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=Heart Rate Drift and Glycogen Depletion=
When [[Muscle|muscles ]] become depleted of [[Glycogen]] a greater percentage of energy comes from fat<ref name="Romijn-1993"/>. However, each liter of oxygen will produce 5 calories of energy when burning carbohydrate, but only 4.6 calories from fat<ref name="Plowman"/>. 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<ref name="Heigenhauser-1983"/>. 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.
=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 Calculator| 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.