High Intensity Interval Training (HIIT) is becoming an increasingly popular way to perform cardiovascular exercise, and for good reason. Recent research suggests interval training improves aerobic fitness to a greater extent than traditional steady-state cardio (SSC), and it also results in greater calorie burning and body fat reduction. HIIT is also a great way to improve running speed for endurance athletes looking to get faster.

What is High Intensity Interval Training?

Before we continue, we need to step back and define HIIT and compare it to traditional SSC. HIIT is characterized by a defined higher intensity “work” period followed by a lower intensity “recovery” period. A simple example of HIIT cardio is running for one minute on the treadmill and walking for 30 seconds. This combination is typically performed for 20-40min. Traditional SSC is what you’re probably used to doing: running (or walking) for 45-60min at the same continuous, moderate pace, or something similar that leads to an even level of exertion throughout.

From a physiological standpoint, the primary difference between these two types of cardio is muscle activation. HIIT utilizes larger muscles and more of those larger muscles compared to SSC. This results in greater levels of lactic acid build-up and biochemical changes in the muscle that signal certain fat burning hormones to be released. The fat burning hormones released as a result of HIIT are in stark contrast to the fat preserving, muscle burning, hormones (namely cortisol) that are released in higher amount during SSC. Clearly this series of physiological events favors HIIT as a great form of cardio for fat loss.

Switching gears — how about endurance performance and HIIT? Even the most novice endurance athlete understands that to be faster in your race you have to train faster in your prep. I think we can all agree that makes sense. So on the surface HIIT would seem to make a lot of sense for any endurance athlete looking to go faster. However, this is only partially true.

Build a Base

Novice and even intermediate level endurance athletes need to be sure they build their aerobic fitness base first before HIIT is integrated as a training modality for building speed. Building an aerobic base is done by more traditional SSC. Failing to base build first, and implementing HIIT training too soon, results in progressing into anaerobic metabolism too quickly (i.e., lactate threshold is reached and exceeded very soon into a HIIT workout) and at that point the workout becomes much less effective for endurance performance. Beyond that, because of the overall training stress HIIT places on the body, there is a greater risk of orthopedic injury if body tissues haven’t been progressed to higher intensities properly. Aerobic base building through SSC not only will aid in developing aerobic fitness, but also helps in preparing the body for more vigorous exercise to come, orthopedically speaking.

In short, HIIT is a more favorable form of cardio when performed for more of a general fitness purpose as well as body fat reduction. That said, endurance athletes should proceed with caution when implementing HIIT into their training plans. If not done properly, injury can occur (due to overuse) and performance can suffer.