Lactate is a byproduct of carbohydrate metabolism. As you increase the intensity of your exercise, more carbs are used for fuel thus producing more lactate. Lactate can be measured in the blood using a handheld meter and a pinprick to get a drop of blood. As your intensity increases, your lactate level also increases.
Physiologists have come up with 20 (or more) different ways to analyze this data that plots lactate level against some measure of intensity like heart rate, power or Vo2 max as measured by a specialized piece of very expensive equipment.
“Lactate Threshold” is therefore defined many different ways…one set of data can give you a handful of numbers. However, all of those will be functionallly close to what people have alluded to above…the maximum level of intenisty at which you can sustain a given activity for a “prolonged” period of time, typically an hour or more.
You don’t need to have it measured to be able to train with the same principals. Field tests like these (20 minute lactate threshold field test protocol, 30 minute lactate field test protocol) will give you the information you need to determine exercise intensity zones.
However having an actual lacate threshold test performed at a variety of times thoughout your trianing cycles can give you valuable information to confirm that your trianing is working and confirm that your field testing is accurate. If you are interested in having a lactate test performed either at my home studio or in your own home, feel free to contact me.