Training intensity is often organised into three zones, allowing us to measure how much time is spent in each zone and therefore at each intensity. One of the most common ways to measure these zones in the lab is via blood lactate levels. Lactate is a product of the metabolism of glucose.

There are 3 lactate zones:

Zone 1– blood lactate concentration remains at resting levels

Lactate threshold/aerobic threshold (LT1)– increase in blood lactate concentration above resting levels, around 1.5mmol/l

Zone 2– blood lactate concentration increases but stabilises at this slightly higher level 

Lactate turnpoint/anaerobic threshold (LT2)– blood lactate concentration start to rise exponentially, around 4mmol/l and approx 85% of max heart rate

Zone 3– blood lactate concentration rises exponentially, lactate production exceeds clearance and fatigue is imminent

References

https://www.espressocoaching.net/blog

https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.1600-0838.2004.00418.x

https://www.semanticscholar.org/paper/Training-intensity-distribution%2C-physiological-and-Neal/05062fc0d5b2ec1c2b3fa72cdee3d5f890ef58bd