Filmed at: Boulder Center for Sports Medicine
Tests performed by Neal Henderson. 

If you want a more precise lactate threshold (LT) number to derive your heart-rate zones instead of the standard field tests, then assuming you have the money and a decent level of fitness, a real lab test is what you want done.  The standard field tests are good, and a lot better than the age based formulas, but they are still prone to inconsistencies and are subjective.  Did you really push yourself the last 20 minutes of the test?

As you exercise harder, your blood lactate levels (a byproduct of metabolism) increase as there are increased demands on your body for energy production.  There comes a point where you are exercising harder than your body can metabolize fuel aerobically and your body then increasingly turns to anearobic metabolism for fuel - which produces a lot of lactate.  This coincides with a spike in blood lactate where a parallel process from the anerobic energy production produces an increased amount of hydrogen ions in your muscles causing your muscles to 'burn' at high, unsustainable intensities.  That point is called your lactate threshold level, it corresponds to a certain heart-rate and is the point where you can then calculate your heart-rate zones from.

The lactate threshold lab tests are done on a stationary bike or treadmill - whichever one you want an LT test for.  The general protocol is to warmup on the trainer and get a blood sample analyzed to get your baseline blood lactate levels before the hard part of the test begins.

The main part of the test consists of a ramp-up in pace every four minutes, hold and test.  So it would be something like this:

Baseline warmup test (to make sure your lactic acid levels start out low)
1 - Increase pace, hold pace for four minutes followed by a blood test of your lactate production.
2-7 - Repeat #1
8 - Your lactate levels should have peaked from prior tests, final test.

Your lactate profile is then generated and gets correlated to your heart-rate.  Precise zones can then be created from this.

The other important parts of this test are that a proper interview is done so that you start out at a pace that is not too easy (on the machine too long) and not to hard (burn out too quick).

Part two will discuss the results and interpreting them.