A tempo run is a type of training workout for runners in which the runner will run at a steady pace for a long period of time. The runner often aims for about 80% effort, or about 20% lower than race pace, during this run. The idea behind a tempo run is to get the muscles used to a constant running pace at a higher speed than a fun run or light training run, which should help the body get used to such a pace during races. It is also useful for improving one's lactic acid threshold.
Lactic acid is a by-product of burned glycogen, which is the body's preferred fuel source during intense physical activity. Lactic acid will build up in the muscles over the course of a workout or run, leading to a burning, cramping, or tight feeling in the legs. The maximum amount of lactic acid buildup one can have in the muscles while still being able to use them is known as "lactic acid threshold." A tempo run helps a runner improve his or her lactic acid threshold by producing lactic acid over the course of the run, and forcing the muscles to continue to perform despite the buildup.
A workout that is more strenuous than the tempo run is the interval workout. An interval workout aims to improve lactic acid threshold as well, but an interval workout can also help strengthen muscles and improve cardiovascular performance. Instead of running at a constant speed as in a tempo run, a runner will start off slower, then sprint or at least increase speed for a set period of time, before falling back to the slower speed. This is done several times throughout the course of the run. Interval workouts will be done for shorter distances than a tempo run, which is a medium to long run.
Like any other running workout, a tempo run can lead to injury if proper precautions are not taken. All runners should spend several minutes both before and after the workout stretching the leg muscles to prepare them for the strain of a longer run. Stretching after the run is just as important, as muscles are easier to stretch when they are warmed up. Proper hydration will also be necessary to prevent cramping and dehydration, and the runner should properly dress for the weather to prevent injury as well as sudden body temperature fluctuations.