Heart Rate and
Cardiovascular Efficiency
By Tiffiny Twardowsky, MS
When starting, changing or maintaining an exercise program it is important
to understand how your body responds to various workloads and conditions.
Monitoring heart rate during exercise can show physiological improvements,
determine whether the intensity should be increased or decreased, and
warn you of stress caused by environmental factors. Understanding maximum
heart rate, target heart rate, and how the heart responds during rest
and exercise can assist in designing an effective exercise program.
Maximum Heart Rate
The maximum heart rate of an individual is the highest heart rate one
can achieve. Accurately measuring maximum heart rate can be done taking
a treadmill test while measuring the volume of oxygen (VO2) consumed and
the volume of carbon dioxide (CO2) exhaled. The level of exercise intensity
is directly proportional to heart rate and the amount of oxygen (fuel)
required by the body. In other words, as exercise intensity increases,
a greater amount of oxygen is required by the working muscles. The heart
rate, which is the limiting factor of the three, increases so that oxygen
can be delivered to the body. When the heart rate reaches maximum, the
volume of O2 that can be consumed by the body reaches its maximum. This
is called VO2 Max. At this point the volume of oxygen consumed is equal
to the volume of CO2 exhaled; there can be no further increase in exercise
intensity, heart rate, or VO2 max.
In general, oxygen requirement for any given workload is the same whether
an individual is an elite athlete or an overweight couch potato. The oxygen
delivery system, the cardiovascular system, is one of two main factors
that limit the body to reaching a certain level of intensity (the other
is how much oxygen is extracted by the muscles from the blood). The efficiency
of the cardiovascular system is extremely different in the elite athlete
versus the overweight couch potato. The sedentary individual will have
a much lower VO2 max and therefore a lower maximum exercise intensity.
This is why the sedentary individual huffs and puffs going up the stairs
while a regular exerciser will climb with greater ease. The sedentary
person’s cardiovascular system is not as efficient in delivering
oxygen to the working muscles and therefore needs to work harder and faster.
Cardiovascular efficiency depends on a number of factors. One measure
is called stroke volume, which is the volume of blood pumped per heartbeat.
A fit individual has a larger stroke volume, which means a greater volume
of oxygen is delivered to the body per heartbeat. This is also the reason
fit people have a lower resting heart rate. As mentioned, muscles require
a given amount of fuel, even at rest, which doesn't change with exercise
training. Because exercise training has increased the volume of oxygen
that can be delivered to the muscles per heartbeat, the heart needs to
beat less to do the same job. Therefore the heart rate is lowered.
Maximum heart rate can also be estimated using a simple equation. MHR=220-age.
As you can see, MHR decreases with age. This means that VO2 max and maximum
exercise intensity also decrease with age. This decrease is due to a number
of cardiovascular changes. A decrease in the number of heart muscle cells
that control heart rate as well as the degeneration of these cells reduces
the strength of the heart. Deposits of fat and other material into the
heart causes heart muscle cells to degenerate and become stiffer. These
changes reduce the strength of the heart and therefore the amount of blood
the heart can pump. The walls of the heart also thicken with age, which
decreases the amount of blood the heart chambers can hold. Another result
of aging is a decrease in the number of blood vessels that lead to the
heart, thereby reducing the amount of oxygen that can reach the heart
muscle. There is also a decrease in muscle mass, which causes less oxygen
to be extracted by the working muscles.
Target Heart Rate
The same changes that affect maximum heart rate will also affect and persons
target heart rate (THR) for exercise. THR represents the minimum and maximum
number of heartbeats per minute (level of exertion) during exercise to
achieve efficient cardiovascular benefits. The American College of Sportsmedicine
recommends an intensity level of 60% to 85% of maximum heart rate. Here
is a simple formula for determining your target zone.
1) Calculate MHR
MHR= 220-age
Example for a 33 year old
220-33=187 (MHR)
2) Calculate minimum THR
Lower limit = MHR X .60
Example
187 X .60=112 (lower limit)
3) Calculate maximum THR
Upper limit = MHR X .85
Example
187 X .85=159 (upper limit)
The greatest advantage in taking an exercise heart rate is the ability
to account for improvement. For example, let’s say a 35 year old
female begins an exercise program of walking for 30 minutes at 3 miles
per hour. In the beginning she may get an exercise heart rate of about
145 beats per minute (BPM). Over time, as the cardiovascular and muscular
skeletal system becomes more efficient, she might get an exercise heart
rate of 130 BPM. This is revealing that her heart doesn't have to work
as hard at the same intensity level. This can also be explained by keeping
the heart rate the same and increasing the workload over time. If this
person wanted to maintain a heart rate of 145, she would have to consistently
increase the intensity as the body becomes more efficient in delivering
oxygen.
The Fat Burning Myth
Target heart rate should be used as a general guideline - an indicator
of how hard you are working. When working at the upper range a greater
number of calories are burned per minute when compared to working at the
lower range. However, when working at the lower range a greater number
of fat calories are burned. So, which is better?
When exercising at a lower intensity for 30 minutes, you might burn
about 200 calories with about 80% coming from fat. This means that 160
calories of fat are burned. When exercising at a higher intensity for
30 minutes, you might burn about 300 calories with about 60% coming from
fat. This means that 180 calories of fat are burned. So, even though the
lower intensity burns a greater percentage of
calories from fat, the higher intensity burns a greater amount
of fat calories.
When it comes to weight loss however, the bottom line is not how many
fat calories are expended, but how many total calories are expended. The
advantage of exercising at a higher intensity for weight loss is that
it will take a shorter period of time to burn calories.
©2002 by Tiffiny Twardowsky, MS All rights reserved.
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