Cardiac drift

Don’t be rigid with heart rate zones!
 If you stay religiously within a 5 beat “zone” for all of your long workouts the net result will be a gradual slow down over time. Not exactly the desired effect or what you want to happen during a race. Typically for longer workouts I stay within a 5 beat zone for the first half and allow my HR to drift upward during the second half while maintaining an equal pace or output for the whole workout. Don’t worry – you are not going to go anaerobic and collapse. For me this drift is only about 10-15 beats over the period of a long run and only about 5 beats higher during my final long workouts approaching Ironman.

Actually in one study the researchers were able to practically eliminate cardiac drift by giving the athletes a glucose and hydration drip.