Interval training is not a new concept, though it has become a much better understood one these days. In its most rudimentary form interval training was referred to as "fartlek," meaning “speed play.” Slow and easy exercise was alternated with short and fast bursts of intensive exercise. Fartlek was an unstructured form of training, but today’s interval training is very specific and highly sophisticated. In fact, it is possible to create special interval to suit the specific requirements of individual athletes.