40/20 workouts are intervals designed around 40-seconds of work followed by 20-seconds of rest or active recovery. They fall under the umbrella of Tabata workouts (named after Dr. Izumi Tabata) and are often referred to as HIIT (high intensity interval training). Over the years, many different work-to-rest ratios have been used for certain workouts, such as 20-seconds of work/10-seconds of rest, 30-seconds of work/30seconds of rest, 40-seconds of work/20-seconds of rest, etc. All of these work on similar principals.

During the intense 40-seconds of work, you will be close to or at your lactate threshold, i.e. the point at which your muscles begin producing lactate as a byproduct of anaerobic respiration