Zone 1: Active Recovery

Easy aerobic training which stimulates circulation and accelerates tissue healing response. Intensity is enough to increase blood circulation and trigger a growth hormone response, but not enough to cause significant muscle damage.

Zone 2: Endurance

This zone should feel very easy (“conversational” effort). Training at this intensity primarily uses slow-twitch muscle fibers, since these fibers provide most of the mobility for events lasting 2 minutes or longer.

Zone 3: Muscular Endurance below LT

In this zone, the body is still primarily functioning aerobically, conversation is possible, and burning in the legs/shortness of breath is minimal. The intensity is too high however for maximal stimulation of the slow-twitch muscle fibers and fat-burning. As intensity increases, oxygen debt becomes greater, and more carbohydrate than fat will be burned for fuel.

Zone 4: Muscular Endurance at LT

Here the aerobic and anaerobic systems are working together in balance to provide energy for exercise. Anaerobic metabolism is slow enough that lactic acid does not accumulate.
At this intensity, you are working very hard, but can still maintain the effort since lactic acid levels in the blood and muscles are steady. Increasing the intensity just slightly causes lactic acid to build up creating rapid fatigue.