by David B. Glover, MSE, MS, CSCS
A client recently emailed me the following question:
I am working through your training program, and just completed another bike field test. I noticed that my Lactate Threshold Heart Rate (LTHR) dropped from 171 bpm to 167 bpm. Is this a sign of improvement, or fatigue? I expected that the opposite would happen as I progressed through the training plan.
Here are my thoughts:
Like many questions in exercise physiology, the answer is, “It depends.”
We ask our clients to do field tests every 3 to 4 weeks in order to set their training zones by determining their Lactate Threshold Heart Rate (LTHR) and to monitor progress. For the purposes of this discussion, we’ll define LTHR as the average heart during a sustained 30 minute bike or run session as hard as the athlete can perform.
A lower heart rate in a second test as compared to a prior test could be fatigue – either fatigue from training or fatigue from more stress, less sleep, etc.
Other factors that may affect heart rate:
- If you did the second test in cooler temperature than your first test, your heart rate will be lower in the second test because your body has to work less to cool itself.
- If you were dehydrated the first test and not the secondtest then this may account for a lower heart rate because there’s more blood available for your heart to pump if you are fully hydrated. During a long race like an IRONMAN, an athlete’s heart rate will often creep up over the duration of the run as the athlete becomes more and more dehdyrated.
- Caffeine will elevate heart rate. If, for example, you did the first test in the morning after a cup of coffee, your heart will likely be elevated compared to doing the test later in the day.
Your more second test LTHR may also be lower because you’ve become more fit – this is what I suspect is happening. Basically, your heart is a muscle and can pump a certain volume (quantity) of blood per beat. As you train your more with endurance exercise, your heart becomes stronger and pumps more efficiently. In other words, your heart can pump a greater volume of blood each beat. As such, for a given amount of blood that the other muscles in your body need per minute, your heart can pump fewer beats because it pumps more blood each beat.
Do you have another metric you can look at to see if your fitness increased with the lower heart rate?
Perceived effort is an excellent metric to compare against because it is always available (although somewhat subjective). Did your perceived effort change?
Did your average speed increase? If you have a power meter, you could look at average power, too.
Finally, as you become more fit, you’re able to maintain all your training zones longer. There is “a rising tide raises all ships” affect with training.
