You guys are both partly right. You don't build up lactic acid becaue the body doesn't get enough oxygen. What I'm arguing may seem like semantics, but its slightly more correct.
Work with me here and assume that training doesn't make a difference. When you move your muscles, the intensity determines what energy system you use. If you need energy quickly, hydrogen ions are produced faster than they can be utilized by the electron transport chain and build up. The way the body gets rid of this is by having pyruvate pick them up, therebye converting it into lactic acid (in skeletal muscle anyway). So, its not the speed of aquiring oxygen that matters (that is the same) its the amount of "left over" or "by product" of making energy quickly.
Now, if he does exercise more, he can increase blood flow to the muscles and then allow more H+ ions to go through the electron transport chain which would delay the backup of them at a given intensity. So with training you can increase the flow of H+ ions and not get as much lactic acid, but this has nothign to do with oxygen intake.
So, my argument is its not a deficit of oxygen, but a buildup of by products in order to get as much energy on a short time scale as possible. When too much latic acid builds up, the muscle must stop contracting, and thats what happens.
Tito, the muscle ALWAYS uses ATP, and glygocen's by product isn't only lactic acid, it can be two pyruvate assuming the buildup of H+ ions isn't present. Under low intensity the pyruvate goes along the krebys cycle to create a lot more ATP in the long run.
Improving O2 consumption for quick energy (such as curls) is pointless as oxygen isn't used, and no matter how much you take in you will always have the backup. For aerobic work or grappling improving O2 intake is possible and will help you workout longer.