The latest gym buzzword is HIIT, but what does it stand for? This popular fitness trend is popping up everywhere — and with good reason. HIIT stands for High Intensity Interval Training. The intervals consist of heavy (90 to 100 percent effort) and light (20 to 30 percent effort) workloads for a pre-determined amount of time. The latter is considered “active recovery.” For example, HIIT would combine a heavy workload like sprints for 45 seconds, with a light workload like walking for 20 seconds.