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.
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.