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"Sport Medicine Journal" No.6 - 2006

Heart rate monitoring and the physiologic basis of using it, in exercise intensity dosage

Ursta Anghel Mihai Relu
August Krogh Institute, Department of Human Physiology, University of Copenhagen, Denmark

The aim of this article was to observe the relationship between heart rate (HR) and maximum oxygen uptake (VO2max), as well as the possibility of using HR in determining intensity level during physic exercises.
Many changes occur prior, at the onset and during the exercise, in response to its duration and intensity. The main reason is to assure in the shortest time the required energy. These adaptations consist in increase of cardiac output (CO) based on increasing HR and stroke volume (SV) and blood flow within the working muscles and further increment of oxygen consumption VO2. In submaximal exercise intensity it was found that the contribution of aerobic energy production increases in time. The relationship between HR and VO2max is not linear within low exercise intensity. It tends to become more linear as the exercise intensity increase. One explanation in this regard, can be the increase in SV found within high exercise intensities.

Key words: heart rate, VO2max heart debit

An expression often used in the sport of performance, before a competition, is that, the warming up starts even from the locker room.

The true of this expression is proved too by the adaptations occurring in the cardiovascular system (CVS) prior to the exercise. These adaptations consist particularly in reducing the parasympathetic influences onto the heart activity and increase the stimulatory effect of sympathetic division of autonomic nervous system (18), which increases HR and myocardial contractility, leading to enhancement of CO.

At the onset and during the exercise, the control exerted on the CVS is the result of afferent impulses coming from muscles, tendons, as well as from baroreceptors and chemoreceptors residing within blood vessels. The control of the CVS however, is the aim of this article.

Thus, this fast response represented by the increase in CO prior and during the exercise, can be the first reason for using HR in determining and dosage of exercise intensity. Moreover as it will be seen further, the adaptations taking part within CVS during the exercise, are caused by its intensity. In the figure 1, can be noted the changes in HR prior, at the onset and during exercise, in this example being about speed runners on 60, 200 and 400 meters.

Relationship between HR and VO2

According to the principle of the german physiologist Adolf Fick, VO2 can be obtained by multiplying CO with oxygen extraction (a-v O2diff). a-v O2 represents the difference between arterial and venous oxygen content. CO is given by the product between HR and SV. Therefore, monitoring the HR during the exercise can provide available information about VO2.

Fig.1. Effort anticipation (r, rest-the dark area) causes already the increase of HR. In the figure it is presented the dynamic of HR to speed runners on 60, 200 and 400 m (18).

Factors influencing oxygen consumption during exercise

The reason why all these changes occur in the homeostasis of CVS, is that somewhere in the body, a new area has come up and needs O2 and energy. That area is accounted by the muscle groups engaged into the effort.
In the rest state, the constrictor effect exerted by autonomic nervous system on the blood vessels by means of catecholamine adrenaline (A) and noradrenalin (NA) prevails. During the exercise however, this effect is suppressed by the local factors whose weight increase significantly diminishing almost completely the vasoconstriction effect of catecholamine.

HR and SV

It is important to mention that exercise duration and intensity as well as the fitness level of the subject determine the extent of the changes occurring in the CO components.

In the intense and short duration efforts, increase both HR and SV. It was estimated that in the exercise with intensity equal to VO2max, the blood flow in the working muscles accounted for 80 – 85 % CO (20). Above 60 – 70 % VO2max, SV decreases (2), reaches a plateau (8) or increases (11). These variations in SV however, were observed in different conditions such as old subjects, in the Fleg et al (1994), submaximal and maximal exercise intensities as well as high trained athletes as was the case of the Gledhill et al (1994) study.

If the evolution of SV during the exercise can vary as it was presented above, the HR increases progressively (fig 2). A plausible explanation might be enhancement of the skin blood flow. Fritzsche et al (1999) found in their study that, the decrease in blood volume, more precise the plasma volume, leads to reduction of SV and therefore, in compensation the HR increases in order to keep the blood pressure constant (7). They did not find any change in the skin blood flow in conditions in which the rise of HR was impeded by administration of small doses of atenolol, which blocks the β1 adrenaline receptors. It is however interesting to note that in a first phase, the peripheral circulation increases. However, soon as exercise intensity rises, skin blood flow diminishes so that the blood can be diverted to the working muscles and the central blood circulation represented by heart, arteries and veins.

