Heart rate training makes use of the fact that the demand
for oxygen rises with exercise intensity. As would be expected heart rate has a
close relationship to oxygen consumption, especially at exercise intensities
between 50 and 90% VO2 max.
Heart rate is easy to monitor and for the majority of
athletes it offers a practical measure for assessing exercise intensity, which
is why it is so popular.
It's important to monitor exercise intensity for a number of
reasons. Firstly, the specific physiological adaptations to training change
depending on what relative work load is employed. It's fundamental that the
athlete or coach understands which type of endurance training (as a reflection
of intensity) is best for their sport or event.
Secondly, monitoring the intensity of individual sessions
allows the coach or athlete to manipulate the overall program, helping to
prevent over training and in order to reach a physical peak for competition.
While heart rate is convenient and practical for most
athletes, for many it can be inaccurate in determining the best exercise
intensity.
The Limitations of Heart Rate Training
Most heart rate training programs are devised around an
estimation of the maximum heart rate. The are two problems with this approach.
The first is that maximum heart rate is estimated with the basic formula
220-age. For a significant number of athletes however, this estimation maybe
out by as much as 25 beats per minute.
The only way to accurately determine maximum heart rate is
perform a short, maximal stress test (to exhaustion). During the test heart
rate will rise steadily until a plateau is reached despite the exercise
intensity continuing to rise (assuming the individual is fit enough to last
until such a time). This is a direct marker that the heart is beating as fast
as possible.
The second problem is that, even if maximum heart rate is
estimated accurately, prescribing exercise on the back of standardized zones
makes no allowances for individual differences. For example, endurance
performance improves when lactate lactate threshold as a percentage of VO2 max
is increased and it can be improved with training (3,4). A standard heart rate
zone of 85-90% of the age-predicted maximum is commonly prescribed to improve
lactate threshold but this may not be accurate. As with maximal heart rate, the
only way to determine the correct heart rate training zone for improvement of
lactate threshold is to measure it during laboratory testing.
Despite these limitations, heart rate training still offers
a more objective method for determining exercise intensity than nothing at all.
Heart Rate Training Zones
Different exercise intensities tax the body's energy systems
in different ways.
Exercising at 60% of maximum heart rate for example, is said
to predominantly tax the aerobic system in most people. If exercise duration is
long enough, the major source of fuel will be fat.
This type of intensity is often favoured by people who want
to lose weight and are generally de-conditioned.
A heart rate training zone of 70-80% maximum will still
predominantly tax the aerobic system in fitter individuals but the main source
of fuel will be carbohydrate, or more specifically, glycogen. This is the heart
rate training zone that endurance athletes typically aim for.
Here is a quick example of calculating a heart rate training
zone using the age-predicted maximum of 220-age:
Rachel is 35 years old and wants to train for a 10km run.
Maximum heart rate = 185bpm (220-35)
Target heart rate zone = 70-80%
Lower target heart rate = 130bpm (185 x 0.7)
Upper target heart rate = 148bpm (185 x 0.8)
Target heart rate zone = 130 - 148bpm
The Karvonen Formula (Heart Rate Reserve)
Simply using 220-age makes no allowances for individual
differences. All 35-year olds will have the same heart rate training zones
according to this formula.
The Karvonen formula takes into account resting heart rate
making it a slightly more specific to the individual. Because resting heart
rate decreases with conditioning it also makes allowances for differing degrees
of fitness to some extent.
Keeping with the example above, here's how Rachel (who has a
resting heart rate of 65bpm) would use the Karvonen formula to achieve a more
accurate heart rate training range for aerobic endurance conditioning.
Karvonen formula:
Maximum heart rate - resting heart rate x heart rate zone +
resting heart rate
185 - 65 = 120bpm (this is called the working heart rate)
120 x 0.7 = 84bpm (70% zone)
84 + 65 = 149bpm (lower limit)
185 - 65 = 120bpm (this is called the 'working heart rate)
120 x 0.8 = 96bpm (80% zone)
84 + 65 = 161bpm (upper limit)
Target heart rate zone = 149 - 161pbm
You can see that the Karvonen formula calculates a higher
training zone than just using 220-age and this is often the case.
It's often a good idea to use a rate of perceived exertion
along side heart rate to make the intensity more specific to the individual.
Rate of perceived exertion, although subjective, has been shown to correlate
with heart rate. Essentially, it is a scale of difficulty that ranges from
6 (no exertion at all) to 20 (maximal exertion). It is often called the Borg
Scale after its creator.
Swimming is a Little Different Maximum heart rate while
swimming tends to be lower than for running events. To adjust for this subtract
13 from your maximum heart rate i.e. use 207-age rather than 220 - age. Use
this adjustment for the Karvonen formula also.
The Conconi Test for Measuring Lactate Threshold
As mentioned earlier, the simplest method for determining
the lactate threshold is to assume it occurs at 85-90% of the maximum heart
rate. An alternative is to use the Conconi test.
In 1982 Conconi et al, stated that the lactate threshold was
linked to a deflection point in heart rate data. Heart rate plateaus briefly
before rising sharply again and this is said to correspond with a sudden rise
in blood lactate concentrations.
There are various protocols used to elicit the
plateau Conconi and co-workers refer to. Here is an example:
Equipment
Treadmill (with metric setting - km/hr and meters)
Heart rate monitor
Assistant to take recordings
Procedure
Begin by warming up at a light pace for 5 to 10 minutes. Set
the treadmill to a 1% incline.
The run should last between 2.5km and 4km to allow
sufficient data to be collected.
Gauge your starting speed. Speed is gradually increased
every 200m so start too quickly and you won't last long enough. Start too
slowly and you'll be there all day.
As a guideline 8 - 10 km/hr is a good starting point.
Increase the speed every 200m by 0.5 km/hr.
Record the heart rate and speed at each 200m interval.
Continue until exhaustion and complete a 10 minute cool
down.
You can now plot a simple heart rate graph like the one
below and read off lactate threshold:
You can see from the graph above the obvious plateau and
deflection in heart rate. It seems to correspond with a heart rate of 172bpm.
In theory, then an athlete could train at or just above this heart rate
training zone and improve their lactate threshold. However, caution is required
when using this test as subsequent research has questioned its validity (7,8).
It has been argued that the deflection point occurrs only in a certain number
of those tested and that it underestimates the lactate threshold exercise
intensity.
Heart Rate Training to Increase Lactate Threshold
Here's a simple heart rate training program to increase
lactate threshold...
Assuming your heart rate at lactate threshold is 170bpm
Start by completing two 6-10 minute runs approximately 5%
below the lactate threshold heart rate. In this case it would be 162bpm.
Rest for 2-3 minutes between runs and complete this twice a
week.
Gradually build up the length of each run or the number of
repetitions (up to 6). Also increase your target heart rate up to your
threshold (170bpm).
The target eventually is to reach a sustained 20minute run
at or just above your threshold heart rate.
Complete a thorough cool down at the end of each session.
Also re-test your lactate threshold every 6-8 weeks.
