[klosey]

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.

• nutrients in >> movement & waste byproducts out.

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.

Common examples:

• O2 to CO2
• The chemical reactions are exothermic .. release heat = sweat!
• The Krebs Cycle deals with converting ATP >> ADP >> ..