Fartlek, which means "speed play" in Swedish, is a form of cardio training which puts stress mainly on the aerobic energy system due to the continuous nature of the exercise. The difference between Fartlek training and "traditional" cardio/interval training is that the intensity or speed of the exercise varies, meaning that both the aerobic and anaerobic systems can be placed under stress. The aerobic system of your body requires oxygen to complete an activity lasting longer than thirty seconds. The anaerobic system is used in activities that are not oxygen dependant and last for only a few seconds. An example of these two systems working together would be going from an aerobic walk into an anaerobic sprint, then returning to a walk/jog. On average, fartlek training sessions last a minimum of 45 minutes and the success is due to its ability to adapt to your specific needs, whether it is for a sport or just for fun.