Changes

Decreases in stroke volume of 10-16% and increases in heart rate of 12-16% are associated with a 9% to 19% reduction in [[VO2max| V̇O₂max]]<ref name="Ganio-2006"/><ref name="Wingo-2006b"/><ref name="Wingo-2006a"/><ref name="Wingo-2005"/>. Fluid intake<ref name="Ganio-2006"/> or body cooling<ref name="Wingo-2006a"/> will mitigate the reduction in stroke volume and [[VO2max| V̇O₂max]]. Reducing the exercise load to keep the Heart Rate constant will also mitigate, but not eliminate the reduction in [[VO2max| V̇O₂max]]<ref name="Wingo-2006b"/>. Heart Rate drift and reduction in [[VO2max| V̇O₂max]] is greater in hot than cooler conditions<ref name="Lafrenz-2008"/>. 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<ref name="Wingo-2005"/>.

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<ref name="González-Alonso-2000"/>. 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% [[VO2max| 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<ref name="Fritzsche-1999"/>. (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 (hypothermia[[Hypothermia]]), with each contributing about equally in most cases<ref name="Coyle-2001"/>.

When 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[[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 [[Breathing]] can be seen in subjects who are exercising when glycogen [[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 [[Breathing]] to loud, labored breathing[[Breathing]], and struggle to stay on pace. [[Glycogen ]] depletion is not the usual cause of Heart Rate Drift, but the symptoms are the same.