If you monitor your intensity with heart rate you need to know about this. Feedback from your heart rate monitor can make you think you are working harder than you actually are. If you were off for a long ride or run at a constant effort or workload your heart rate can drift up after 10 to 15 mins. It can drift by as much as 20% even though you are doing the same work (or consuming the same amount of oxygen). If you respond by easing off your workload you could make your session unproductive ie. the training effect will be lost. For the cyclists who are really serious, your preference should be to train with power. This does mean investing in a power meter. Initially this is expensive, but it will transfer to all your future bikes. The not quite so serious cyclist should at least have an occasional session with power feedback to see if you are working hard enough. BodyLab provides this service. For runners, your device of choice should be a GPS watch so you can monitor your running speed in real time.
Brendan Roach MSc, Exercise Physiologist