Gathercole wrote

I know something about lactic acid in weightlifting, at least. The pain that you get a day after lifting is called DOMS (delayed onset muscle soreness) and it doesn't have anything to do with lactic acid. It's caused by microscopic tears in the muscles' connective tissue. It usually sets in almost exactly 24 hours after lifting, as long as you lift heavy enough.

Lactic acid burns briefly if it builds up due to the rate of production exceeding the rate of clearing. It only burns briefly, while you're still lifting, and stops a few seconds after you set the weight down because with no more being produced, the leftover is almost immediately cleared from your muscles. Lactic acid burn is the worst with intermediate weight reps. If you're lifting at your 5-rep max, your muscles' power will give out before you get a significant amount of lactic acid buildup. Likewise, if you're doing something like pushups, where you can do 30-40 reps, your muscles aren't doing enough work for lactic acid to be building up quickly enough. Right around 10-15 reps, that's where lactic acid is the worst. You're pushing the weight, and you feel the burn, you know you have the strength to keep going but often it's the pain that makes you stop.