It doesn't matter how conditioned you are or how long you've been a runner, nobody who runs is impervious to occasional fatigue. Fatigue, your body's normal response to physical exertion, is an accepted element of any endurance sport. Runners, especially those who are competitive, must understand the causes of fatigue, how to overcome it, and when it's a sign to back off, if they want to put their best leg -- or legs -- forward.
Types of Fatigue
There are two main types of fatigue among runners: metabolic fatigue and exertion fatigue. Metabolic fatigue occurs after hard, anaerobic efforts, such as sprints and interval training, and is the result of oxygen debt and acid buildup in the muscles. Exertion fatigue happens after long runs, when the muscles begin to run out of fuel and the nervous system starts wearing out. Sprinters are likely to experience metabolic fatigue; however, exertion fatigue is far more common for distance runners and the average folks who run for fitness.
Affect on Running
Continuing to run past the point of fatigue comes with associated risks. When your muscles begin to grow tired, your running mechanics often become altered. Usually, runners slow their cadence and increase stride length when they're at or nearing exhaustion, which reduces running economy, further amplifying fatigue. Other changes in gait, such as increased pronation or foot impact, can lead to injury. Moreover, running past the point of fatigue feels awful.
When Fatigue Becomes Excessive
If you continually train past the point of fatigue, you're probably flirting with an over-training injury. Not only can over-training drastically reduce performance, increase irritability and cause you to hurt yourself, but it totally drains the fun from running. Remember that you don't need to run hard or long everyday to become a better runner. In fact, including rest and recovery days will help your body repair damaged tissues so your next workout will be even stronger.
If you're in the middle of a race and fatigue hits, it's unlikely you'll stop for a catnap before you resume -- so it's important to understand how to stop fatigue before it starts. Assuming you haven't been overtraining and the fatigue you experience is normal exertion fatigue, a few things may help you stay energized. First, make sure you get at least eight hours of rest each night and include recovery days in your training routine. Drink plenty of water before, during and after runs to stay adequately hydrated, and fuel your body with nutritious foods such as whole-grain carbohydrates, fresh produce and lean protein sources. For runs lasting more than an hour, consider drinking a carbohydrate beverage or consuming an energy gel or bar mid-run to refuel your energy stores.