Heart Rate Drift
Heart Rate Drift is the reduction in the amount of blood the heart pumps with each beat (Stroke Volume) and the resulting rise in Heart Rate as the heart attempts to maintain the flow of blood (Cardiac Output). This drift starts happening after about 10 minutes of moderate (50-75% V̇O2max) exercise. Heart Rate Drift is also called cardiovascular drift.
1 Heart Rate Drift, V̇O2max and Maximum Heart Rate
Decreases in stroke volume of 10-16% and increases in heart rate of 12-16% are associated with a 9% to 19% reduction in V̇O2max. Fluid intake or body cooling will mitigate the reduction in stroke volume and V̇O2max. Reducing the exercise load to keep the Heart Rate constant will also mitigate, but not eliminate the reduction in V̇O2max. Heart Rate drift and reduction in V̇O2max is greater in hot than cooler conditions. Heart Rate Drift also increases the Maximum Heart Rate, with one study showing the average Maximum Heart Rate going from 189 BPM to 194 BPM after 45 minutes of exercise.
2 Mechanisms of Heart Rate Drift
The traditional explanation for Heart Rate Drift is that blood flow to the skin is increased, which reduces available blood volume for the rest of the body and therefore the ability of the heart to fill with blood, reducing the stroke volume. However, in hydrated subjects, the stroke volume is maintained even though the blood flow to the skin increases. An intriguing study gave subjects either a placebo or a beta blocker then looked at heart rate and stroke volume over the course of 60 min cycling at 57% V̇O2max. With the placebo, the subjects showed the classic heart rate drift, but with the beta blockers, the heart rate and cardiac output remained steady. (In both conditions, the subjects received a carbohydrate drink to prevent dehydration and both conditions had the same perceived exertion.) There is reasonable evidence that the reduction in stroke volume is the result of dehydration and/or overheating, with each contributing about equally in most cases.
3 Heart Rate Drift and Glycogen Depletion
When muscles become depleted of Glycogen a greater percentage of energy comes from fat. However, each liter of oxygen will produce 5 calories of energy when burning carbohydrate, but only 4.6 calories from fat. This means that burning fat produces about 10% less power than burning carbohydrate. If the energy expenditure stays constant then oxygen supply (heart rate, Breathing) has to go up by 10%, which will use a greater percentage of exercise capacity (feeling harder). On the other hand, if oxygen supply (and effort) stays constant, energy expenditure (and therefore pace) will have to drop by 10%. This change in Heart Rate and Breathing can be seen in subjects who are exercising when Glycogen depleted. How would this feel? For a 3:10 marathon runner, it would turn the 7:14 min/mile marathon pace into a 7:54 min/mile pace, or make the marathon pace feel like a 10K pace. (That's 4:31min/Km and 4:56 min/Km.) You can often hear this change in a marathon runner; they will go from relaxed, inaudible Breathing to loud, labored Breathing, and struggle to stay on pace. Glycogen depletion is not the usual cause of Heart Rate Drift, but the symptoms are the same.
4 Detecting Heart Rate Drift with Efficiency Calculations
Because Heart Rate Drift can either show as a rise in Heart Rate for a constant effort, or a stable Heart Rate and a declining effort, it can be hard to see in a simple graph of Heart Rate. A more useful measure is to look at Running Efficiency, which is the number of heart beats taken to run a given distance. This normalizes the heart rate against pace, making heart rate drift more obvious.
5 Heart Rate Drift and the SportTracks Plugin
The SportTracks Dailymile Plugin will calculate Heart Rate Drift and Efficiency Drift given a recording of Heart Rate and pace. The approach uses a least squares calculation to determine the slope of a best fit line through the values.
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