The short answer is: lack of oxygen or shortage of glycogen.
This site gives a bit more in depth explanation: http://news.softpedia.com/news/Why-Do-Muscles-Get-Tired-60179.shtml
when you get tierd
YOUR BRAIN REACTION,YOUR muscles, tired or no tired,even weather.
How fast muscles get tired vary from person to person. It will vary based on the persons activity level, how hydrated they are, as well as how much muscle mass they have compared to others. While some peoples muscles won't get tired too quickly, others who aren't very active may get tired quickly.
there is force that makes your muscles tired
All exercises eventually make muscles tired.
You can sooth tired muscles by getting a massage, or massaging the effected muscles yourself. You can also use a warm compress or icy hot which can help soothe your tired muscles.
Even the strongest muscles can get tired if they are overworked. When muscles get tired, they may become cramped or experience pain. It is best to refrain from exertion when the muscles start hurting.
The human body is composed of muscles (and all else). When you use those muscles they need oxygen. Playing "soccer" raises the muscle usage. Alas, you get tired!
Rest.
Masturbation does not make you muscular, your method might. EX. squating while masturbating- will increase your leg muscles -Males stroking at a fast rate will increase your upper muscles It is like an exercise- That's why you sweat and end up tired.
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A cold compress is good for swollen tissues. For tired muscles the best thing is heat or massage.