So I'm borrowing portions of a great post that my blogging friend, Karen wrote. (And I added in some comments in purple.)
FATIGUE is one of the most disabling symptoms of Multiple Sclerosis and 80 to 90% of people with MS are affected by it.
MS can cause an overwhelming fatigue which can be very persistent, and often causes other symptoms such as mobility issues, spasticity, muscle spasms, difficulties with concentration, memory, and visual disturbances to worsen.
MS fatigue is not the same as the tiredness that is felt by most of the population. When you are tired you can usually alleviate that by resting or getting a good night's sleep. When you have MS, the fatigue may be lessened with rest and sleep, but it won't be eliminated.
The fatigue of MS is broken down into two types: Primary fatigue, (which includes heat sensitivity and localized fatigue) and Secondary fatigue.
Primary fatigue is caused by the disease process itself, as a direct result of damage (demyelination) to the central nervous system. This damage can cause overwhelming tiredness (lassitude) that is not directly related to an increase in activity.
Then there is the "heat sensitivity" fatigue that is caused by a rise in body temperature due to hot humid weather, a hot bath or shower, illness or exercising. Luckily I do not seem to be too affected by this.
Oh and lets not forget “localized” fatigue, which is extreme weakness of a particular muscle group after a short period of use. An example of that would be; I walk up a flight of stairs in my house and when I get to the top, my legs feel like rubber with ten pound bags of cement attached to them, I then crumple to the floor and crawl to the nearest chair.
On to Secondary fatigue.