Exercise Thresholds: Misnomers and Confusion in Endurance Training (Part 1)

By Scott K. Ferguson, Ph.D. | Co-Founder and Head Coach

Scott Ferguson performs a VO2 max under the expert supervision of Dr. Paul Morgan (Photo Credit: Abigail Ferguson)

Part 1: Lactate

Imagine yourself in bed. Its 5 am, your alarm has just gone off and you grab your phone to see what your coach has scheduled for your morning training session.

“Today we have another easy endurance capacity run to keep building your aerobic base. After a 10-minute warm-up jog, increase your pace up to your aerobic threshold (or just under) and run for 30 minutes.”

You think “Awesome, I need an easy run after yesterday’s interval session!”

You meet your training partner and discuss what your coach has prescribed but find that their coach planned a tempo run and has them running at a pace just below their lactate threshold. She’s really psyched about this session because she just had her VO2max, lactate threshold and critical running speed measured in a lab, and this is the first time her coach has recommended training based on these results.

Wait…a tempo run just below lactate threshold? Didn’t your coach say easy intensity for base building should be just below lactate threshold…or was it aerobic threshold? Also, what the heck is critical running speed, and can I even train with my partner today, or do we have two different goals for this session?

Dr. Paul Morgan takes a blood sample from Dr. Ferguson during a VO2 max test. (Photo Credit: Abigail Ferguson).

Confused? Don’t worry, I was too when I started studying exercise science a little over a decade ago. There are no shortage of thresholds and acronyms in exercise science and each of them can be confusing to the uninitiated. In this series of articles I hope to break down some of the confusion that surrounds each of the primary intensity thresholds so that you can have a better understanding of what these thresholds mean and how you and your coaches can use them to prescribe your training. This first article will discuss lactate, since it is often misunderstood by sports commentators and even many coaches and athletes.

Lactate was once considered to be the consequence of oxygen lack within muscle and has gained a reputation for its association with fatigue. Physiologists and coaches have proposed and debated various thresholds in which the rapid accumulation of lactate occurs during exercise and many use these thresholds to guide their training. However, while coaches and physiologists may have come to a consensus that, at a given exercise intensity, lactate production exceeds clearance, there is still a striking amount of ambiguity surrounding the terms used to describe this phenomenon. A vital concept to understand is that there are two distinct thresholds we hit while increasing exercise intensity: one at the junction between moderate and heavy exercise intensity and a second when we transition from heavy (sustainable) to severe (unsustainable) exercise intensity. While the lactate threshold, aerobic threshold, anaerobic threshold, maximum lactate steady state threshold, and critical speed/power may all ring a bell as terms used to describe these transitions, these terms do not define the same phenomena.

What is lactate?

Professor George Brooks from UC Berkeley (photo credit: Stephen McNally)

Simply put, lactate is a bi-product of glycolytic energy production within cells. When glucose is metabolized, it produces pyruvate, which can then be converted into lactate. This reaction occurs almost universally across various cell types, including yeast and plant cells. In fact, those post-training sour beers you may (or may not) enjoy derive their sour flavor from lactate-producing yeast. Given these associations with anaerobic metabolism, it was originally believed that lactate was produced solely as a consequence of oxygen lack within the skeletal muscle. This notion stuck around for nearly a century as the researchers who came to that conclusion (e.g. A.V. Hill, O. Meyerhof, D.B. Hill and others) won the Nobel Prize, and achieved positions of eminence within the field of physiology in the 1920’s and 1930’s. But enough history for now.

Perhaps the most common misconception surrounding lactic acid is the idea that it alone causes exercise fatigue. It is true that the over-accumulation of lactate can impact muscle contractile function during exercise. However, this is also true for many other metabolites, including hydrogen ions (acid), inorganic phosphates (cleaved from the breakdown of ATP), and the buildup of potassium ions. Thus, lactate itself is not the bad guy, but instead is utilized throughout the body, particularly by our well-conditioned slow-twitch skeletal muscles, which are the foundation of endurance performance. The fact that lactate is continuously produced and metabolized (even at rest) and is utilized as the primary fuel (even more so than glucose) for the heart during exercise underlines the importance of lactate in human exercise physiology. If you’re interested in reading more about lactate, you should check out "Lactate: Darth Vader or Jedi Knight of exercise physiology?" by George Brooks (pictured above), the scientist who helped identify lactate's beneficial role in human physiology.

Now that we have established that lactate isn't a bad guy, our next article will focus on establishing the domains of exercise intensity and explaining what thresholds are important to know and how they can be used to inform your training.