Weight loss is ruled by energy balance; to lose weight your caloric intake must be lower than your caloric output. Caloric restriction is used to decrease the caloric intake portion of the equation, while exercise can be used to increase the caloric output portion. For several years, Low-Intensity Steady State (LISS) cardio has been used to enhance fat loss phases by increasing energy expenditure. Recently, High-Intensity Interval Training (HIIT) has been famed for being a shorter, more fun type of cardio that causes your body to burn calories throughout the entire day. The purpose of this article is to look at the scientific literature to critically analyze each type of cardio and determine their benefits and drawbacks.
LISS and HIIT: What Are They?
LISS is cardio done at a low-intensity, typically low enough that it would allow the individual to hold a conversation without gasping for air. The intensity of the cardio is not changed throughout the duration of the exercise. Due to the low-intensity, LISS can typically be performed for long periods of time. Examples of LISS include walking on the treadmill, riding the elliptical at a low intensity, exercising on the stair master, and many others.
HITT is a type of cardio that contains short bursts of intense work with periods of low-intensity or even rest in between. Typically, a 3:1 or 4:1 rest-to-work ratio is used. For example, in a 3:1 approach on a bike, you would pedal at a low intensity for 45 seconds and then burst into an all-out sprint for 15 seconds. Examples of HIIT include: bicycle sprints, regular sprints, sled pulls, battle pulls and many others.
Excess Post Exercise Oxygen Consumption: The Truth
HIIT has been famed for its effect on Excess Post-Exercise Oxygen Consumption (EPOC). When you perform a burst of exercise at a high intensity, your body must use the anaerobic pathway to generate energy, meaning that it is generating energy without using oxygen. After the exercise is over, your body uses the aerobic pathway to replenish this anaerobic energy that was used. Thus, oxygen is consumed after the exercise to “repay” the oxygen debt. Hence the name, excess post-exercise oxygen consumption. HIIT creates an oxygen debt by demanding a large amount of energy rapidly. As a result, an “oxygen debt” is created and must be replenished throughout the day. For this reason, it has been claimed that HIIT causes your body to be in a fat-burning state all day. However, the effects of HITT on EPOC have been greatly exaggerated.
In reality, the effect that HIIT has on your energy consumption throughout the day is minimal. Gore et al. (2006) have shown that HIIT only creates a post-exercise energy consumption of 6-15% of the energy cost of the actual exercise. If you were to burn 120kcal in a typical 10-minute HIIT session, you would only burn 7-18kcal post-workout – which is not very significant by any means.
The Interference Effect
Cardio and strength/hypertrophy training interfere with each other’s ability to progress. The interference effect occurs because endurance training and strength training require opposite adaptations from your muscles. When performing cardio, a protein called AMPK is secreted (King-Himmelreich et al., 2016). This protein is a signal of energy-depletion and helps your body to conserve energy. As a result, it inhibits muscle-building by inhibiting mTOR, the master regulator of protein synthesis, or muscle building (Bolster et al., 2002). This pathway makes sense, as muscle-building is highly energy demanding and should be down-regulated if your body is attempting to conserve energy. While this adaption might be beneficial for survival, it interferes with your gains!
A recent study showed that a group of men who did resistance training plus HIIT grew their biceps 35% less and gained 31% less strength than a group who only performed resistance training (Kikuchi et al., 2016).
HIIT and Recovery
Each athlete has a total amount of exercise volume they can do and still benefit from. If you do more than this, your body will not be able to optimally recover and improve – in fact, it may even digress. As a physique/strength athlete, you obviously want most of your volume to come from lifting weights. However, you must still consider the fatigue that comes from doing cardio. HIIT takes a greater toll on your recovery than LISS and may interfere with your ability to properly recover for your next training session. If you are in dieting phase, then you will already have reduced recovery abilities; it will likely be a detriment to decrease your recovery abilities even further by performing HIIT cardio.
The Case For LISS
A major benefit of LISS is that it can be performed for long periods of time due to its low-intensity. You cannot perform HIIT for a long period of time while maintaining adequate intensity and proper form. However, you can walk on an incline treadmill or ride the elliptical for an hour or more when performing LISS. For this reason, you are able to burn a greater number of total calories from the LISS session.
Remember that weight loss is a result of energy balance, your caloric intake must be lower than your caloric output. The purpose of cardio for a strength or physique athlete is to increase the caloric output to potentiate fat loss. You want to use the cardio session to burn as many calories as possible. While HIIT may cause you to burn an extra few calories, LISS can be performed for longer and will cause you to burn more calories. Further, LISS will have a lower effect on your ability to recover.
Avoid Black-or-White Thinking
While the evidence seems to show that LISS may be more beneficial than HIIT for strength and physique athletes, this does not mean that you should never do HIIT. If you truly do not like doing LISS, then you can alternate between both styles of cardio. However, you likely want to limit HIIT to 1-2 times per week, according to your ability to recover, nutrition, training, injuries, goals and lifestyle.