Fatigue is a natural consequence of hard work and exercise. If you overstep your energetic boundaries while running, your body makes energy anaerobically, or without oxygen. This process often occurs due to overtraining and inadequate nutrition. Rest, proper diet and the right training can improve your endurance. If your fatigue is extreme -- to the point of sleep or immobility -- your condition may be more serious, so consult your physician about your symptoms.
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Your muscles use aerobic or anaerobic metabolism or respiration to create energy. Aerobic respiration, as opposed to anaerobic, uses oxygen to continuously synthesize ATP, an energy molecule. Oxygen allows for the breakdown of glucose, or sugar, to make carbon dioxide, water and energy. This process is ideal for running long distance, as aerobic metabolism allows your muscles to work for long periods of time without fatigue. Taking breaks along your run or reducing your intensity help keep your muscles working with oxygen. Needing more oxygen than you have causes your body to shift into anaerobic respiration.
Overtraining is one of the main causes of fatigue after running. Exceeding your aerobic capacity -- running too far or too hard -- leads to anaerobic metabolism. This process causes increases in several substances that tire muscles and sabotage performance. Concentrations of hydrogen and potassium ions rise as do levels of inorganic phosphate and adenosine diphosphate, both of which break down ATP. As a result, ATP production falls and calcium release -- which is necessary for muscle contraction -- is inhibited. Lactic acid builds up as a byproduct of anaerobic respiration. Muscle strength declines and your overall speed slows.
Diet and Nutrition
Food fuels your body; without enough calories and the right balance of nutrients, your body cannot perform optimally. Fatigue during and after runs may indicate a deficiency in calories or in certain vitamins or minerals. As a runner, your diet should consist of 50 to 70 percent carbohydrates, 20 to 30 percent fats and 10 to 20 percent protein. Eating a variety of foods can help you ingest adequate amounts of important vitamins and minerals like B12, B6 and iron. B vitamins are needed for energy metabolism, red blood cells and nerve health while iron is used to transport oxygen to muscle tissue. Talk to your doctor or a nutritionist about your diet and whether it relates to your fatigue.
Because endurance running is so demanding of your body, training properly is paramount for remaining healthy and performing optimally. Fatigue is often indicative of needing some time off. Training should include rest days and easy runs to allow muscles and other tissues adequate time to repair and rebuild. Muscles can be trained, however, to resist fatigue. Performing tempo runs at the lactate threshold -- the pace where lactic acid builds and muscles tire -- may decrease muscle fatigue. Adding sprinting and plyometric exercises to your training regimen can also increase your endurance capacity.