Why "Cardiovascular Drift" is something you should know abo

Cardiovascular drift refers to the increase in heart rate that occurs during prolonged endurance exercise with little or no change in workload. During steady-state aerobic exercise, heart rate should reflect the intensity of the work being performed. If intensity (workload) remains constant, heart rate should remain constant as well. With prolonged exercise, however, it is common to see heart rate "drift" upward during the workout, even with no increase in workout. Sometimes the increases can be significant.

Cardiovascular drift is mostly caused by increased body core temperature. This increases heart rate, but decreases stroke volume so that cardiac output (and oxygen uptake) remain the same. The effect can be enhanced by the decrease in plasma volume that is caused by water loss during exercise. Both causes of cardiovascular drift will be more pronounced when exercising under thermal stress--e.g. hot room, high humidity. The effect can be blunted somewhat by ingesting sufficient fluid during the workout.

It is important to emphasize that cardiovascular drift results in an increased heart rate WITHOUT AN ASSOCIATED INCREASE IN OXYGEN UPTAKE OR CALORIES BURNED. You can observe this phenomenon by paying attention to your breathing during a longer, low to medium intensity cardio workout. If you are attuned to your breathing pattern, you will notice that, even though heart rate increases steadily during the workout, breathing remains relatively constant.

I tend to "run hot" during exercise and lose a decent amount of water weight (3-4 lbs/45 min) so I have been aware of this phenomenon for many years. I was thinking about it again last night during my workout. I try to include one (relatively) extended, lower-intensity (50%-60%) cardio workout every week. Sometimes it 's hard to do because you really have to force yourself to stay at a low level and just grind it out, but it is an important part of an overall conditioning program. Since last October, I have been kind of spinning in circles due to a variety of physical and life issues. For the past month or so I have gotten back on track and have started ramping my program back up.

Last night I did 60 minutes on the cross trainer, with a goal intensity of 50%. I kept the workload constant the entire time (that's what I mean by "grinding it out"). Once I steadied out, my heart rate settled at around 113, right in my goal of 110-115. As expected, at around 20 min it started to drift upward. Because my endurance levels are still relatively untrained (I had only been beyond 30 min twice in the previous 2 months), the increase was larger than usual. After 30 min the HR went up even more and by the end I was pushing 140.

So that was the "drift"--113 to 140, with no increase in workload and no significant change in breathing. Consistency of workload is demonstrated by the calories burned shown on the machine display: 434 the first 30 min, 441 the second 30.

So what are the practical implications of cardiovascular drift and why should you even pay attention?

One goes back to some of my issues with HRM calorie totals. Since HRMs only measure heart rate and estimate calories based on heart rate, an increased heart rate with no increase in oxygen uptake (as occurs during cardiovascular drift) could cause an HRM to OVERestimate exercise calories. This was clearly demonstrated during my workout. I wear a Polar F11. The HRM calorie number for the 1st 30 min of my workout was 353, for the 2nd 30 min 460. That's a 30% difference--with NO change in workload intensity. Just one more reason why one must exercise caution when incorporating HRM exercise calorie estimates into an eating plan.

Two: Cardiovascular drift is the main reason why I advise clients NOT to use HR-controlled programs on cardio machines. Cardio programs use the HR data from the machine sensors or a chest strap to automatically adjust workout to keep you in a specific HR "target" range. On paper, it sounds like a great idea--let the machine automatically keep you in your desired "zone". However, in reality, once you reach your target and "drift" above it, the machine will start to reduce your workload, thus steadily decreasing the quality of your workout. Workload intensity is the stimulus that produces the results you want (whether the goal for the day is easy, hard or in between). You can use heart rate to help gauge and control intensity, but you don't want to reduce intensity solely on inflated HR numbers

Three: since cardiovascular drift is enhanced by thermal stress, any activity that occurs under those conditions (e.g. "hot" yoga, working out in sweatsuits) will result in exaggerated and overestimated calorie counts that are virtually useless.

Bottom line: Don't be a slave to your HRM numbers --either heart rate or calories. As your progress in your workout program, learn to use breathing as a way to intuitively assess your intensity and use that along with heart rate to control your overall effort.