Use heart rate as a measure of your exercise intensity. The basics
are that 1) heart rate serves as a measure of exercise intensity during
steady state activities, and 2) If we estimate maximal heart rate as
220 minus age we can use this value to gauge the intensity of our training.
That is not the whole story though. Here are a few details that can
be important in your training and racing.
The "220 minus Age" formula is Only an Estimate. Your actual
maximal heart rate in a given activity could be 10 to even 20 beats
higher or lower than the estimated value. This has important implications
for judging training intensity.
Your Maximal heart Rate Differs in Different Activities. Cardiac hemodynamics
and maximal sympathetic drive are influenced by 1) body position during
exercise and 2) muscle mass involvement. So, a triathlete with a max
heart rate during running of 180, may only hit 176 on the bike, and
171 during swimming. In this case we call the running heart rate "Maximal
Heart Rate" and the highest heart rate observed in cycling and
swimming, "Peak" heart rate, for that event. Knowing your
peak heart rate for each discipline will help you to more accurately
guage the intensity of your training. If the activity is restricted
to upper-body muscle mass, peak heart rate will generally be considerably
lower than in whole body activities. Examples include kayaking and double
poling during cross-country skiing. Highly trained athletes can achieve
a higher percentage of true max heart rate when performing small muscle
mass activities.
A Better Method for Gauging Exercise Intensity with Heart Rate For a
given exercise mode, heart rate will increase linearly with exercise
intensity, and therefore, oxygen consumption. However, the resting heart
rate creates an offset between % of HR max and the associated % of "peak"
Oxygen consumption for that activity. For example, running at 65% of
Heart rate max corresponds to approximately 50% of VO2 max. At 87% of
HR max, you are at about 77-83% of VO2 max, depending on your resting
heart rate, heart rate and VO2 percentage finally converge at 100%.
I prefer to use HEART RATE RESERVE as my training intensity guide. To
do this I need to know 1) my resting heart rate, and 2) my peak heart
rate for that specific activity. The first one is easy to determine.
The second one may sometimes be a slight estimate. My current resting
heart rate is about 36 beats/min. My peak heart rate during rowing is
about 181. So my heart rate range is 181-36 or 145 beats. Now, if I
want to train at 85 % of my peak VO2 for rowing, I will take 85% of
my heart rate reserve (0.85 x 145=123) and add it to my resting heart
rate (123+36 = 159). PERCENTAGE HEART RATE RESERVE will give a better
approximation of % maximal oxygen consumption then just % max heart
rate. And, it is more accurate because you can adjust for changes in
your resting heart rate.
Body Position on the Bike will Influence Heart Rate. Let's say I am
riding on an indoor bicycle trainer with my upper body parallel to the
ground (Hands on the drops) at a heart rate of 145. Raising upright
while continuing to cycle at the exact same workload will result in
an increase in heart rate of about 5 beats per minute. Trust me I have
experimented with this effect on many a winter evening! This is due
to decreased venous return in the more upright position. Heart rate
increases to compensate for the slightly decreased venous return and
stroke volume, keeping cardiac output constant. Whe I return to the
drops, the heart rate drops again.
Temperature Will Greatly Influence Heart Rate. Above about 70 degrees
farenheight (21C), Heart rate at a standard submaximal intensity will
be increased about 1 beat/min per degree F increase in temperature.
Thus, a steady state run at a heart rate of 150 on a 70 degree Spring
day, may have you close to maximal heart rate on a scorching 95 degree
day in July, if you try to maintain the same speed. I am from Texas,
so I remember these days well. The oxygen demand doesn't increase in
the heat, but the thermal stress load does. As a result, your cardiovascular
system must divert blood flow to the skin to enhance heat dissapation.
Since you only have so much cardiac output, this means a lower maximal
steady state speed in the heat, or early exhaustion. You choose. My
choice is generally to avoid running in 95 degree heat.
Humidity Hurts Too For the same reasons, a higher relative humidity
will increase heart rate at a submaximal workload. Increased humidity
decreases the evaportation rate of sweat. This means the body has to
resort more to heat removal via increased skin blood flow. Data from
Wilmore and Costill "Physiology of Sport and Exercise" shows
a 10 beat increase in heart rate from 165 to 175 when running in 90%
humidity compared to 50%. This is the difference between a morning and
afternoon workout in many parts of the country.
What about the Time of Day? Our bodies show diurnal (time of day) variations
in many physiological responses. Within the normal range of times that
you might be training, this can result in a 3-8 bpm difference in heart
rate at rest, during moderate exercise, and during recovery. The differences
during maximal exercise are probably smaller. Data demonstrating this
effect is in the literature. However, I suspect the exact pattern of
these changes can be altered by your specific exercise pattern. For
example, after several years of rowing before sunrise, I am sure my
diurnal response pattern was modified. I have no data to support this
assumption, but I do know that I was transformed from an afternoon exerciser,
to a morning guy! So, my best guess is that you should not be too surprised
by small differences in heart rate response if you do your training
at an unusual (for you) time of day.
What is Cardiovascular Drift? If you begin a 90 minute steady state
ride on your bicycle trainer at a controlled intensity, your heart rate
may be 145 after 10 minutes. However, as you ride and check your heart
rate every 10 minutes, you will notice a slight upward "drift".
By 90 minutes, your heart rate may be 160. Why is this happening if
intensity is held constant? There are two explanations. As you exercise,
you sweat (dah). A portion of this lost fluid volume comes from the
plasma volume. This decrease in plasma volume will diminish venous return
and stroke volume. Heart rate again increases to compensate and maintain
constant cardiac output. Maintaining high fluid consumption before and
during the ride will help to minimize this cardiovascular drift, by
replacing fluid volume.
There is also a second reason for the drift during an exhaustive exercise
session. Your heart rate is controlled in large part by the "Relative"
intensity of work by the muscles. So in a long hard ride, some of your
motor units fatigue due to glycogen depletion. Your brain compensates
by recruiting more motor units to perform the same absolute workload.
There is a parallel increase in heart rate. Consequently, a ride that
began at heart rate 150, can end up with you exhausted and at a heart
rate of 175, 2 hours later, even if speed never changed!