A tempo run is a faster-paced workout also known as a lactate-threshold, LT, or threshold run. Tempo pace is often described as "comfortably hard." Tempo running improves a crucial physiological variable for running success: our metabolic fitness. Why tempo runs work By increasing your LT, or the point at which the body fatigues at a certain pace. During tempo runs, lactate and hydrogen ions—by-products of metabolism—are released into the muscles. The ions make the muscles acidic, eventually leading to fatigue. The better trained you become, the higher you push your "threshold," meaning your muscles become better at using these by-products. The result is less-acidic muscles (that is, muscles that haven't reached their new "threshold"), so they keep on contracting, letting you run farther and faster. But to garner this training effect, you've got to put in enough time at the right intensity. Finding the right tempo To ensure you're doing tempo workouts at the right pace, use one of these four methods to gauge your intensity. Recent Race: Add 30 to 40 seconds to your current 5K pace or 15 to 20 seconds to your 10K pace. Heart Rate: 85 to 90 percent of your maximum heart rate. Perceived Exertion: An 8 on a 1-to-10 scale (a comfortable effort would be a 5; racing would be close to a 10). Talk Test: A question like "Pace okay?" should be possible, but conversation won't be.