Muscle Fatigue and its Causes by Duncan Stewart

First in an series

Most, if not all, endurance athletes have had the unpleasant experience of “hitting the wall.” I know I have, and it’s no fun! By knowing the factors that cause this condition, you can help delay the onset of the wall, by taking actions to reduce its effect. Here are the prime contributors and what you can to combat them:

1) Dehydration. Exercise generates a lot of heat. The more intense the exercise, the more heat is generated. This is why a runner on a cool day can be dressed in only shorts and a singlet, yet still be sweating, while a casual walker out in the same weather may be wearing long pants and a long-sleeved shirt. To get rid of the heat, your body perspires. Evaporation of the perspiration creates a cooling effect. However, when your blood gives up this fluid, it becomes thicker and therefore harder to pump through your body. Solution? Drink plenty of fluids, before, during, and after exercise. Thirst is a poor indicator of dehydration, so don’t wait until you’re thirsty before replenishing those fluids. Also, it’s difficult to take in enough fluids to offset what you’re sweating off, so for a long duration event, you really need to be “topped off” beforehand. Experienced runners know this, which is why the lines for the bathroom are so long before a marathon! J

2) Overheating. As you exercise and your body temperature rises, it becomes more difficult to provide enough blood to your muscles AND regulate your temperature. In response, you’ll sweat more and more profusely, while running more and more slowly. Rising temperature and humidity exacerbate this out-of-balance condition, which is why there are no summer marathons in Texas! Solution: dress lightly (you should feel somewhat chilled for the first couple miles of your run), take in plenty of fluids, and minimize your runs in hot and/or humid weather. When it’s sunny, a lightweight, brimmed cap can also help. Get one that’s vented, so heat and perspiration can escape.

3) Glycogen depletion. Muscle glycogen is the energy source that allows you to exercise at high levels, due to the efficient way in which your body is able to convert it into energy. Carbohydrates are the prime food source to boost glycogen levels. When running at a moderate level (such as in a marathon), you use both glycogen and fat. Running fast burns proportionately more glycogen vs fat than running slowly. This is why starting too fast in a marathon will cause you to pay double later on. Eventually, the glycogen stores become depleted. As this happens, your body turns to alternate sources of energy, such as blood glucose and muscle protein. As more and more blood glucose is used, fatigue sets in because your blood sugar becomes low. Your body uses muscle protein as a “least best” energy source. Both fat and muscle protein metabolize slower than glycogen, which is why it becomes harder to run at the same pace, even if you can push past the fatigue. Solution: carb up before a long run, maintain an even pace, and take in plenty of fluids containing carbohydrates.

4) Lactic acid buildup. Lactic acid is a substance produced by your muscles when they’re working hard, and its presence causes the “burning” sensation. The more intense the exercise, the more lactic acid is produced. Although your body is able to metabolize it back into glucose, running over your lactate acid threshold will cause a lactic acid build up, which will adversely affect your performance. Training can raise this threshold. Solution: train diligently, find out where your lactate threshold is, and don’t spend much time over it.

With proper training, hydration, carbo loading, and pacing, you can run a marathon without hitting The Wall. It really makes those final miles much more fun!