Muscles become sore from the lactic acid production being increased from strenuous output of energy on the muscles.
You muscles could become sore after sleeping when you do not exercise for many reasons. The most common reason is that you stayed in one position to long which made the muscle stiff and tired.
The muscles in the posterior can become sore due to overexertion. For instance, running or walking long distances can cause a great deal of soreness in the legs and buttocks.
When muscles are over-exerted, the muscle cells carry out anaerobic respiration to compensate for the oxygen deficit. Lactic acid produced as a result of this causes muscles to become sore or develop cramps.
Stretching is a good way to help sore muscles to feel better.
your muscles will get smaller and you will be sore all the time, and your immune system will become weaker.
Lactic acid created during a process of "lactic acid fermentation."
Too much use of your muscles.
Your muscles tightening up
because your muscles are adapting to the workout you did