SCUBAPRO-UWATEC HEARTRATE MEASUREMENT User Manual page 5

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Let's use the car metaphor again. In a car
(blood) certain passengers (oxygen and
metabolites) are to be quickly transported to
the requested location (muscles). Because
the car can only be enlarged to a limited
extent (limited stroke volume) and therefore
can only accommodate a limited number
of passengers, the pilot (heart) has to drive
more often and therefore steps on the gas
(heart rate). Responsible for the reaction
to such changes –(that cannot only be
caused by strong temporary exertion but
also by a change in position ) are special
stretch receptors on the aortic arch and at
the bifurcation of the carotid artery. They
deliver the necessary information that is
processed by the central nervous system to
quickly adapt the blood pressure to the new
situation. However, it is also crucial whether
it is static work done over a longer period
of time or dynamic work that allows for a
continuous blood supply to the muscles.
This type of muscle activity occurs with all
types of locomotion, so with finswimming
in particular as well. Here, the performance
level is the decisive factor for the heart rate
setting: if a person swims faster or has to
fight strong currents he or she needs more
oxygen in the muscles used than somebody
who calmly glides over a coral reef with an
occasional fin stroke.
This greater need for oxygen does not
mean that the breathing rate needs to be
increased immediately to raise the oxygen
uptake, but rather that more oxygen needs
to be transported to the muscles used.
WHILE DIVING
On the previous two pages we've
explained what means the body uses to
adapt to certain situations. Of course, these
mechanisms are also effective in diving,
both before the dive and later underwater.
Carrying and putting on heavy equipment
is usually already the first physical strain,
which does not only exercise the muscles
but also the cardiovascular system.
HEART RATE
Even more strenuous is when you are in the
water with the equipment, which gives a
strong water drag (much stronger than the
air resistance during jogging or bicycling)
and you are moving along by finning, your
muscles and heart are being exercised.
Obviously, the faster you swim the
more physical effort is required. But this
relationship is also heavily dependent
on the diving equipment worn and the
movement technique employed. A diver
who has a refined, economic finning
technique needs to exert himself a lot
less to achieve the same velocity than a
swimmer with less training. However, in this
context the heart rate can't be used as an
indicator for the absolute metabolic value.
Heart rate is a useful tool to measure and
prove changes in one's physical fitness, but
it is not the best and only one.
Maximum
depth
The heart rate behaves
independently from the depth
HEART RATE
INFLUENCE OF BODY POSITION
ON THE HEART RATE
The body position also influences the
heartbeat. In particular it determines the
venous return to the heart: in a horizontal
position for example, when the body lies
more or less on an even level, more blood
can flow to the heart. Why? The answer is
very simple: when standing up the blood
moves to the lower extremities. The blood
has to flow against the hydrostatic pressure
gradient – meaning virtually against the
force of gravity – back up to the heart. But if
you are lying down and thus move your legs,
torso and head at the same level, the blood
volume shifts: blood from the legs moves to
the heart whereby initially the stroke volume
increases and the heart rate decreases.
The reason for this is the increased blood
supply to the right heart chamber (ventricle,
atrium), which the body interprets as a
signal for a higher fluid balance –, since
this is exactly what also happens when you
drink a lot and the blood volume rises as a
result. In both cases, another compensation
mechanism takes effect: more urine is
produced. This increased urine production
and the resulting blood volume reduction
will over time compensate for at least part
of this effect. When you stand up again the
exact opposite happens: the stroke volume
decreases and the heart rate increases.
The blood volume increased once again
through fluid supply.
WHILE DIVING
As just described, this effect arises from
the influence gravity has on the body. If
the body moves to a horizontal position it
neutralizes gravitation to a certain extent.
And exactly the same thing happens
when the body is submerged in water up
to the neck, because the water pressure
When submerging into the water
the venous blood volume shifts
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