If you’ve been training for triathlon for even a short time you’ve probably heard the term “threshold.” Maybe you read it in a book or magazine or overheard a coach mention it in practice. Maybe you even think you’ve experienced it in a training session. Unfortunately, it is not only one of the most commonly used terms in triathlon, it is also one of the most misunderstood.

To start with, you don’t have one threshold, you have (at least) three. This is the source of much of the confusion, because when people speak of threshold they often don’t create a distinction (or don’t understand the difference) between them. Knowing the difference between these thresholds will help you more effectively target your efforts to get the most out of your training and racing.

The threshold you are working at is defined by the lactic acid concentration of your blood or the rate of lactic acid accumulation. Lactic acid is a marker of anaerobic energy production, the kind of short-term energy production that is often associated with sprints or high-intensity work. Increased lactic acid concentration is associated with discomfort, increased effort and fatigue. Your body also produces some lactic acid at rest and as such you have a baseline, or resting, lactic acid concentration.

First Threshold
The threshold that occurs at the lowest exercise intensity is called your aerobic threshold. This threshold occurs at the highest level of intensity that you can sustain while maintaining the baseline concentration of lactic acid in your blood. If you tried to work even a tiny bit harder you would feel your breathing rate and sensation of effort increase more than expected for the small increase in speed or work. This is the best intensity for long, sustained efforts and is the intensity that many age-group athletes will use for their half-iron and iron distance races.

Second Threshold 
This threshold occurs at a slightly higher intensity than your aerobic threshold. It is often called maximal lactate steady-state (MLSS), function threshold power (FTP), or critical power (CP). At this effort level the concentration of lactic acid in your blood has increased slightly from your baseline level, but your body is able to metabolize the lactic acid fast enough that the concentration does not continue to increase. This threshold is marked by an increased breathing rate, increased sense of discomfort and a higher demand on your concentration. The fitter you are, the longer you can maintain this intensity. Many professional athletes will complete half-iron or iron distance races at or very near this intensity. For age groupers it is more commonly used for sprint or international distance races.

Third Threshold 
This threshold occurs at the highest intensity of the three. At this intensity your blood lactate concentration has increased substantially from baseline and your perception of effort and discomfort is pronounced – you are breathing hard and it feels like a lot of work but you can keep going for up to 20-30minuntes depending on your fitness. However, if you worked any harder your lactic acid levels would shoot up very quickly and you would have to stop exercising within 2-5 minutes. For most triathletes, this level of intensity is reserved for harder training sessions, fast 5km races, or a final surge. Some professional athletes will complete international or sprint distance races at or near this intensity.

Now the next time you hear a training partner talk about their “killer threshold session” you can ask them “which one?”

Darian Silk is a triathlon coach and Certified Exercise Physiologist based in Toronto. Read more about Darian at www.teamatomica.com or email him at darian@teamatomica.com.