Fig 2. In this figure can be noted very well the extent of changes in the functional parameters of the heart in response to different exercise intensities (■) SV, (●) HR (4). After an initial and fast increase SV lower while HR rises progressively.

a-v O2diff

There are two parameters that condition the extent of a-v O2diff, the amount of O2 transported in the blood and the oxygen requirement at the working muscle level. The arterial O2 varies litle relative to the rest level of 20 ml dl-1 even in the large variation of exercise intensity. The main responsible for a-v O2diff is the O2 from the venous blood which at rest is about 12-15 ml dl-1 and decreases in the maximal exercises to 2-4 ml dl-1 (18).

Once the O2 reached the working muscles, myoglobin, the correspondent from muscle to hemoglobin, together with mitochondrial content and aerobic enzymatic apparatus of the muscles will condition further a-v O2diff depending on exercise intensity. In figure 3 can be seen a-v O2diff evolution at different level of VO2.

In order to detail the aforementioned statement, the muscle mass size and also the type of muscle fibers recruited in sustaining the effort influence categorically the O2 requirement. Although one could argue this by the fact that the exercise intensity causes the recruitment pattern, it should not be omitted that inside the muscle the capillarisation is higher around the oxidative type I fibers and that the type II muscle fibers are less efficient in producing energy aerobically.

Therefore the physiological reason in using HR for exercise intensity determination lies in the way by which the heart activity offsets the changes of peripheral blood flow. Without adjustments occurring in HR and SV the central blood flow would be affected significantly.

Fig 3. Arterial and venous O2 content as well as the O2 transport capacity of the blood are presented. It can be observed that the arterial O2 varies little while the venous O2 content decreases significantly, in parallel with increase in exercise intensity. (data are from ref 18)

Methods of HR monitoring

For many hundreds years, HR was addressed by placing the ear on the subject chest. About 200 years ago, by means of stethoscope invented by Rene Laennec, the man was able to obtain more information about heart activity.

At the beginning of XX century, the dutch physiologist Willem Einthoven improved the first electrocardiograph (ECG). Over the years, in the 80’ came out the first models of heart rate monitors (HRM) capable of measuring HR without being necessary wire connection.

As for these HRM, it can be said that there are two methods by which HR can be measured. These are palpating directly the pulse by HRM that are applied at the wrist and recording the heart activity by HRM consisting on a receptor applied on the chest of the subject by means of an elastic belt and the receiver resembling a regular watch (1).

The pulse can also be palpated by the sportive himself either at the wrist by pressing softly the radial artery or at the neck level by palpating the carotid artery. The later could give some errors in measurements due to the pressure receptors which reside in the carotid. These are susceptible at changes in pressure within the arterial system and exerting pressure on them, could make them to signal to the medullar region of the CVS control which in return decreases the HR. Therefore HRM using is more appropriate.

HRM accuracy

The accuracy of the data obtained by using HRM has been verified by comparing the HR values recorded by both HRM and ECG (14).

In the study of Godsen et al (1991), the HR recorded by HRM was 6 beats lower than that recorded by ECG on the same time course. In Goodie et al (2000), HR was monitored by both methods in 30 subjects who carried out isometric contractions as well as mental activity by solving mathematic calculations. HR was 80.7 ± 10.4 in using ECG and 81.3 ± 10.4 b min-1 in the case of using HRM with r – 0.98 with probability of errors P < 0.001. These findings support HRM utilization in measuring HR during exercise within high indices of accuracy.

Exercise intensity determination and dosage

One of the methods used in establishing exercise intensity is to express it as percentage of VO2max. In this regard the values of VO2max and maximal heart rate (MHR) must be known. Although both parameters can be estimated by theoretical calculations, in high performance is extremely necessary to obtain their values within special designed laboratory tests. These special evaluations should take place periodically because of the cumulative and qualitative effects of well designed and carried out training sessions.

VO2max measurement

The golden rule in carrying out this measurement is that the runners should run, the swimmers should swim and so on. The physiological reason for that is that each sport field assumes a specific effort which causes a certain way and size of muscle group recruitment.

VO2max evaluation can be carried out by submitting the subject to an effort that could be running on the tread mill or pedaling on an ergonomic bicycle as the exercise intensity is increased steeply. The intensity level at which the subject can not maintain the exercise for a certain period established before is taken as representing the maximum oxygen consumption value (Fig 4). There are many ways to increase the intensity during exercise and establishing the VO2max value (see 21), but the principle remains the same.

It has to be specified that reaching VO2max occurred within exhaustion exercise intensity; therefore the way and the level of subject motivation can affect VO2max final value.

