Lactic Acid Is Not Muscles' Foe, It's Fuel: It has always been thought that they have to learn to work out at just below their "lactic threshold," that point of diminishing returns when lactic acid starts to accumulate.

But that, it turns out, is all wrong. Lactic acid is actually a fuel, not a caustic waste product. Muscles make it deliberately, producing it from glucose, and they burn it to obtain energy. The reason trained athletic horses & dogs can perform so hard and so long is because their intense training causes their

muscles to adapt so they more readily and efficiently absorb lactic acid.
The lactic acid is taken up and used as a fuel by mitochondria, the energy factories in muscle cells.

Mitochondria even have a special transporter protein to move the substance into them. Intense training makes a difference, because it can make double the mitochondrial mass.

Through trial and error, Racing & competition horse/dog trainers have learned that athletic performance improved when animals were worked on endurance, running longer and longer distances.
That, it turns out, increases the mass of their muscle mitochondria, letting them burn more lactic acid and allowing the muscles to work harder and longer.

Just before a race, trainers often tend to train their horses/ dogs very hard in brief spurts.

That extra stress increases the mitochondria mass even more and is the reason for improved performance.