Any interpretation of LT and the associated heart rate is an estimate and should be viewed as just that. It is not a number cast in stone.
While several studies confirmed the existence of a deflection point during testing, a la Conconi, others did not see a deflection in all subjects. This is not to say that LT does not exist, rather a marked deflection point did not exist for some of the test subjects.
This is similar to what I have seen with my own athletes in field testing. Some athletes exhibit a deflection point on the graph of pace vs. heart rate, while others have not. Additionally, I have accompanied athletes to laboratories for LT tests and the laboratories have all had to interpret the results.
Who Has the Highest VO2 Max: Runners, Cyclists or Triathletes?
In one of the research papers I reviewed, it noted that triathletes could have LT heart rates that were equal in cycling and running, while other studies found LT heart rate values to be different for different sports.
Running typically elicits the highest VO2 max of the two sports--cycling vs. running. However, trained cyclists can achieve VO2 max values on the cycling ergometer as high as trained runners can achieve on the treadmill. Interestingly, when triathletes are tested in cycling and running, the results are conflicting.
One study found that LT heart rates varied with individual athlete sport history, combined with specific sport. In other words, people that came from a single-sport background to triathlon could drive high LT heart rate values in their primary sport. People that were not highly trained in a single sport and were trained as triathletes could drive LT heart rate values that were near equal in cycling and running.
Heart Rate Variables
One issue with heart rate is that it does vary with environmental conditions such as heat and humidity. It also varies with an individual's hydration, nutrition and sleep status. Training sessions from previous days and weeks influence heart rate, including the positive effects. (More on that in the next section.) Illness, stress and injury can affect heart rate. Stimulants such as caffeine, or any other drug, whether it is prescription or over-the-counter, can affect heart rate and LT values.
When you use heart rate as a training tool, you have to consider any variables that affect your status from day to day. A heart rate of 150 may or may not produce a 6-minute-per-mile pace from one day to the next.