The lactic acid that makes yogurt tangy is the same lactic acid that builds up in our muscles when we exercise strenuously. Instead of bacteria fermenting the sugar in milk to make energy for themselves, our muscles ferment sugar in our diet to produce energy to contract. If, like when we're sprinting, lactic acid builds up in our muscles faster than it can be removed we can end up with a burning sensation in our muscles, forcing us to stop.
Credit: Flickr/Pressebereich Dehner Garten-Center
Now if we train we can increase the number of blood vessels in our muscles and clear out the lactate faster. For example, if you take some "overweight sedentary women" and start them on an aerobic training program of running and walking, at the end of three months their lactate levels during exercise dropped 17%. But those on the same program who drank 2 cups of orange juice a day dropped their levels 27%. They did the same exercise program, but the citrus group experienced a significant decrease in blood lactate concentration, indicating an improvement in physical performance with less muscle fatigue.
I don't recommend drinking juice, though, because you're losing all that wonderful fiber that slows the rate of fruit sugar absorption into our system. If you click on the above video, you can see the blood sugar spike one might expect after drinking Coca-Cola. Compare that to the spike you see with orange juice? No difference. However, if you eat the same quantity of sugar in the form of orange slices you experience a significantly smaller spike in blood sugar.