This is running at a pace that’s faster than “moderate” but not exactly “hard- A pace you can run for an hour. Tempo run increase the body’s lactate threshold, or “LT.” which is the point at which lactic acid begins to accumulate in muscles. Lactic acid is a by-product of the body’s metabolization of glucose, which is the primary energy source for running. This accumulation causes increased levels of acidity in the muscle tissue, causing the fatigue and soreness that runners experience in races. When runners increases their LT, they are able to reduce the accumulation of lactic acid and run at higher speeds without suffering muscle fatigue.