Fartlek means ‘speed play’ in Swedish. Although, both HIIT and FIT use changes in intensity, Fartlek training uses RANDOM changes in both time and intensity, which means that you do not know what is coming and cannot prepare for it. Fartlek therefore, does give your body any time to adapt, so you will not know whether a faster interval or a slower interval is next and your metabolism and energy stay high! Check if the machines in your gym are/ can be pre-programmed to create random workouts. Many think that “Fartlek Interval Training” (FIT) is the future of HIIT.
Fartlek: Casual, unstructured training
HIIT: Precisely measured intervals