Lactate is an intermediary in the metabolism of glycogen, which is a stored form of 'sugar' in the muscles and liver. It's produced in our bodies all the time, at a linear rate (meaning it gets cleared away as fast as it gets produced), so we don't feel it's presence.The lactate threshold (LT) is the point where lactate accumulation is greater than lactate clearance. When we surpass the lactate threshold, we reach what's called the Onset of Blood Lactate Accumulation, or OBLA. Essentially lactate is now being produced faster than it cab be cleared, and rises exponentially in the blood. Of course, it reaches a point and the person will have to stop doing whatever it is they're doing that causes them to have this OBLA. Lactate as a molecule can dissociate in to it's anion (La-) and acidic H+ ions (I know, nerdy chemistry. Bear with me). Since H+ ions in the blood are acidic, they cause the pH level of the blood to drop. At rest, lactate has already dissociated into its H+ and La- parts, so there's already H+ ions floating around. But they get cleared as fast as they are produced, so we don't notice. It's when the production rate exceeds the clearance rate that we feel it.  Lots of lactate = lots of H+ ions = low pH = high acidity. This is where the "burn" feeling comes in that people mention. It's not due so much to lactate itself (or, incorrectly known as 'lactic acid'), but to the accumulation of H+ ions in the blood. Why does this suck? Well, it's well-known in the literature (i.e. geek speak for things I read) that reduced pH in the blood impairs muscle function, as it prevents the micro-processes that make our muscles work, from happening. So, too much acidity = muscles say 'to hell with this!'  and crap out.

How do we get rid of those nasty things? Breathing heavy, for one. Oxygen happens to be a fantastic buffer of H+ ions. Also, as we work hard with exercise, we increase the amount of CO2 (caron dioxide) in the blood, which is a stimulus for increased breathing rate. La- itself can be shuttled out of the muscles and into the blood, heading to the liver to get reformed into new energy, but this can only be done if blood flow is sufficient! If you work hard to the point where you want to puke, and then just sit down, you're not facilitating circulation throughout the body, and La- will just sit there, as will H+ ions, and you will be in agony for much longer. Trust me, it's to your benefit to move around actively during a recovery period following lactate training.
Why do lactate training? In BJJ, we use a high amount of type II muscle fibers to create strength and explosiveness, and we do this repetitively. Type II fibers (as opposed to Type I, of which we have both), have an easier time producing La- and H+ in the blood. BJJ also involves high amounts of anaerobic (without oxygen) work. Now we are throwing high work demands onto the body with diminished oxygen supply and increased buildup of H+ and La-. If you can't deal with this, metabolically speaking, you're gassing out and tapping. But... learn to push through this and increase the level of work you can do before you reach that OBLA point, then your conditioning improves, your gas capacity goes up, and you get tired a lot less. We do this through specific styles of lactate threshold training. If you want to know more, and how to conditioning yourself so you don't gas out, feel free to contact me.

For those of you who are really into this topic, I suggest you read a little paper by Gladden (2004) called "Lactate metabolism: a new paradigm for the third millenium", in the Journal of Physiology (vol 558).