Notice the word "estimate". In reading through the study done by Karapetian GK, et al., it was noted that even in the laboratory, graphical results of blood lactate values were visually interpreted by two trained researchers, and a third researcher reviewed differences between the first two. Of the 28 volunteers in the study, four participants' results required discussion and interpretation from the three researchers.
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.
Why Worry About Heart Rate and LT Anyway?
The good news about LT, and associated heart rate, is that it turns out LT is very "trainable". Lactate threshold typically occurs at 55 to 65 percent of VO2 max in healthy, untrained people. In highly-trained endurance athletes, lactate threshold is often greater than 80 percent of VO2 max.
In other words, you can train your body to process lactate at higher percentages of VO2 max, which means increased levels of speed before the onset of discomfort forces an end to the effort. In short, you can go faster for a given distance.
Studies have shown lactate threshold to be a reliable predictor for endurance race performance. VO2 max is not nearly as reliable. So, if you have been tested for VO2 max and your numbers weren't stellar, do not panic.
Can Heart Rate Determine the Maximum Fat Burning Zone?
The study by Carey verified what other researchers have found in past studies and that is, similar to LT, there is not a single "jump off the page" heart rate value that is your maximum fat-burning number. This value can be determined by laboratory tests, but few people want to go through frequent laboratory tests to keep tabs on that maximum fat burning number.
Though there is considerable difference between individuals if you are doing your workouts aerobically (between roughly 60- and 80-percent of maximum heart rate), more than likely you are doing good things for your heart and your hiney.
The research indicates that heart rate remains a valid tool for estimating the body's response to exercise and LT. Heart rate training zones can be used to design training plans for improved performance and to assess progress.
For triathletes, though you can estimate your LT in cycling from a test in running (or vice versa), it is best to confirm the values by testing in both sports or collecting data during workouts. This data can include rating of perceived exertion for similar workouts in both sports. Also consider your personal sport history when estimating your training zones.
I could not find any current studies validating the use of age-based heart rate training zones. Perhaps they exist, but I could not find one.
Your heart rate training values and LT heart rate should be based on your history and physiology. You can do very simple field tests to get started. As you gain fitness and experience, obtain the best field estimate of LT heart rate by using a combination of graphical test results, rating of perceived exertion, breathing rate and how long you can continue tolerating increases in pace before voluntarily stopping. LT is usually near the end of your pace-increase tolerance. Continue to refine your estimated LT values as you gain more experience.
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