling rate constant at 60 rpm until the final 10 minutes where the subjects were allowed to accelerate. During the trial heart rates were monitored, venous blood samples taken and expired gases collected.

Results of the experiment showed VO2 levels were maintained with the maintenance of heart rate at the specified levels, however work rates and lactic acid concentration decreased after about 40 to 50 minutes of the prolonged exercise. These results indicate that heart rate and VO2 are good indicators of intensity, while lactate levels and power or work output may not be ideal with prolonged exercise.

Monitoring heart rate can also be effective in reducing the problem of overtraining. Elite athletes often suffer from overtraining due to too much training and too little recovery. Symptoms of this include fatigue, irritability, sleep problems and lack of motivation. Jeukendrup et al (1992) did a study on eight well-trained cyclists in which the weekly training duration was increased by 45% and the duration of high intensity training was increased by 350%. After 2 weeks, performance decreased significantly in all subjects, there was a significant drop in mean heart rate during exercise and maximum heart rate, notably there was also an increase in sleeping heart rates. Within one week recovery the subjects' heart rates and performance levels had returned to previous levels. By monitoring heart rate during training and staying within specific zones the risk of overtraining can be reduced. One way to prevent overtraining is to have both easy and hard training sessions throughout the week, these sessions can be set through setting heart rate training zones.

Heart rate monitoring can also be valuable in measuring fitness levels, especially in the untrained athlete, as heart rate has a linear relationship with oxygen consumption. An individual who wants to begin an exercise regimen but does not want to place too much immediate load on themselves may do a submaximal exercise test such as a 12 minute walk/run test, measuring their heart rate at the beginning, throughout and at the end. By monitoring their heart rate they can establish a baseline from which they can launch their exercise campaign.

Are Heart Rate Monitors Valid and Reliable

Leger and Thivierge (1988), performed a study in measuring the validity of 13 commercially available heart rate monitors. They compared the monitored values with simultaneous ECG readings in subjects performing bicycle, treadmill and step tasks. They found the monitors measuring electrical activity of the heart using chest electrodes to be the most valid while the models using non-conventional electrodes such as earlobe electrodes to be less valid and often unreliable. This is consistent with Macfarlane et al (1989) who suggested earlobe and finger electrodes to be highly unreliable.

The study described above is well over ten years old, since then there has been technological advances in heart rate monitors such that they now both reliable and valid. It is generally accepted that, heart rate monitors measure heart rate accurately under a variety of free living conditions (Noakes et al 1998).

Factors Affecting Heart Rate

Temperature

It well known that heart rate is affected by temperature changes, especially by extremes in temperature. In a study by Stannard and Thompson (1998) heart rate was investigated as an indicator for exercise intensity under different environmental conditions. In their study seven highly trained cyclists performed a 50 minute cycling session consisting of five 10 minute workloads of 150,250,350,250,150 Watts. The tests were performed at 37degrees Celsius and 20 degrees Celsius on the same day in a random order with at least 2.5 hours between them. A constant wind flow of 14.5m/s was provided. The higher temperature induced a significantly greater heart rate than the cooler conditions. At the end of the fourth stage, HR was an average 26 bpm higher in hot conditions. The 150 watts load at the end showed significant higher heart rates (average 18 bpm) than the same workload at the beginning of the task. But in cool conditions heart rate for the same workload were similar at the beginning and at the end of the exercise.

From this study the authors concluded that in the hot conditions power output is reduced for the same heart rate in cooler conditions. Thus an athlete may underestimate the intensity of their training session while training in the warmer conditions. Also of note was that once elevated the heart rate stayed raised in the warmer conditions, even when the workload was reduced.

Exercising in cold environments seems to be less conclusive with regards to changes in heart rate. Theoretically, baroreceptor reflexes would attempt to lower blood pressure (which would be high due to vasoconstriction from exposure to cold air or water) through a parasympathetically mediated reduction in heart rate. Thus a decreased heart rate at rest and during exercise would be expected. Stevens et al (1987), found that males exposed to 5 degrees Celsius air had lower heart rates at rest and during exercise than in 22 degrees Celsius, but observed no significant changes in females during exercise. Sink et al (1989) and Therminarias et al (1989) found heart rates to be relatively lower in males during submaximal exercise in cold air. While, Jacobs et al (1985) found no difference between warm and cold air.

Similarly with exercise in water there seems to be little evidence to show that there is either increase or decrease in heart rate with warmer or cold water. Doubt and Hsieh (1991), found no difference in exercise heart rate for the same workloads in 28 degrees Celsius versus 18 degrees Celsius water or in 31 versus 20 degrees Celsius water.

