Exercise intensity

Exercise intensity refers to how much work is being done when exercising. The intensity has an effect on what fuel the body uses and what kind of adaptations the body makes after exercise (i.e., the training effect).

Fuel used
The body uses different amounts of fuels (carbohydrate or fat) depending on the intensity and heart rate. The Heart Rate column below is an estimate for a 20 year old with a Max Heart Rate (MHR) of 200.

Training Effect
The effect achieved by exercise is affected by the intensity.

Low Intensity (up to 80% MHR)
This has the effect of improving general fitness and improves the muscles' ability to use the oxygen delivered to them via the blood. Way of doing this is by increasing the number of capillaries in the muscle.

Medium Intensity (80-85% MHR)
This intensity is around anaerobic threshold (AT) and improves an individual's AT - allowing the body to work at a higher intensity while still working aerobically.

High Intensity (85-90% MHR)
The heart is a muscle and can be trained to be come stronger and more efficient. This intensity of exercise places a load on the heart causing it to become stronger so it can pump more blood in one contraction.

Flat Out (90-100% MHR)
In this region the body works completely anaerobically - causing a quick build up of lactate (lactic acid). This kind of training, despite being very painful, improves the body's anaerobic capacity - the production, removal, and tolerance to lactic acid.