Fartlek training or 'speed play' training involves varying your speed and the type of terrain over which you run, walk, cycle or ski. It improves aerobic and anaerobic fitness, it improves your general fitness and isnt really specific to one sport or area

Fartlek training is a type of interval training designed to improve cardiovascular output. Interval training in its broadest sense (periods of intense work interspersed with rest periods).

Most team sports will use interval training in their conditioning - the nature of say, a football match is that there is little need for the players to maintain a steady pace for 20 mins plus - the sport they play places different demands on their bodies, so when they train they use short, intense sprints followed by rest periods, which is designed to miminc the demands of a football match.

Many runners, cyclists, swimmers etc will use interval training as well. There is a solid body of evidence that highlights the benefits of interval training on VO2 max, cardiac output, lung capacity etc.

Using intervals properly will improve your cardiovascular fitness and your performance in many sports. Think broader than just fartlek training though - there are infinite ways you can apply intervals to your training.

I use this myself as a trainer/conditioning coach, however I only use it to throw in some variety in my clients program. I don't use it as a main program focus.
I do agree that in sports, it mimics often what the fartlek training does... however, it's hard to determine improvement with that type of training. So using it as a variety tool is usually my recommendation.

Derek Arsenault(CSEP-CPT)
Personal Trainer/Conditioning Coach
ONE TO ONE Health & Fitness Centre
Belleville, ON