Interval training is a type of training exercise that involves a series of high-intensity 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 at more intense levels. Interval training can refer to the organization of any cardiovascular workout (e.g., cycling, running, rowing). It is prominent in training routines for many sports, but is particularly employed by runners. Variations Fartlek training Fartlek training, developed in Sweden, incorporates aspects of interval training with regular distance running. 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 2 km; rapid walking for 5 minutes (recovery); sprints of 50-60s interspersed with easy running; full-speed uphill for 200 m; rapid walking for one minute; repeating this routine until the time schedule has elapsed (a minimum of 45 minutes). The development of aerobic and anaerobic capacities, and the adaptability of fartlek - to mimic running during specific sports - are characteristics it shares with other types 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 predetermined time intervals - e.g. 200 metre sprint, walk back, and sprint again, every 3 minutes. The time interval is intended to provide just enough recovery time. 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. The acronym DIRT is sometimes used to denote the variables : D = Distance of each speed interval, I = Interval of recovery between speed intervals, R = Repetitions of speed intervals, and T = Time of each.