Q: What assumptions are typically made with submaximal aerobic fitness tests and what impact do such assumptions have on the accuracy of the test results?
A: The validity of the results of all submaximal fitness tests rests on several assumptions:
- That a steady-state heart rate is obtained for each exercise workload.
- That a linear relationship exists between heart rate, oxygen uptake, and workload.
- That the maximal heart rate for a given age is predictable.
- That the biomechanical efficiency of the physical activity performed (i.e., oxygen uptake at a given workload) is the same for everyone.
Unfortunately, it is often quite difficult to conduct a test that meets all the requirements for the four listed assumptions.
For example, exercising at a given workload for only a few minutes can involve an insufficient amount of time for many individuals to achieve a true steady state. To ensure that a steady state has been achieved, the heart rate should be measured after two minutes of exercise at a given workload and again after the third minute of exercise at that workload.
These two heart rates should then be compared. If a difference of more than five beats per minute between the two is found, an individual should continue to exercise at one-minute intervals at the same workload until two successive heart rates differ by less than five beats per minute.
It is also important that the submaximal heart rates obtained be between 115 and 150 beats per minute, because research shows that within this heart rate range that a linear relationship tends to exist between heart rate and oxygen uptake or workload for most adults.
When the heart rate is less than 115, many external factors (e.g., talking, laughing, apprehension, etc.) can greatly influence heart rate. Once a person’s heart rate reaches a level between 115 and 150, external factors no longer influence heart rate, and a linear relationship exists. As the heart rate of an individual being tested rises above 150, the heart-rate oxygen-uptake relationship becomes curvilinear.
The third assumption involves maximal heart rate. Maximal heart rate is the greatest heart rate that can be measured when individuals are exercising to the point of volitional fatigue (i.e., stopping because they cannot exercise longer) during a graded exercise test. Several equations have been developed to estimate maximal heart rate:
- Maximal heart rate = 220 minus age (low estimate)
- Maximal heart rate = 210 minus [0.5 x age] (high estimate)
- Maximal heart rate = 226 minus age (estimate for older individuals)
Maximal heart rate can, however, vary greatly among different individuals of the same age. The standard deviation for the aforementioned estimates is +12 bpm. This statistic refers to the fact that two-thirds of the population varies an average of plus or minus 12 heart beats from the estimate given by the equation.
If an individual's age-predicted maximal heart rate is higher than that person's true maximal heart rate, then his/her estimated VO
2 max will be an overestimation of the correct or actual value.
The final assumption addresses the issue of biomechanical efficiency. Oxygen uptake during exercise at any given work rate can vary by approximately 15 percent between different individuals. This factor occurs because some individuals are more efficient at performing a given task than others.
As a result, the average oxygen consumption associated with a given workload may vary significantly from one person to another. Thus, VO
2 max predicted by submaximal exercise tests tends to be overestimated for those who are biomechanically efficient when exercising and underestimated for those who are inefficient.
Although submaximal exercise testing is not as precise as maximal exercise testing, it does have advantages. For example, the results of submaximal testing can provide a fairly accurate reflection of an individual's fitness status without the cost, risk, effort (on the part of the individual), and time involved in max testing.
Submaximal testing is also a good tool for monitoring fitness changes over time. If an individual is given repeated submaximal exercise tests and his/her heart rate response to a given workload is found to decrease over time, it is reasonably safe to conclude that the individual’s level of aerobic fitness has improved.
Source: Bryant, Cedric X. 101 Frequently Asked Questions about "Health & Fitness" and "Nutrition & Weight Control". Sagamore Publishing, 1999.
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