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Exercise, like being chased, puts a high energy demand on the body. You needed ATP to power the contraction of sarcomeres by myosin that lead to muscle contraction. The first step in producing ATP is glycolysis. However, glycolysis only produces a limited amount of ATP (2 ATP and 2 NADH = 2+2x3= 8 ATP). For more ATP you need to enter into the Kreb's cycle (or citric acid cycle). From the Kreb's cycle and glycolysis you can derive a total of 38ATP. However, to get these ATP (largely formed from the reduction of NADH to NAD+) you need oxygen. Now if you exercise to exhaustion or you can't breathe in enough oxygen the electron transport chain that is responsible for converting NADH to NAD+ backs up. Now you don't have any NAD+ to accept protons from the Kreb's so the whole cycle backs up. When this happens the end product of glcolysis (pyruvate) gets converted into lactic acid. Lactic acid can be fermented to produce NAD+ but this process is energetically wasteful and lactic acid will build up. This acid is of course acidic and will accumulate within muscles causing the muscles to be tougher and more pungent.
So the deer that has not been chased will have no lactic acid in its muscles and will be more tender. The deer that has been chased to exhaustion, on the other hand, will have lots of lactic acid in its muscles and will be less tender and more pungent tasting. Check the web for information.
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