This workout was designed specifically for the treadmill. Unlike traditional threshold runs, which are done at a steady, moderately fast pace, this one includes frequent, small fluctuations in pace. The purpose of these fluctuations is to make the workout more interesting, as running indoors can get a little boring. Like traditional threshold runs, this one increases fatigue resistance at moderately high intensities. It’s a somewhat challenging session that is appropriate for all runners except beginners.