Whenever a muscle contracts, it needs energy. This energy is released through a process called respiration. Respiration is the process by which glucoseis converted to  energy. (http://en.wikipedia.org/wiki/Cellular_respiration)

The release of this energy is actually the breakdown of a chemical called Adenosine Triphosphate (ATP) During this reaction, two new chemicals are formed: Adenosine Diphosphate (ADP) and phosphate (P).Since there is a limited amount of this in cells, ATP must be rebuilt from these two products. (http://www.s-cool.co.uk/gcse/pe/how-the-body-obtains-its-energy/revise-it/muscle-respiration )

This is known as muscle respiration, and can happen in two ways.

Aerobic: glucose + oxygen    →    carbon dioxide + water (+ energy)

Anerobic: glucose    →    lactic acid (+ energy)

(http://www.bbc.co.uk/schools/gcsebitesize/science/ocr_gateway_pre_2011/ourselves/0_fit_for_life2.shtml )

Aerobic respiration is by far the most efficient (we get more energy from it) and is what almost all organisms use to survive. Glucose, from the digestion of food (usually carbohydrates), and oxygen from the air inhaled, is delivered to cells in the bloodstream. Carbon Dioxide is then breathed out, and water is left in the blood/cell or breathed out as vapor.

(http://www.biotopics.co.uk/newgcse/aerobicrespiration.html )

When excercising hard, your heart may not be able to get enough Oxygen to your muscles, and in this case respiration becomes Anerobic. This is the incomplete breakdown of glucose and  releases only 5% of the energy per molecule of glucose. The waste product is lactic acid, not Carbon Dioxide and Water. This is how your muscles get energy in the event of an oxygen shortage.

However, long periods of anerobic respiration may cause muscle fatigue. This means they stop contracting efficiently. One cause of this is the build up of lactic acid in the muscles.

(http://www.bbc.co.uk/schools/gcsebitesize/scienceadd_aga/respiration/respirationrev4.shtml)

This is harmful, as a buildup od lactic acid interferes with electrical signals in your muscles and nerves, impaire muscle contraction and slows energy reaction. This hydrogen ion concentration is what causes the burning sensation you feel when experiencing fatigue.

(http://www.delano.k12.mn.us/high-school/academic-departments/science/mr-b-wiesner/cross-country/10-things-you-should-know-about-lactic-acid)

After exercise, muscles need to recover. They go through a two-stage process of Oxygen Debt Recovery in order to do this. An Oxygen Debt is the amount of enery needed to oxidise Lactic Acid to Carbon Dioxide and Water

The first stage is Excess lactic acid materials are either sent into the bloodstream or processed within the muscle cells. The amounts that remain in the cells are broken down inside a chemical pathway called the TCA cycle--also known as the citric acid cycle. This cycle happens when muscles are in oxygen debt. The TCA cycle converts lactic acid materials into ATP molecules for use as energy by the cells. The ATP molecules produced through the TCA cycle are used to manufacture glucose molecules. Glucose molecules are the raw materials used to fuel aerobic respiration processes.

The Cori cycle takes place when muscles are in a state of oxygen recovery. About 70 percent of excess lactic acid materials are sent into the bloodstream and absorbed by the liver. The liver's job is to convert these materials into pyruvic acid and send them back to the affected muscle cells through the bloodstream. The pyruvic acid materials enable cells to rebuild their glucose or glycogen reserves until normal oxygen supplies are restored within the cells.

http://www.ehow.com/about_6554800_oxygen-debt-recovery.html#ixzz2VLTZj1r6