Z2 - blood lactate down but HR up?

I started a new block of training a couple weeks ago and have noticed something interesting. For my z2 work(all indoor on rollers at 70% of current FTP) my blood lactate(tested with a lactate plus meter in 10 minute increments throughout the rides) has been trending down(from 1.8 to around 1.4-1.5) but my HR has been trending up(from low to mid 130’s to low 140’s).

It just seems weird to have a blood lactate level of around 1.4-1.5 with my heart rate creeping into the 140’s(I’m a 37 year old male).

I have had a fairly high training load(for me at least) these past two weeks of around 10.5 hrs and 550-600 tss each week but I’ve gotten plenty of rest(8+ hrs of actual sleep each night) so I feel plenty recovered. I just wonder if anyone else has experienced anything like this and if it seems normal?

Is it warmer in your training space compared to previous tests?

Nope. The conditions are basically identical.

That’s the easy one then. The rest could be hydration, life stress, what you ate recently, anxiety/excitement about the test, etc to infinity in any combination. If nothing stands out, I personally would chalk it up as noise and see what happens next time.

Blood lactate concentration during exercise is highly correlated with the rate of glycogenolysis. In turn, the rate of glycogenolysis is influenced by muscle glycogen concentration. The reduction in lactate you have observed therefore could simply be due to diminished glycogen stores, rather than increased fitness.

This well-known effect is one reason why it is unwise to rely on absolute lactate values, and why the Dmax method of determining LT was invented.

Thanks for the explanation. I was hoping I’d hear from you. Is this effect as likely to occur even with relatively short 1-2 hour Z2(70% FTP) rides that never go above those levels?

As likely? No, because the rate of glycogenolysis (and therefore blood lactate levels) increases exponentially as a function of exercise intensity. However, glycogen utilization also increase with duration, and you indicated that you had been training more recently, so…

Glycogen in muscle (and liver) is associated with (a variable amount of) water… have you by chance been monitoring your body mass?