From the information presented it is clear that when exercising in warmer conditions increases in heart rates may result, this can cause a decrease in relationship between intensity and heart rate thus making monitoring of heart rate less effective in these conditions. It must be noted however that with training the increased heart rates in warmer temperatures can be reduced. Gisolfi and Cohen (1979), described a study in which 6 college women were assessed by performing a 4 hour walk at 30% of their maximal oxygen uptake in dry heat (24 degrees C) before and after 11 weeks of interval training and heat acclimation. The training program consisting of running 50-60 min/day 4 days/ week in a 21 degrees Celsius controlled room. They found that after the training program the women had improved cooling as indicated by lower skin temperatures and higher sweat rates, and lower heart rates.

Thus when using heart rate monitoring to measure intensity it may also be important to know if the athlete is used to warm conditions or has trained in the conditions on a regular basis.

Caffeine

Caffeine is found in most everyday diets, it is in tea, coffee, chocolate and many cola flavoured drinks. There has been controversy with regard to what effect caffeine has on the endurance athlete by way of performance. Some people suggest a positive effect, while others report no effect at all or a negative effect. Robertson et al (1978), demonstrated that in caffeine naïve subjects the heart rate decreased during the first hour after ingestion of 250mg then increased above baseline during the next 2 hours.

Flinn et al (1990) performed a study on nine male recreational cyclists. Prior to the start of the experimental sessions, each subject underwent an incremental cycling test in order to determine their VO2 max. Each subject started at a work rate of 100 Watts for a period of three minutes, and this was then increased by 50 watts every three minutes until the subject could no longer maintain the workrate. Respiratory data was collected and heart rate was monitored continuously during the test. The subjects were divided into three groups, a control, a placebo and a caffeine group. The subjects performed the same test as described before on three occasions. Three hours before the commencement of trials the placebo group and the caffeine group were give a blackcurrant drink with the caffeine group having 10mg/Kg of caffeine in it. Diets of all the subjects were controlled so that no caffeine was ingested 24 hours prior to the trials.

A comparison of the times to exhaustion and amount of worked perform between the three trials showed that the subjects that were in the caffeine group worked longer therefore performed more work. However there were no differences between either resting or exercise heart rates throughout the course of the experiment. The authors also found increase in free fatty acids (FFA) in the caffeine group, they suggested that this accounts for the increased endurance performance my decreasing the rate of muscle glycogen utilisation. The authors also found that there is a decrease in lactate levels, the lactate threshold, as measured by 4mM level has been moved to the right as a percentage of the VO2 max, thus allowing more work to be done. This finding may have some bearing on studies such as Gilman and Wells (1996), as they correlated heart rate at hard intensities to lactate threshold. Thus they may be working at intensities which are different to what is to be expected if someone had ingested caffeine 3-4 hours prior to performance. It should be noted that these results are for caffeine naïve subjects.

One possibility for the lack of variation in heart rate hypothesized by the authors, is that the strong stimulation of the cardiac muscle by the caffeine may be negated by the opposing effect of caffeine on the medullary nucleii.

Cardiac Drift

Cardiac drift is the term given to the phenomenon by which there is an increase in heart rate during exercise over time. Mognoni et al (1990), performed a study on 34 male subjects to assess the change in heart rate with prolonged exercise but maintaining a steady work rate. The subjects underwent an initial incremental test to determine their lactate thresholds. They then were asked to perform a prolonged exercise test between 2 and 6 days later. In the prolonged exercise tests subjects were asked to cycle at their lactate threshold work rates for as long as they could manage with 60 minutes being the aim. During the test heart rates, oxygen consumption and lactic acid levels were monitored. They found that heart rates increased significantly after 20 minutes of prolonged exercise, with threshold heart rates being between 17 and 22 bpm lower than those measured at the conclusion of the prolonged exercise test. The authors suggest that these changes may be due to a slow increase in body temperature.

In the study described earlier Boulay et al (1997), where they had their subjects exercise at a fixed heart rate by continuously adjusting the work rate. A reduction in work rate (17% reduction from about 220 to 183 W) was seen after 40 to 50 minutes of a 90 minute task. This reduction could possibly be accounted for by cardiac drift.

Altitude

Heart rate will increase in areas of high altitude, McArdle, Katch and Katch (1996) suggest that submaximal heart rate may increase by up to 50% above sea level values in the early stages of altitude adaptation. This is important to note as if someone normally trains a sea level at a specific heart rate, then they do some training at altitude they must allow for the change in heart rate to avoid any risk of overtraining. Thus in when training at high altitudes heart rate may not be a very good measure of intensity, and percentage of VO2 max may be appropriate.

Position

In sports such as cycling the position of the cyclist may cause a change in heart rate. The use of aerobars to attain a more aerodynamic position by cyclists has been found to cause an increase in heart rate. Gnehm et al (1997), found heart rate to be an average of 5 bpm higher in an aerodynamic position as opposed to an upright position. This was attributed to the increased contribution of the shoulder musculature and a less efficient hip angle.

