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What causes muscle fatigue?

I'm an athletic woman who just turned 30, and I've always had good endurance. Recently, though, I started getting bouts of muscle fatigue that it seems like I just can't break through. I'm training pretty hard, though. Could I just be pushing myself too hard, or is there an illness that causes muscle fatigue?
Muscle fatigue is a common symptom. As it is such a subjective experience--it is hard to tell if this is normal (from exercise) or if this is caused by a medical condition. I recommend you see your primary care doctor who can rule out some of the medical causes of fatigue and therefore reassure you if this is normal. As you mention--muscle fatigue is an expected consequence of training. Our body's muscles use oxygen to make energy. Once the muscles have used up the oxygen and energy stores--it starts doing second line energy making process--and this second line process makes "lactic acid" as a waste product. As this acid builds up in your muscles, your body feels fatigue and pain. This sensation is a defense mechanism that makes you stop exercising and replete your stores of energy. However there are some diseases that make you more fatigued then you should be. Some conditions to rule out are (1) anemia--the most common cause of fatigue in women of reproductive age. Anemia is a low red blood count--which can occur from heavy menstrual cycles. If you have a low blood count then you cannot get enough energy to your muscles. (2) Hypothyroidism--the thyroid hormone is the "energy hormone." If this is low you would feel fatigued. (3) Myositis--this is inflammation of the muscles. A simple blood test called a CPK can test for this. (4) Depression. Honestly, depression can sometimes present ans just fatigue. There are MANY other causes-but these are some of the more common. Talk to your doctor. He or she can help your rule out these causes as well as see if there may be any other cause of fatigue.
This answer is for general informational purposes only and is not a substitute for professional medical advice.
If you think you may have a medical emergency, call your doctor or (in the United States) 911 immediately. Always seek the advice of your doctor before starting or changing treatment. Medical professionals who provide responses to health-related questions are intended third party beneficiaries with certain rights under Zocdoc’s Terms of Service.

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