The short answer is no. Although there are many websites that talk about training thresholds using the word limit in their definition but there are in fact no limits. A limit means you cannot go past it, so the only real limit would be found in a VO2max test, which makes you perform at your limit to identify the maximum amount of oxygen your body can use.

If you Google threshold you will actually get the following definition:

the magnitude or intensity that must be exceeded for a certain reaction, phenomenon, result, or condition to occur or be manifested. eg) “nothing happens until the signal passes the threshold”

Training Thresholds

So a training threshold is an intensity that must be exceeded for a physiological adaptation to occur. Aerobic threshold – training intensity to cause aerobic adaptations. Anaerobic threshold – intensity needed for anaerobic adaptations.

Training thresholds are best understood by learning about blood lactate levels and what they tell us about which energy systems are being used.

Aerobic threshold

The aerobic threshold is the intensity or workload at which lactic acid begins to rise above resting levels in the blood. This is NOT the lactate inflection point, just the point when your muscles begin to put lactate into the blood for it to be dealt with. Generally at this intensity, you can still maintain a conversation with someone but might need to take a break to breathe every now and then.

The lactate begins to appear in the blood because your muscles are beginning to create lactic acid because they are now using the lactic acid system at a rate that is producing so much lactic acid your muscle needs to get rid of it, which it does by putting the lactate and the hydrogen ion into the blood for your body to deal with.

At this point, your body is crossing into the aerobic training zone. In this zone, you can continue to perform for a long period of time as your muscles and body can handle the lactic acid levels. It is not building up in your muscles, they are not sore, and you feel good.

When people use the word limit to describe the aerobic threshold it is being used to say the body has reached the limit of the aerobic energy system, but this is not quite true. While your body has begun to use the lactic acid energy system, this does not mean the aerobic energy system is limited to this intensity. It will continue to be used for all intensity levels and may well increase the speed at which it produced ATP as your body increases its ventilation rates further. Your body has also been using the lactic acid energy system already but has not transported the waste products into the bloodstream yet and your muscles have also been using the ATP/PC system, which is anaerobic but does not produce lactic acid. So it is hard to say this is technically a “limit” for the aerobic system. However, it is an indicator that seems to tell us that our muscles have turned on the lactic acid energy system and are using it consistently, but not yet at its maximum capacity.

Now, let’s begin to apply this to training. This aerobic threshold, where lactate begins to be transported into the blood from the muscles marks the point which needs to be exceeded to produce aerobic physiological adaptations. These include:

increased haemoglobin levels,
increased capillary density in the muscles and around the alveoli,
increased myoglobin,
increased mitochondria,
increased aerobic enzymes, and
increased stored fats and carbohydrates to be used for energy production.

So in order to produce these changes, we must push our workload past the aerobic training threshold and into the aerobic training zone. This is because at this intensity and beyond it we are stressing our body’s ability to deliver and use oxygen to produce ATP. It is now working at or close to its capacity or “limit”.

Now, we want to train our athletes so that their aerobic threshold increases. We want them to adapt physiologically so that they rely less on their lactic acid system and can use their aerobic system at higher workloads. As they train and push the aerobic capacity of the body adaptations occur that result in an increased aerobic threshold (as is seen in the 2 graphs below).

Graphs comparing trained and untrained athlete’s aerobic and anaerobic thresholds

Anaerobic threshold

The anaerobic threshold is the workload needing to be exceeded in order for the body to produce anaerobic physiological adaptations. These include adaptations such as:

increased anaerobic enzymes
greater capacity to transport out and break down lactate and the hydrogen ion
The ability to mentally manage the pain that comes with the accumulation of the hydrogen ion so that “fatigue” occurs with a higher concentration of the ion.

As a side note, anaerobic training will also produce all the aerobic adaptations because in order to get to the anaerobic threshold you have to pass the aerobic one. Aerobic adaptations will continue to occur, and may in fact occur faster and at greater levels by doing anaerobic training above the anaerobic threshold.

The anaerobic threshold is marked by the lactate inflection point. This is the workload at which blood lactate levels begin to increase rapidly. This rapid increase indicates that the body is no longer able to manage the lactic acid at the same rate at which it is being produced by the lactic acid energy system.

Above the aerobic threshold, intensities cannot be maintained for very long because of this imbalance between the production and breakdown of lactic acid. Instead, training intensity varies by using intervals where the athlete will exercise above the aerobic threshold for a period of time but then needs to decrease the intensity in order for the body to manage the lactic acid levels. These intervals above the threshold are enough to stress the body’s systems used to manage the lactic acid being produced. This stress then leads to the physiological adaptations needed to manage the lactic acid.

One of the added benefits of this training is that it also produces aerobic adaptations because an increased aerobic capacity will also reduce the amount of lactic acid being produced.

In trained athletes then we can see (in the graphs above) that both their aerobic and anaerobic thresholds have increased. This is because of the physiological adaptations in the body that have happened because of the training. These adaptations mean the lactic acid energy system is not needed until a higher workload and that the body can better manage the amount of lactic acid being produced, which increases the anaerobic threshold.

By increasing the workload of the anaerobic threshold there is an increased workload able to be maintained by the aerobic energy system before the lactate inflection point, beyond which fatigue will occur as the lactic acid builds up in the body.