Many training plans use HR to determine training zones. Other training plans use direct measurement of blood lactate levels to determine training zones. Inevitably, there are questions about how well correlated these two measures are. In other words, can you heart rate as an accurate indicator of blood lactate level.
To a first order, there is a rough correlation. A step test will show a increasing lactate level and increasing heart rate as the steps increase in intensity, as shown below above 195W.
The question is, can HR be accurately used to determine a good training power to ensure that blood lactate is below 2.0mmol/l. Over the past month or so, I’ve taken lactate readings after 20 minutes of steady state. In all cases, I was aiming for a lactate level of 1.6 to 2.0mmol/l. As you can see, I often miss on the high side, because I have trouble containing my exuberance. The data shows very little correlation between HR and lactate level in this narrow range.
Heart rate data is not entirely useless. If the average HR over the last 5 minutes of the piece is above 140BPM, then it is very likely that blood lactate is above 2.0. The problem is that a low HR is not necessarily indicative of low lactates, so if you train to a HR limit in a session you might end up with higher than ideal lactates in some of your sessions. No real harm done, but you might not get the ideal training effect.
A more serious issue is that HR changes constantly through a steady state session as dehydration and other factors induce CV drift. So training to a HR limit will generally cause a slow down as the session progresses. If one is using HR as an analog for lactate, then you would probably want to use a HR limit that increases as the session continues. For me, 140 after 20′, 145 after 40′, 150 after 60′ and 155 after 80′. At the end that would be right at the top end of my UT1 range. Ideally, a session would run about 5 beats lower than that at each point.