A much better explanation of the training zones and z-notation

from http://www.cptips.com/hrmntr.htm

HEART RATE TRAINING ZONES

There are 5 training "zones" or heart rate ranges. These are arbitrary divisions and can differ from article to article or coach to coach. They are based on the increase in heart rate (and cardiac output) as the oxygen consumption of the exercising muscle increases, and the concept of the benefits of variable stress in developing the exercising muscle (heart or skeletal). As one moves up the hierarchy of training zones, exercise intensity increases and there is a shift from the use of fat as an energy source for the muscle cell to carbohydrate (below 70% MHR fat is burned preferentially). And as the MHR is reached, there is a shift in the muscle cell towards anaerobic (without oxygen) metabolism with increased lactic acid production.

The Heart Rate Intensity Zones are divided as follows:

• Zone 1 65% of MHR (recovery rides)
• Zone 2 65-72% of MHR (endurance events)
• Zone 3 73-80% of MHR (high level aerobic activity)
• Zone 4 84-90% of MHR (lactate threshold(LT,AT); time trialing)
• Zone 5 91-100% of MHR (sprints and anaerobic training)

If you always train at low heart rates, you will develop endurance with no top end speed. Conversely if you train hard most of the time, you'll never recover completely and chronic fatigue will poison your performance. The solution is to mix hard training with easy pedaling in the proper proportions.

The best approach is to stay below 80% of maximum heart rate (zones 1 to 3) on your easy days to build an aerobic base while allowing day to day recovery, and then push above 85% when it's time to go hard to improve your high level performance. But avoid training in the no man's land or mediocre middle at 80-85% of MHR where it's too difficult to maintain the pace for the long rides needed to build endurance and allow some recovery time, but not hard enough to significantly improve your aerobic performance and increase your lactate threshold.

Training programs should be individualized, but once a good base is developed early in the season with Zones 1 and 2 exertion, most programs contain the following elements.