Just like any other form of training, cardio is effected by the General Adaption Syndrome and The SAID Principle. In order to continue with effective progression, variety needs to be incorporated. Stage training is a great way to do that.
In order to continue the adaptation cycle, a cardio program must be progressive and organized to minimize the risk of over-training and injury. Stage training is a three stage system that uses different heart rate zones. The three different stages mimic the three stages of training in the OPT model.
Zone One consists of a heart rate of 65-75% of a heart rate max. This zone is a recovery zone for those working in higher intensity programs. It's a great zone for beginners to start in. This form of training coincides with the stabilization training.
Zone Two consists of a heart rate of 80-85% of a heart rate max. This is near the anaerobic threshold. Anaerobic threshold is the point where the body can no longer produce enough energy for the muscle with normal oxygen intake. As a result, it begins to produce higher levels of lactic acid (part of what causes post-workout soreness) than can be removed from the body. Training and staying at this level will burn more calories with a higher percentage of those calories coming from fat (but this is not a "fat burning zone"). Zone Two coincides with the progression from stabilization training to strength training.
Zone Three consists of a heart rate of 85-90% of a heart rate max. This is a true high intensity workout and cannot be sustained for long periods of time (more than 10-60 seconds). Staying in Zone One or Two will cause an individual to hit a plateau. Working out in Zone Three about once a week is typically enough of a change to continue the adaptation cycle. One must be careful to not spend too much time in Zone Three because it can lead to over-training. Zone Three coincides with the progression from strength training to power training.
By organizing these zones to fit in with the appropriate strength training program for your goal, you will effectively achieve your goal while improving your performance.