>95%
innermost layer
It's not unless you do things too strenuous. Exercise is encouraged.
Because the blood is diverted to the working muscles instead, as they need the more blood in order to provide oxygen for aerobic respiration
During strenuous exercise, lactic acid or 2-hydroxypropanoic acid is formed. Lactic acid buildup occurs during strenuous exercise, the body is incapable of removing the waste product from the muscle tissue. When lactic acid buildups it creates a burning sensation, however over recent years and studies, scientists have been doing more research on the acid and believe that is actually doesn't cause or increase muscle soreness but rather is an important fuel source for active muscles. But, more research is needed in this particular field for more definitive proof and information to be discovered.
The muscles continue to get enough energy during high levels of activity when there is not enough oxygen through the absorption of water. It is a good idea to drink water frequently when doing strenuous exercise.
lactic acid
Lactic acid makes your muscles sore. Its the product of glycolisis, which your body uses to create energy in the absence of oxygen.
oxygen
Yes. Strenuous exercise can kill the muscle fibers. Such muscle fibers are replaced by fibrous tissue.
with proper form you can.
Skeletal Muscles
When you exercise you use your muscles and your muscles contain Glycogen which could be considered your "energy source". during exercise your muscles use glycogen depleting your energy levels making you feel tired.