HIIT vs. Steady-State Cardio

First, let's define what HIIT and Steady-State Cardio are.

HIIT stands for High Intensity Interval Training. It is characterized by short bursts of all out max effort (on a scale of 1-10 of your perceived effort, a 7+ or 70%-90% of your max heart rate), followed by a short period of rest where your heart rate drops to 60%-65% of your max heart rate, repeated over and over. HIIT workouts are generally 20 minutes in length, but can vary depending on the person and what they are training for. HIIT activities can include running, cycling, rowing machine sprints or heavy weight lifting. These types of workouts are anaerobic ("without oxygen"), meaning the workout is generating more demand for oxygen than can be supplied. To compensate, the body breaks down glucose stores for energy, which leads to the buildup of lactic acid in the muscles. Lactic acid causes the "burn" you feel in your muscles during a hard workout.

Steady-State Cardio, by contrast, is aerobic ("with oxygen") meaning the body uses oxygen to generate the energy required to sustain the activity. That energy comes from fat stores. These types of activities have a low-to-moderate intensity (on a scale of 1-10 of your perceived effort, between 3-5 or 40%-70% of your max heart ) that can be sustained for a period of 30 minutes or longer. Common forms of Steady-State Cardio include brisk walking, jogging, hiking, cycling and swimming.

So which is better for fat loss? It depends on how you define "better". Steady-State Cardio is more efficient at burning fat stores. Most of the calories burned during aerobic activities comes from fat. However, HIIT burns MORE calories overall, and even though it is less efficient at burning fat stores for energy specifically, you will typically end up burning more fat calories overall during a HIIT workout. Here is an example of how that concept works that is generalized for ease of math to illustrate my point:

Steady-State Cardio Activity for 30 minutes: Client burns 200 calories, 60% of which are from fat = 120 fat calories.

HIIT Activity for 20 minutes: Client burns 350 calories, 35% of which are from fat = 122.5 fat calories.

It's close, but the HIIT activity was done for 10 minutes less and still burned 2.5 more fat calories. If HIIT was performed for the same duration of time (30 minutes), the client would burn approximately 140 fat calories, or 20 more than with the Steady-State activity.

What is also not being taken into consideration is that anaerobic HIIT workouts produce EPOC (Excess Post-Exercise Oxygen Consumption), meaning your body continues to burn calories after your workout is completed. Aerobic Steady-State Cardio does not produce EPOC.

So HIIT is better, right? Not necessarily. Like most things in life, the answer is not "black and white". HIIT is tougher on the body than Steady-State Cardio, which makes it harder to perform as often, as consistently, or for as long. Additionally, HIIT works Type II muscle fibers, also known Fast Twitch Muscle Fibers. These are primarily used for quick and powerful movements for short duration. Type II fibers tire quickly. Steady-State Cardio works Type I muscle fibers, also known as Slow Twitch Muscle Fibers. These are primarily used for long duration endurance focused activities. Type I fibers take longer to tire. If you only utilized HIIT for your cardio workouts, you would beat the crap out of your body, not be able to do as much work in the long run, and would only work one set of muscle fibers.

The answer than is a blend of HIIT and Steady-State Cardio should be utilized to maximize fat loss, recovery, and to work both types of muscle fibers. How much of both and the programming involved will be different for everyone and this is where a personal trainer can be helpful in guiding a client on the right mix of both types of cardio.