i've hearad that the accumulation of lactic acid in the muscle causes pH levels to drop and The 'muscle burn' is due to the neural ( nerve ) irritation associated with an altered pH ( pH = measurement of acidity, more below ) within the worked / exercised muscle A snippet of what i've found says
The chemical reactions cause muscle fiber shortening = muscle contraction.
The chemical reactions ( glycolysis / Krebs Cycle amongst others ) convert the nutrients into waste byproducts to be dealt with by the excretory systems.
The waste products are normally easily removed from the muscle.
As exercise (more muscle contraction = more chemical reactions ) increases the amount of byproducts produced also increases .. eventually the amount of byproducts produced exceeds the bodies ability to remove them from the muscle group.