Fartleks - spontaneous running training sessions without planned intervals - are recommended by coaches to improve stamina and speed. Most interval timers just tell you when to start and stop running - no use if you're being spontaneous. Fartlek Interval Trainer puts you in control. It tracks when you're running, walking or stationary, turning your workout into a visual breakdown showing when you ran hard, and when you rested. Now you can run intervals without having to plan in advance.