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Introduction
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Athletes often use the results from physiological stress testing (metabolic or cardiopulmonary stress test) to determine exercise zones that can determine their level of effort while training or racing. Indeed, the test data from systems like ORCA can help an athlete determine whether his or her training is appropriate. Proper use of the results is important. Several factors can influence the relationships of heart rate to VO2 max to anaerobic threshold to maximal effort. Some of these factors have been well studied. Others remain to be evaluated in detail.
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These factors are typically well known to professional trainers, but if ignored, can lead to simplification that may lead the athlete to question the results. Generally, these factors are at work after the test, when the athlete is out on the road, training or racing, while the athlete is in a endorphin or adrenaline modified state of mind, and fails to consider the effects. It is wise for the testing facility to advise these athletes of these effects and to also verify that they are minimally at work at the time of testing. Among the factors affecting the results are:
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Temperature
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The ambient temperature when performing at a high stress, maximal level activity is a significant factor in heart rate. It has been shown that heart rate is elevated by 26 bpm for athletes performing at 98 deg F compared 68 deg F. (Stannard and Thompson, 1998: Heart Rate Monitors: Coaches' Friend or Foe? Sports Coach) Since many events and training occur at temperatures higher than a typical lab setting, this effect is certain to be a continual factor.
Note that these researchers and others have found little evidence of similar effects at colder temperatures.
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Time of Day
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Our bodies do not normally adjust to the changes from sleeping to waking upon our conscious will. An athlete's cardiopulmonary system will take some time to wake up and his metabolic system may not quite be caught up with his intentions. Different subjects will take various lengths of time before their system can produce the same level of effort upon waking that they did in a test performed at three o'clock in the afternoon.
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Altitude
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When an athlete who normally lives at sea level, for instance, travels to an event in the mountains of Colorado, she will initially find her heart rate much higher than usual for a given level of exertion. The magnitude of this effect diminishes with time at altitude, depending on the ability of the person's body to adapt to rarified air. (McArdle, Katch, Katch, 1991: Exercise Physiology)
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Type of Activity
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Different types of activity using different muscle groups may produce different heart rates at the key physiological indicators. Stressing many muscle groups simultaneously produces a greater response from the cardiopulmonary system. Running is generally seen to produce the highest heart rate for any given individual, cycling slightly lower, and swimming below that. Even in a single activity like cycling, different cycling positions, standing vs. seated, pedaling style, ("circles" vs. stomping) cadence, hand positions on the bars, etc. can have an effect.
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Hormones
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The hormones produced by the human body can have an enormous effect in some situations. Adrenaline output varies within each individual and with different amounts of stress. An athlete in competition against a powerful rival will find a heart rate well above that in training or in a lab setting for the same power output. The effect of adversity and mental stress can have a significant metabolic effect.
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Caffeine
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Caffeine is one of society's most prevalent drugs. Studies have shown various results from the use of caffeine upon test subjects. Some have shown heart rates varying and others have shown lactate levels reduced at certain times after consuming caffeine. Since the use of caffeine is not regulated in the sports world, it is very likely so commonly used, that with many individuals, their metabolism has become dependent on it. Thus the athlete may see her "norm" after consuming the refreshing beverage and also an associated drop in performance and metabolic activity when the blood levels of the drug are reduced. Studying the effects of caffeine on metabolism can be complicated by it's common use as well. As with any drug, its use in an athletic setting can produce unknown results on your athlete's metabolic system.
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Cardiac Drift
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Cardiac drift is the tendency of a person's heart rate to increase with the passage of time. We are not taking about old age here, but the change after 20 minutes would be noticed. The same test subjects had lactate thresholds that dropped during the same period. (Grassi, etal: Power and Peak Blood Lactate at 5050 m with 10 and 30 s `all out' Cycling. Acta Physiol Scand 2001)
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Smoking
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Some research has indicated that smoking causes a delayed heart rate response. For any athletes who smoke, this should be a consideration.
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