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Interval training is a type of physical training that involves a series of low- to high-intensity exercise workouts interspersed with rest or relief periods. The high-intensity periods are typically at or close to anaerobic exercise, while the recovery periods involve activity of lower intensity. Varying the intensity of effort exercises the heart muscle, providing a cardiovascular workout, improving aerobic capacity and permitting the person to exercise for longer and / or more intense levels.
Interval training can refer to organization of any cardiovascular workout (e.g., cycling, running, rowing, etc.), and is prominent in training routines for many sports. It is a technique particularly employed by runners, but athletes in many disciplines use this type of training.
Fartlek training, named and developed in Sweden, incorporates aspects of interval training with regular distance training. The name means 'speed play', and consists of distance running with "bursts of harder running at more irregular points, lengths and speeds compared with interval training". For example, a Fartlek training session might consist of a warm-up for 5-10 minutes; running at a steady, hard speed for 2km; rapid walking for 5 minutes (recovery); sprints of 50-60m interspersed with easy running; full-speed uphill for 200 m; rapid walking for one minute; repeating this routine until the total time schedule has elapsed (a minimum of 45 minutes).  The development of aerobic and anaerobic capacities, and the adaptability of Fartlek to mimic running activities during specific sports are characteristics that Fartlek shares with other variations of interval training. 
Sprint interval training
"Walk-back sprinting" is one example of interval training for runners, in which one sprints a short distance (anywhere from 100 to 800 metres), then walks back to the starting point (the recovery period) to repeat the sprint a certain number of times. To add challenge to the workout, each of these sprints may start at a predetermined time interval, e.g. 200 metre sprint, walk back, and sprint again every 3 minutes. The time interval provides just enough recovery. A runner will use this method of training mainly to add speed to their race and give them a finishing kick.
High-intensity interval training
High-intensity interval training attempts to decrease the overall volume of training by increasing the effort expended during the high-intensity intervals.
Some experts believe aerobic interval training may benefit exercisers by allowing them to burn more calories in a shorter period of time, and by improving aerobic capability at a faster rate, when compared with continuous-intensity exercise. In addition, some exercisers find interval training less monotonous than continuous-intensity exercise.
Interval training can be an effective means of improving many physiological aspects of the human body. In athletes this can enhance lactate threshold and increase VO2max. Lactate threshold has been shown to be a significant factor in determining performance for long distance running events. An increase in an athlete's VO2max allows them to intake more oxygen while exercising, enhancing the capability to sustain larger spans of aerobic effort. Studies have also shown interval training can induce endurance-like adaptions, corresponding to increased capacity for whole body and skeletal muscle lipid oxidation and enhanced peripheral vascular structure and function.
There has been increasing evidence that interval training assists in managing risk factors of many diseases including metabolic syndrome, cardiovascular disease, obesity, and diabetes. It does this by improving insulin action and sensitivity. Generating higher insulin sensitivity results in lower levels of insulin need to lower glucose levels in the blood. This helps individuals with type 2 diabetes or metabolic syndrome control their glucose levels. Experts have also made it know that a combination of interval training and continuous exercise increases cardiovascular fitness and raises HDL-cholesterol, which reduces the risk of cardiovascular disease.  This type of training also decreases waist circumference, waist-to-hip ratio(WRH), and the sum of skin folds on the body.
This method of training may be more effective at inducing fat loss than simply training at a moderate intensity level for the same duration. This is due to the metabolism-boosting effects of high intensity intervals.
- Hypoventilation training
- Continuous training
- High-intensity interval training
- Long slow distance
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- Short-term sprint interval versus traditional endurance training: similar initial adaptations in human skeletal muscle and exercise performance
- Two weeks of high-intensity aerobic interval training increases the capacity for fat oxidation during exercise in women
- NYTimes Article on Interval Training "A Healthy Mix of Rest and Motion"
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