Understanding the relationship between the intensity of a training session and its optimal duration is poorly understood. Endurance athletes derive energy from three different energy systems. This means that an endurance athlete needs to train all three.

Muscles are fuelled by different energy systems. The systems are:
- The creatine phosphate system
- The lactate system
- The aerobic system

When we train we want to target these energy systems in a way that reflects the demand of the the event we are training for.

Simplified, the three training zones are:
- Zone 1- aerobic, easy, recovery, long slow distance. I can keep going and going. Heart rate is under 80% but better around 70-75% of maximum. Energy system used is aerobic system.
- Zone 2 - Threshold training. Moderately hard. Heart rate is 80-85% of maximum. Energy system is aerobic system with use from the lactate system. 
- Zone 3 - Very high intensity, race pace. Training is very hard. Heart rate is over 85% of maximum. Energy system used is lactate with use from the creatine phosphate system.

Knowing when and how to train these systems based on the event we are training for is when improvement in performance is made. In addition, it is important to understand how much time is spent training in these zones based on the distance of event we are training for.