The lactate threshold (LT) is the exercise intensity at which blood lactate levels start rising rapidly. This happens when more lactate is produced than can be metabolized. It is sometimes called onset of blood lactate accumulation (OBLA) or anaerobic threshold.
Here is what happens. After a warm-up, you exercise at increasingly intense levels for several minutes. At each intensity, someone takes a blood sample. They measure the lactate levels in the samples and plot them on a curve. Usually they plot lactate versus power/intensity. You can also express LT versus heart rate.
One can buy small lactate meters (they look similar to glucose meters that diabetics use) for anywhere from $200-500. These meters use disposable test strips.
With increasing intensity the lactate level will be more or less steady or increase gently. At some point the increase will jump and there will be a distinct inflection in the curve. From here on, lactate levels will rise quickly. At about the same time, the person will start experiencing difficulty maintaining the intensity required. Although everyone will experience difficulty at some point, in some individuals no clear inflection point in lactate levels is seen. In those cases, an absolute value of lactate is used. 4mM/L is often used as the "threshold" value.
During the test, one also measures the heart rate, and in some set-ups, the amount of CO2 produced to O2 consumed. The latter ratio changes when the anaerobic threshold is reached. In most cases the anaerobic threshold and the lactate threshold occur at about the same heart rate.
In general, the better trained you are the closer your lactate threshold is to your maximal heart rate. There are however, individual variations and some individuals can get their lactate threshold much closer to their maximal heart rate than others.
A non-invasive (no needles) way to estimate LT is the Conconi test. The test was made popular because it was reportedly used by the Postal Service/Discovery Channel team to check fitness for the Tour de France. The Conconi test is easy to do. All you need is a treadmill or ergometer and a heart rate monitor. I should note that the dependability of the Conconi test has been challenged in many studies. Nevertheless it remains a very popular test.
Again, you warm up and then progressively increase the load while measuring your heart rate. You do so until you reach a "plateau" where your heart rate no longer increases (or increases ever so slightly) while the exercise load is going up. Once again, in most individuals there will be a distinct inflection point, where the curve levels off and your heart rate no longer increases despite the increased load.
As described earlier, at or near that intensity, you will experience a distinct increase in difficulty. Furthermore, you won't be able to go very far past that point. It is important to make repeat measurements to get a reproducible number. Preferably you rest in-between trials.
A lot has been written about lactate threshold tests. One thing is for sure: accurate measurements are difficult to obtain. They are also of limited value. But that has not stopped athletes from pursuing these tests with great determination or practicing what is known as lactate threshold training.
It also has not stopped many gyms, health clubs, and other organizations from performing LT tests and charging handsomely for it too. In nearly all cases, you get one shot for your money. So, despite the high tech equipment and professional setting, chances are that your measurement will be pretty meaningless.