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ARTICLES
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
Abstract.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.
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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). |
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Factors
influencing oxygen consumption during exerciseThe
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.
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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. |
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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. |
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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) |
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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. |
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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) |
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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.
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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.
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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.
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Table 1.
Relationship between MHR
and VO2max|
% MHR |
| % VO2max
| | 50 |
| 28
| | 60
|
| 40
| | 70
|
| 58
| | 80
|
| 70
| | 90
|
| 83
| | 100
|
| 100
|
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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.
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6. Based on the MHR and VO2maxrelationship the HR
corresponding to
target exercise
intensity can be obtained |
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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. |
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Fig
7. As it can be noted at low intensities catecholamine level
is constant. Increasing intensity stimulates catecholamine secretion.
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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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