High Intensity Interval Training, or H.I.I.T., has been clinically proven to burn more fat than steady state cardio training. H.I.I.T. is superior in that the extremely high intensity bursts create an oxygen deficit in the body which triggers a spike in metabolism for many hours after training ceases as the body attempts to adjust for this increased demand on the system. This is call EPOC, or excess post-exercise oxygen consumption. This means you can do cardio for less time and subsequently burn more fat, great right? Not always. The downside of H.I.I.T is that it is just that, high intensity, and as such, it places a substantial load on your body, specifically your central nervous system.
Most overtraining from a workout routine is not the result of muscle soreness, but rather of central nervous system (C.N.S.) fatigue. While sore muscles need time to heal and repair, the damage is localized. Thus, having sore pecs does not have any influence on when and how you train other muscles. Conversely, C.N.S. fatigue is far more ubiquitous. A fatigued C.N.S. affects the entire body and will result in decreased ability to fire muscles, all of them. This is the reason you may have a blistering squat day and come in the next day for chest and feel weak. While your chest muscles themselves are fresh, squatting places a tremendous load on the C.N.S. and as a result your brain is unable to send powerful signals to muscles to contract and in turn your muscles do not completely fire. You feel lethargic, drained, and overall just blahhh.
H.I.I.T., being as intense and explosive as it is, places additional strain on the C.N.S. above and beyond weight training, resulting in the need for more recovery time, and less frequent and intense weight training sessions. This may not be an issue for some people, depending on multiple factors such as age, muscle mass, workout intensity and style etc., however for an advanced, heavily muscled lifter who goes hard and heavy in the gym, H.I.I.T. is never recommended due to the fact that their weight routine will tax the C.N.S. so much on its own.
Slow, steady state cardio, on the other end of the spectrum, has the opposite effect on the C.N.S. and recovery times. It has been clinically shown that 20 minutes of light, post-training steady state cardio will help you recover faster by means of improving blood flow and nutrient exchange through the body, and slow or moderate steady-state cardio places virtually no undue load on your body.
Overall, H.I.I.T. is a fantastic tool for weight loss, but you must evaluate your workout program as a whole as well as your individual recovery needs before incorporating it. If your workouts aren’t too intense or loaded with heavy compound movements, you stand a much better chance of being able to incorporate H.I.I.T. without issue. However, for those with lots of muscle mass and have brutally intense workouts, it is probably best to stick with steady-state cardio so you can have more focus on weight training.