Other Factors

Hormones circulating in the blood can have a direct affect on heart rate. Adrenaline and noradrenaline secreted from the adrenal glands are the most effective in changing heart rate. These increases in hormones may be a result of psychological changes such as anxiety or nervousness, or increase in motivation levels before a competition which is often seen to cause an increase in pre performance heart rate.

The type of sport, as mentioned earlier cycling down hill will have greater power outputs and speeds than up hill, but the heart rate's will be greater going up hill. Another example in cycling is when someone is drafting behind another cyclist they will be going at the same speed but there work rate, ie energy expenditure and heart rate will be decreased in comparison to what they would be using if they were riding alone at the same speed.

Smoking is another factor that can alter heart rate. McArdle et al (1996) suggests that with smoking there is a blunted heart rate response. He describes a submaximal test where smokers were able to perform for significantly longer time period before reaching the preset heart rate of 130 bpm. It was suggested that these results may be due to an altered sensitivity in autonomic neural control due to cigarette smoking. This is another important factor to be concerned with when setting training zones and target heart rates, as over estimation of fitness levels could result.

Clinical Implications

There are a few clinical implications with regard to monitoring heart rate. Firstly with athletes who are returning from injury there is a need for them to ease back into training, thus monitoring of their heart rate may be one way to make sure that they are not working too hard. Coinciding with this is when working with athletes who may exhibit symptoms of overtraining or other problems such as chronic fatigue, monitoring of heart rates so that the athlete can work in easy to moderate zones may be important to prevent further problems arising. If working with clients who have a very low exercise capacity it may be of value to give the person some objective markers to aim for, these markers can act as both motivation as well as limits to prevent over work in the untrained.

The main area that concerns Physiotherapists is probably for those working in the area of cardiopulmonary rehabilitation. It is imperative that when working with clients where small changes in heart rate can have large effects on the body, it is important that constant monitoring of heart rate is maintained. It is also of great importance to be aware of factors affecting heart rate such as heat, cardiac drift, caffeine and cigarette smoking, so that too high intensities are not set.

The final area where monitoring of heart rate may have some implication in physiotherapy would be when setting up a preseason training program. When setting a program, training zones can be set, with some modification taken into account to allow for warmer conditions.

Unless working in cardiopulmonary rehabilitation, as a Physiotherapist there may not be many situations where heart rate monitoring is of use. However with the increasing team approach to health care, especially when working with athletes, it may be important for the Physiotherapist to become involved with the Exercise Physiologist and Sports Physician to set and modify an athlete's program. Thus it is essential that there is an understanding of what is trying to be achieved and what limits should be set.

Conclusion

From all the information presented it would seem that there are a number of reasons for supporting the monitoring of heart rate and a number against. Firstly when we look at measuring heart rate, it is very easy to do, cost effective and non-invasive when you compare it to blood lactate levels and blood gas levels. It is also achievable in a number of environments not just a laboratory. It can be seen to correlate closely to changes in lactate levels and relates closely to VO2 max tests. Thus it would seem that as a measure of intensity it seems a valid measure. Similarly, heart rate measures in laboratory testing also relate well when compared to heart rate measures during competition, this is an important factor when using heart rate to set training program.

Secondly the monitoring of heart rates allows for the prevention of problems such as overtraining. This problem can be reduced significantly by setting target heart rates and remaining within them, allowing the athlete to have appropriate amounts of recovery time.

However when looking at both of these statements one thing must be taken into account, is the heart rate that is being measured a true indication of intensity or has it been affected by other factors. For instance if you were to base a training program purely on heart rate and taking no other measures you may in fact be underestimating the intensity achieved. This may be seen through increases in heart rate due to such things as cardiac drift and increased temperatures or training at altitude or possibly a blunting of heart rate due to cigarette smoking.

In an ideal situation, when setting heart rate training zones it would be better to set them for each individual in relationship to other metabolic measures such as VO2 and OBLA. Although this may be viable for an elite athlete it is neither cost or time effective for the everyday person or recreational athletes.

Generally I think that while there are some limitations to monitoring heart rate during exercise, in moderate conditions and for exercise periods of under an hour it is definitely worthwhile, so that you can plan your training to achieve what you want and not overtrain.

References

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Caffeine ingestion prior to incremental cycling to exhaustion in recreational cyclists. International Journal of Sports Medicine 11:188-193.
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Short Answer Review Questions

  1. What measures best depict exercise intensity and energy expenditure?
  2. What are the training zones related to in terms of energy sources?
  3. How does onset of blood lactic acid relate to training zones?
  4. What are the effects of warm and cold climates on exercise heart rate?
  5. Does caffeine cause an increase in heart rate?
  6. Does caffeine increase endurance levels? If so, how?
  7. What is cardiac drift?

Exercise Physiology Educational Resources 2000