One reason I ‘run’ on a treadmill once a week is to evaluate my heart rate with a heart rate monitor under controlled conditions. There are no headwinds, gradient changes and temperature variations on a treadmill. I alternate my weekly treadmill workouts. On one week, I do one-quarter mile ladder runs followed by one-quarter mile cool down walks at 3 miles per hour. For example I might run a quarter mile at 5, 5.5, 6, 6.5, 7, 6.5,6,5.5 and 5 mph. In between each quarter mile run, I recover with one-quarter mile walks.
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On the alternate weeks, I run a set pace for one hour and record my average and maximum heart rates plus the total increase in beats per minute (bpm) during that time. The amount and rate of increase during that hour is what I call cardiac creep. I set the treadmill for a 4.5-mile per hour pace today and went for one hour. As one gets closer to his maximum heart rate, the rate of heart rate also increases. In other words, if you go beyond your comfort zone where your heart rate remains relatively steady, all of a sudden the heart rate will increase and not level off. It will continue to climb and climb.
What I have noticed over the years in my journals is that my top speed and average pace have declined and one factor that explains this is a significant drop in my ability to sustain and endure a high average heart rate and an increase in cardiac creep. It is simply too uncomfortable to hold as high an average heart rate over a given distance and my heart rate climbs faster at a given speed. I have also noticed that my average heart rate during a marathon has declined by 20 bpm. While my predicted maximum heart rate is 150 (220-70 [age] = 150. I have a maximum heart rate of about 175 and this has not dropped more than 10 bpm since age 50. In my final marathon this fall my average heart rate was 131 with a maximum of 153. When I qualified for Boston the first time in 1994 (age 50) my average heart rate then was above my maximum heart rate this year. Cardiac creep is often explained as a drop in blood volume via loss of water but I also think that the heart muscle and the skeletal muscles fatigue over time even with adequate O2 supplied. I am also the same weight and percent body fat as 20 years ago.
So many running performance articles base predictions off from maximum heart rate and/or VO2 Max but it seems to me that the average endurable and sustainable heart rate and rate/amount of heart rate climb are also something to consider. Today, at a constant pace, I had an average heart rate of 125 with a maximum of 140. I would say that the less the difference between the two (average and maximum) the more one could sustain a given pace. I was well within even my predicted maximum and far from my maximum heart rate yet my perceived level of effort was high near the end of the hour. The amount/rate of cardiac creep I experienced would be acceptable for a 10K but would it be acceptable for a marathon? I think not. The amount and rate of creep during a given run is edging you ever closer to what is referred to as the lactate threshold. That is essentially when the body can no longer keep pace with the buildup of lactate acid in the muscles and one has to slow down.
One thing that tells me if I am running at a sustainable pace is the ability to still get what I would call a ‘deep cleansing breath’. It seems like when I am in the groove, I can get one about every minute. If I am on a treadmill at the time, I can see my heart rate drop about two beats for a few seconds.
The “so what” part of this is that cardiac creep is like taking money from your bank account. How many withdrawals can you take before you are broke?