Signs Of Fatigue - Energy And Fatigue: Chronic Fatigue

Content provided by the Faculty of the Harvard Medical School

Signs of fatigue

Ask any two people what fatigue feels like, and you're likely to get two sets of answers. That's because the experience that people call "fatigue" may refer to very different things, for example, muscular weakness, lack of endurance, slow reaction times, poor concentration, and so on. The experience of fatigue is often classed as one of two general types: muscle fatigue and central (or brain) fatigue.

Muscle fatigue is the weakness you feel in your muscles when you've tired them out. Make a fist with one hand, then open and close it as fast as you can for one minute and you'll feel some weakness in your hand muscles. Then repeat the experiment a few more times. Chances are, your muscles won't contract as fast as they did when you first started the experiment. Central (brain) fatigue involves the central nervous system and is a general feeling that encompasses a lack of concentration or alertness as well as a sense of lethargy and loss of motivation.

Muscle fatigue and central fatigue are closely related. Every muscle contraction relies on stimulation from the central nervous system. When you feel fatigued, you're usually feeling the effects of both muscle and central fatigue. But you can also experience one without the other. Although the mechanisms are not well understood, the two types of fatigue look different inside the body.

Last updated:	January 23, 2007