effects of exercise

when you exercise your muscles slide against each other that crate friction and create little rips in the muscles and when they repair the muscle gets thicker and stronger.

If the rate of exercise is increased due to greater muscular activity, a point is reached at which not enough oxygen is supplied to meet the requirements of the muscle cells for aerobic respiration, as described earlier.
Anaerobic respiration cannot go on for more than a few seconds as the lactic acid/lactate builds up in the muscles and is said to cause cramp, or at least fatigue and soreness. In fact it is better to say that there is an equilibrium between the rate of production and the rate of removal of lactic acid/lactate.
http://www.biotopics.co.uk/newgcse/exerciseeffects.html i got some of this imformation from this website.