This is a continuation of this post about aerobic (low intensity) vs anaerobic (high intensity) training.
A quick review of the two types of activities:
Aerobic: the energy is created by burning fat and carbs. This produces CO2 and water (breathing and sweating).
Anaerobic: to keep up with the additional energy requirements, the body burns sugar supplies (glycogen) in addition to the carbs and fat. This produces lactic acid (in addition to CO2 and water), and when this acid is produced faster than it can be metabolized, the muscle pain appears.

The anaerobic threshold (AT) is the point where the aerobic system can no longer keep up with the energy requirements. After this threshold, the anaerobic metabolism kicks in. Because of the lactic acid production, the AT is also known as lactate threshold.

The fitter you are, the longer you can fuel your body with the aerobic system before the anaerobic system needs to take over.
Interval workouts are effective for raising the AT. For the best results, vary your workouts between aerobic work (where duration takes priority over high intensity), and higher-intensity intervals (where you will be just under or at your Maximum Heart Rate).
http://www.concept2.com/indoor-rowers/training/tips-and-general-info/anaerobic-threshold

The AT is generally linked with the heart rate.
A quick estimation of your AT is 85% of the maximum heart rate (MHR). The MHR can be in turn estimated to 220 - age. So for a 36 year old person, the MHR=184, and the AT is 157bpm. Basically this tells that once this hearth rate is reached by a 36 year old, his body switches to anaerobic metabolism.

In order to push the AT, you can either:
– do HIIT (high intensity interval training), where you alternate low intensity with high intensity intervals (aerobic vs anaerobic)
– or do ATT (anaerobic threshold training), where you train just around the AT value.

Again, these types of training are generally linked with the hearth rate. A widely used concept is the training hearth rate (THR) (some gym machines also refer to the target heart rate).
In determining the THR, the following indicators are being used:
– the resting hearth rate – RHR. It can be determined with a heart rate monitor or Apple watch right after you wake up.
– the maximum hearth rate – MHR. It can be either measured with an ECG in a controlled environment, or estimated as 220-age (other formulas exists).
– the heart rate reserve – HRR defined as MHR minus RHR

Using the indicators above, each type of training can be associated with a certain THR range:
– the aerobic training (low intensity), 50–75% HRR + RHR
– the AT training, 80–85% HRR + RHR
– the anaerobic training (high intensity), 85-95% HRR + RHR

If RHR=52, MHR=184, HRR = 132 and age=36, then
– THR range for low intensity training: 118-151 bpm
– THR range for AT training: 158-164 bpm
– THR range for high intensity training: 165-177bpm

Image: sportograf.com