How To Structure a HIIT (or SIT) workout

Interval Training

Interval Training is an umbrella term used to describe both High Intensity Interval Training - HIIT, and its more intense cousin Sprint Interval Training - SIT. Interval training consists of repeated bouts of high work rates followed by rest or active recovery. These intervals of work and rest are repeated several times during a workout to promote time efficient adaptations and health benefits. Interval training can be a great tool to help you achieve your fitness and health goals. If it is deployed improperly it can lead to overtraining, fatigue, and burnout, so getting the prescription right is important! This post will go through the basics of how to construct and properly program a HIIT or SIT session into your training routine.

*The mode of training (bike, rower, running/treadmill etc.) does not necessarily matter unless you are training for a specific event. If you want to improve your 5K time for a fun run, most of your training should be running outside or on a treadmill (ie specificity is king). If you are just looking for metabolic health improvements, any mode will work, so choose the one you enjoy most!

The Variables

The four variables you can manipulate are: Work time, rest time, work intensity, and the number of intervals to perform in a given session. Here are brief descriptions and examples of each to provide some additional context.

Work Time: the amount of time spent during the harder or more intense work bout.

Sprint Interval work bouts are typically 10-60 seconds, while High Intensity work bouts typically last between1-5 minutes.

Rest time: the amount of time spent during the active recovery time. Sometimes this can be referred to as “Off”

Sprint interval rest time is usually 2 to 4 times the work time while HIIT rest time is between 25-50% of the work time.

Work Intensity: This is how hard you are working during the work time. This can be expressed in multiple ways, including percentage of VO2 max, percentage of heart rate max, percentage of maximal work rate (power output or speed), or even as a rating of perceived exertion. Ultimately what you choose to use depends on your goals and what tools you have available to you to measure or estimate your exercise intensity.

Number of Intervals: This is simply how many intervals you do in a session, where one interval consists of the work time and rest time combined. So when you have completed one interval, you have done one work bout and one rest bout. This typically ranges between 3 and 10 intervals per session.

Example of a HIIT Workout

This is a 1 minute work bout at 100% VO2 max with a 2 minute rest (3 minute interval) for a total of six intervals (18 minutes). *Both the absolute work rate (watts or speed) and RPE will vary depending on the person and their training status.

An additional term you will see often is Work:Rest ratio. If an interval workout is said to have a 1:3 work rest ratio with a 30 second work time then the amount of rest will be a minute and thirty seconds (90 seconds) because the work bout is one third or 33% of the rest bout. Another way of saying it is a thirty second work bout on the 2 minute mark (i.e. 30 seconds of work plus 90 seconds of rest = 2 minute intervals).

Even though there are only four variables to consider, you can combine them and manipulate them for a virtually infinite number of distinct workouts, which can make it overwhelming and potentially difficult to implement in your training. Below are some guiding principles to help focus the interval workout construction.

Interval Construction

In general the more intervals you do (≥6) and/or the longer each interval is (>90 Seconds) the more endurance type adaptations you will get (increased aerobic capacity, increased mitochondrial content, improved lactate clearance, etc). Sprint intervals that are more intense and shorter in duration (<60 seconds) promote sprint capacity, maximal power output, and fatigue resistance to repeated sprints. Both can be useful for sport performance and overall general health. Which you decide to prioritize depends mostly on your goals and what you like to do, although I would encourage everyone to employ both in their training.

Weekly Prescription

How often you do intervals in a week depends on your current fitness level and how familiar you are with them. For general health and physiological function, I would not recommend more than 3 Interval sessions per week, even in very fit people becuase of the risk for burnout and overtraining. With intervals, a little goes a long way. I generally try to do 1 HIIT session and 1 SIT sesison a week. All my other aerobic and endurance type training is low intensity long duration work. I have found this training distribution is sustainable and has worked well for me, but as with all matters of training, you may find something different works better for you. Play around with different interval types, intensities, and the weekly training frequency and let me know how it goes!

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