Originally Posted by Camilo

YOu can actually get your LT measured with blood work during controlled effort, but really, it is defined as the average heart rate over a ~1 hour race pace effort. When you think about it, this makes total sense. If you were exceeding your LT, you could not sustain it for an hour. Look up definitions. Some use the actual blood values, but many training-based definitions use the effort sustained for an hour (or some other similar time period).