What are the physiological effects of interval training, and how do they increase fitness and performance?
Imagine that your muscles are engines that burn fuel (fat and carbohydrate) to keep you going, and in that engine there are two energy systems, aerobic (more fat-burning) and anaerobic (more carbohydrate-burning). Athletes and others who play sports that demand stopping and starting, or individuals who participate in endurance events that include hills or a sprint at the end, require that the muscles switch quickly between both systems. For example, say you’re on a long-distance bike ride and you come to a large hill. Along the flat road your heart rate is at the low end of your training range and you’re working aerobically and burning lots of fat (comparable to the active-recovery of your interval training session), but then you hit a very large hill. Now your heart rate increases and you start breathing harder (the work interval of your interval session), and so your muscles must make the switch to the anaerobic system where you burn more carbohydrate than fat. If you’ve put your tim