Fig 4. In this figure is presented a ramp or incremental test in which the intensity is increased steeply.(the values in the figure are informative)
MHR determination

In measuring MHR it is important that duration and intensity of the effort to be enough long and high respectively. A suggestive example is in the figure 5. There are also theoretical ways to obtain the MHR values. The most known is  MHR = age – 220

These theoretical values are only approximations. For instance the MHR obtained by the above presented formula may contain an error of ± 10 b min-1.

Fig 5. A model of testing MHR. The test consists of running 20 x 10 m followed up by 40 x 10m in a tempo of 2/4. This combination is repeated increasing the tempo (3/4, 4/4). All running schemes can be seen as overlapped, only the running speed (intensity) raises.

Interpretation of HR and establishing the values for aerobic capacity improvement

Before interpreting the HR values during the exercise it should be also underlined the aspect of the variation within the time period between heart beats. It is because it was found that a high variation is associated with higher VO2max (19). Table 1. displays the correspondence between percentages of MHR and VO2max.

The errors in expressing VO2max % based on MHR % are ± 8 % (18). As it can be seen in the table, the differences between MHR and VO2max percentages are larger at low intensities but the corresponding values of both parameters tend to even as they approach the maximum level.

Table 1. Relationship between MHR and VO2max
% MHR  % VO2max
100   100
In the figure 6 can be seen an example of determination of the HR corresponding to the percentage of VO2max, for a subject with 200 b min-1 MHR.
It also should be mentioned that HR is different for exercises carried out by upper and lower part of the body being lower within exercises engaging the upper part of the body (18). It is explained by the smaller size of the muscle groups sustaining the effort. In swimming, MHR is 13 b min-1 lower compared to running (10). Reduction in the cost energy necessary for maintaining the vertical position can be the explanation in this case.

Fig 6. Based on the MHR and VO2maxrelationship the HR corresponding to target exercise intensity can be obtained
An explanation for discrepancy in terms of MHR and VO2max might be that within submaximal exercises energy production is predominantly aerobe. Thus one of the factors causing the raise in HR is the local vasodilatation determined by the higher requirements of O2.
The incremental role in supporting the effort of aerobic sources was also observed by Bangsbo et al (2001) within maximal exercise intensity study.

They found that within the same exercise the mechanical efficiency decreases. This finding assumes that in order to maintain the intensity, a shift in the recruitment patter has occurred toward type I muscle fibers. It is known that these are less efficient within the maximal exercise intensity. Also increase of temperature and decrease of pH enhance the oxygen unloading by hemoglobin (Bohr effect). Moreover they found that in the repeated exercise energy turnover is unchanged but energy production became predominantly aerobic. This important characteristic in terms of HR during submaximal exercise intensity should be taken into consideration by coaches or sport teachers when they are working on improving the aerobic capacity by using interval method.
As it was stated above, increase of O2 demands causes blood flow to rise in the working muscles due to local vasodilatation. Krustrup et al (2001) obtained a 10 times increase in thigh blood flow during knee extension exercise. At rest the blood flow was 0.43 L min-1 reaching 4.32 L min-1 by the end of exercise.
Thus the low VO2 at rest and during low exercise intensities explains the reduced peripheral blood flow. In this state, arterial pressure and the blood flow within the body are sustained by a relative high HR compared to VO2 value and reduced SV.

Within submaximal and maximal exercise intensity the increase of capillaries blood flow raises the venous blood return to the heart, which is also sustained by skeletal and respiratory muscle contractions. This increased venous return enhances the ventricular filling (Frank-Sterling mechanism) which will result into increased myocardial contractility. Thus SV becomes larger and CO meets the higher exercise intensities requirements in terms of oxygen delivery.
It was mentioned that A and NA are the hormones by which the autonomic nervous system controls CVS activity. In the figure 7 can be noted the dynamic of their secretion during exercise at different exercise intensities. Compared to resting state, NA secretion increases at lower intensities than that of A. This is due to the fact that the general vasoconstriction occurred throughout the body during exercise, in order to enhance blood supplying to the working muscles, is mainly imposed by NA while A increases HR and myocardial contractility as well as stimulates glycogen phosphorylase that support the higher exercise intensities.

Fig 7. As it can be noted at low intensities catecholamine level is constant. Increasing intensity stimulates catecholamine secretion.
In summary can be said that using HR in determining and dosage exercise intensity is a good means because it reflects quite accurately VO2 during the exercise. Maintaining intensity during submaximal exercises is accomplished by increasing the contribution of aerobic sources in energy production along with increase in local blood flow. Relationship between HR and VO2max is not linear. It is because CO adaptation to VO2max occurs by variations of both HR and SV.

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I thank to Professor Colov Rozalia from Lugoj-Timis county for all good advices she has given to me.
Ursta Anghel Mihai Relu

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