Causes of Fatigue During Exercise and How to Deal With Them

istock_000010278535xsmall-1Some days working out feels easier than others. There are times where you feel like you could go for hours and other times where you feel like stopping 30 minutes into a workout because you’re wiped out. Sometimes there are physiological reasons why you’re experiencing fatigue. In other cases, “fatigue” can be psychological. It may be your brain’s way of telling you you’re overtraining or your workout has become “stale” and needs some shaking up. What causes early fatigue with exercise, and what can you do about it?


Exercise performance becomes impaired when you reach a dehydration level of 2% or more. According to research, dehydration causes premature fatigue by its effect on muscle metabolism, temperature regulation and by the additional burden it puts on your heart. It also reduces the motivation to exercise, possibly as a protective mechanism to ensure that you don’t lose more water. To avoid this problem, drink between 5 and 10-ounces of fluid every 15 minutes during a workout. If you’ll be working out for more than an hour, an electrolyte-rich sports drink is a better choice to replace lost sodium and electrolytes.

Glycogen Depletion

For moderate to high-intensity exercise, muscles depend on carbohydrates and fat as their primary fuel source. The more vigorously your work out, the more important carbohydrates are. If you exercise in a fasted state at a moderate intensity or greater for a long period of time, your muscles can become prematurely fatigued as carbohydrate stores in the form of glycogen are depleted. At this point, the liver steps up to the plate to form glucose from amino acids, a process called gluconeogenesis, but during long exercise sessions, the liver may not be able to produce enough glucose to meet the needs of the hardworking muscles. This can lead to a drop in blood glucose level and premature exercise fatigue. That’s why exercising in a fasted state makes a workout seem harder.

If you work out in the morning at a moderate intensity or greater, eat a snack containing around 0.5 grams of carbs per pound of body weight an hour or so before lacing up your exercise shoes. If you plan on working out for longer than 90 minutes, take along an energy bar or sports drink containing carbohydrates to refuel. It’ll help you train better and reduce the fatigue that comes from glycogen depletion.

Other Reasons for Premature Exercise Fatigue

If you notice a drop off in performance and are feeling more fatigued when you work out, check an iron level. Low iron levels can lead to iron-deficiency anemia that can cause reduced exercise tolerance and fatigue. Iron deficiency is a relatively common problem among young female athletes, especially those that restrict calories. Other medical problems such as diabetes, an underactive thyroid, and some medications can cause early exercise fatigue.

Fatigue and decreased performance can also be a sign that you’re overtraining. When you’re overtraining, a workout that would typically feel easy becomes a challenge. Overtraining not only causes fatigue, but it also reduces immunity to infection, so you’re more likely to catch a cold or flu virus. How do you know if you’re overdoing it? Check your heart rate first thing in the morning before getting out of bed. If your heart rate is elevated by 5 or more beats per minute above its usual range, you may be overtraining. The solution? Rest for a few days, and then return to training at half your usual volume and intensity for a week or so. Also, make sure you’re getting 7 or more hours of sleep a night and practicing good nutrition.

Sometimes exercise fatigue isn’t physiological but a sign of boredom with your current exercise routine. If you’re doing everything else right and you’re not overtraining, try doing a different routine, and see if it reinvigorates you. For example, try doing one of my exercise DVD s  that you haven’t tried in a long time for a change. If you usually strength train using machines at the gym, do a bodyweight circuit workout instead. When your workout becomes stale, you’ll feel less motivated and your workout will feel harder.

The Bottom Line?

Keep these factors in mind if you’re not performing as well as you should and you’re feeling fatigued during a workout. Sometimes something as simple as drinking more water and eating more carbs before you start can make all the difference.



J Am Coll Nutr October 2007 vol. 26 no. suppl 5 542S-548S.
Sports Med. 1984 Nov-Dec;1(6):446-58.


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