Numerous studies have shown that short, ferociously intense interval work leads to greater fat loss than long, slow cardio; they've also shown that it builds endurance equally well, and without destroying strength gains, which is a known side effect of longer low-intensity aerobic exercise. The all-time classic is the Tabata interval: eight rounds of 20 seconds of all-out work broken by 10-second rest intervals. Ideally, trainers like their clients to do two full Tabata cycles in a single workout, twice a week, in addition to strength training. This can be done on a bike, on a rowing machine, or by running sprints – but getting the maximum benefit out of interval training requires going full speed.