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Fitness testing
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I have a standard test that I do to determine running fitness. On a treadmill do 60 minutes at 8 miles/hour on a 2 percent grade. I have been doing this test once a week for the past 2 months. My average HR for the test has been gradually dropping over the weeks (I am using a Polar S710 to measure HR). Over the hour test my HR slowly increases. I.e. If I split the 60 minutes into 4 15 minute splits my average HR for each split is as follows: 0-15min = 156bpm average, 16-30min = 160bpm average, 31-45min = 163bpm average, 46-60min = 165bpm.

What causes this gradual increase in HR? What does it mean is it good/bad/just natural?
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Re: Fitness testing [74] [ In reply to ]
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What you experience is called "cardiac drift": The gradual rise in heart rate at a given constant level of exertion.

It is the limiting factor for all aerobic activity. With training the "drift" becomes less pronounced and more delayed.

Keep up the testing,

CM
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Re: Fitness testing [MTL_QC] [ In reply to ]
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It could also be due to increasing dehydration with time.

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Frank,
An original Ironman and the Inventor of PowerCranks
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Getting tired [ In reply to ]
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It is called getting tired among us less educated athletes. This process is very pronounced with me in the heat. The same level of exertion will slowly lift my HR from 150 to the high 160s. At that point I stop unless I am specifically planning a killer workout. I think hydration helps, but I have not really tested it.

I wish I could maintain that pace at that HR for that long.
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Re: Fitness testing [74] [ In reply to ]
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Short answer: all the posts so far are absolutely correct - its natural - don't worry.

Long answer: The heart uses the available blood volume to supply the tissues. The amount of blood pumped per beat (Stroke Volume) multiplied by how many beats per minute (HR) gives you cardiac output (Q) (SV X HR = Q) As you exercise you will slowly decrease your blood volume due to water loss via sweating and increased movement of water from the blood into the tissues. This will decrease SV at a given intensity (overall it will still increase throughout exercise). In order to make up for this the heart will gradually increase HR in order to maintain Q. AKA cardiac drift. Much more pronounced in hot, humid conditions.

Good test. Personally, I like to test my fitness only about once a month. Any more than that and I feel like I'm concentrating too much on just seeing good test scores.
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