Lactic Acid - Part 2

It is a common misnomer that Lactic Acid is the cause of fatigue and cessation of high intensity exercise, yet training plans built around your individual Lactate Threshold are highly effective despite the debunking of the “Lactic Acidosis” rationale. Let’s learn why…

By Matt McNamara

Last month we looked at the intricacies of Lactic Acid/Lactate production and its role in limiting performance. The short summary of that article is to say that Lactic Acid production is NOT the limiter in high intensity exercise, and the science behind that belief was founded on an inferred cause and effect relationship between lactate production and cessation of exercise that, ultimately, proved to be untrue.

While lactate production may not be a limiter, it is clearly a marker of overload and does play a role in athletic development and performance. Lactate Threshold based training, when paired with use of a powermeter, is seen as the gold standard for endurance based performance improvement. So let’s explore the real meaning and value of Lactate Threshold based training.

What Does Lactate Threshold Really Mean?
First off, Lactate Threshold is commonly defined as “the exercise intensity at which lactate production exceeds lactate removal, and thus begins to accumulate in muscle and hence in the blood.” Unfortunately, the definition of what constitutes “Lactate Threshold” is highly variable.

Many researchers establish threshold as the point when lactate concentration rises 1 mmol above an exercise baseline. Others use a fixed value, for example 2.5 mmol per liter, as the threshold point. Still another approach is to use D-max which takes the mid-point between the baseline and maximal lactate concentrations. In the end the most important consideration isn’t the way threshold was determined, so much as the concept of Lactate Threshold (and associated terms) as illustrating the non-linear relationship between lactate concentration and exercise intensity.

It is also important to acknowledge that terms like Maximal Lactate Steady State (MLSS), Onset Blood Lactate Accumulation (OBLA), Ventilatory Threshold (VT), Individual Anaerobic Threshold, Critical Power, etc are talking about roughly the same range of intensity. Each of these, MLSS and OBLA in particular, correlate well with the power training concept of Functional Threshold Power (FTP), which is itself defined as your maximal sustained power output for approximately 60 minutes.


Now that we have a clearer idea of what is meant by Lactate Threshold, and we know that Lactic Acid is not the cause of fatigue, let’s look at other factors that might play a role.

Other Causes of Fatigue
In 2005 researchers from Edith Cowan University in Western Australia set out to do just that. Models to Explain Fatigue During Prolonged Endurance Cycling, Chris Abbiss and Paul Laursen’s comprehensive review of fatigue literature, looked at no fewer than ten different explanations of fatigue.

Abiss and Laursen point out that fatigue is usually defined by the type of research being done. For example, if one is looking into psychological causes then they will tend to classify fatigue as “a sensation of tiredness,” while a biomechanist might look more at changes in force output to qualify fatigue. Fatigue research is also driven by a reductionist approach; those doing the research tend to look for a single ‘answer’ to the question of fatigue.

Among the different paradigms and models explored were the anaerobic/cardiovascular model, the energy supply/depletion model, neuromuscular fatigue, biomechanical, thermoregulatory, and muscle trauma models. In addition the psychological/motivational model, central governor, and complex systems models were also reviewed. A quick summary of characteristics might demonstrate that:

Neuromuscular fatigue tends to be divided into a question of where along the neuromuscular pathway inhibition occurs, while the muscle trauma model seeks to explain fatigue as coming from damage to the muscle itself, or to alterations in the chemical homeostasis.

The biomechanical paradigm seeks to define fatigue as the result of decreased efficiency of motion, where increasing efficiency lowers the production of metabolites (like lactate) and energy consumption, helping attenuate increases in core temperature. This segues nicely into the thermoregulatory model which looks at the role of core temperature and the increased demands on the physiological systems brought about as a result of increased core temperature towards critical points at which exercise capacity is reduced or terminated.

While psychologically no single variable appears to be responsible for motor output alteration due to afferent (outgoing) signals, it is thought that numerous mechanisms are responsible for the subconscious perception of fatigue and alterations in central activation and perceived exertion.

The central governor and complex systems theories seek to explain fatigue as a function of oversight by an as-yet-undefined central mechanism, or through the complex inter-relationship of multiple feedback loops seeking to maintain homeostasis, respectively.

Their net conclusion is that any number of systems may contribute to fatigue in a specific way for a specific situation, but in general the limitation of the system is derived from oxygen delivery to the muscles, especially at high intensity.

To further clarify in the Abiss and Laursen article fatigue was defined as “tiredness and associated decrements in muscular performance and function.” This is an important point as much research has looked at performance to exhaustion. The relevance comes when we look at how to best apply some of the factors above into the creation of a responsible training program. Many of the changes we seek are built around the optimization of oxygen delivery and increasing metabolic efficiency during the training year, so how does Lactate Threshold help?

Threshold As Proxy
An individual’s Lactate Threshold is the single most important physiological determinant of endurance exercise performance. It is trainable, reliable, and a sort of proxy for other important metabolic processes that underlie performance.

For example hormone production, like epinephrine/norepinephrine, shows a similar curvelinear relationship with increasing exercise intensity. Plasma potassium concentration, catecholamine concentration, plasma ammonia concentrations, growth hormone, cortisol and many other elements also demonstrate the same threshold type trends as lactate.

Power at Threshold
Now that we’ve established what Lactate Threshold is, how it is determined, and what processes it parallels, let’s spend a little bit of time on what advantages threshold level training can bring to your performance.

For untrained athletes the Lactate Threshold benefits of training can be seen at a wide range of intensities. Simply getting on the bike regularly will bring about many changes including increased mitochondrial density, blood lactate response, and reductions in lactate concentration at a given intensity.

For the trained athlete however, continuous training at intensities around Lactate Threshold has been shown to be beneficial since the time of the fabled East German sports machine in the twentieth century. The East Germans were famous for doing extended hours of training at OBLA!

In a similar vein, Gorostiaga et al in 1991 compared a continuous training group at circa-threshold intensity to one that did only structured high intensity VO2max type intervals (of the type that are all the rage today) and found some compelling differences. While the VO2max group did show a two fold increase in percentage change in VO2max (16% increase v 8% increase), the continuous training group had a ten fold increase in citrate synthase production compared to the VO2max group (25% increase v 2.5% increase). Citrate synthase is one of the main markers for muscle mitochondrial capacity, and is a good reference for total metabolic efficiency.

Both of these examples (the first decidedly anecdotal) serve to illustrate the value of continuous training at an intensity around Lactate Threshold. This has most recently been termed ‘sweet spot’ training, but the idea has been advocated by Lydiard, Coggan, and others in various forms or years. Typically “sweet spot” is defined as approximately 88-93% of your Lactate Threshold power, however the true measure of intensity should be determined by your ability to repeat them over multiple days in a training block.

These circa-threshold efforts should be at least twenty minutes in length, but can last up to two hours or more for advanced athletes. A key determinant of the duration and intensity is your ability to replicate the workout intensity/duration again the next day. A well prepared, motivated athlete doing 60 minutes at 88-93% of threshold power (FTP), should be able to replicate that workload again the second and third days. If you can’t then you probably went too hard, too long, or don’t have a good estimate of your FTP and need to adjust. My suggestion is to start doing some field testing to establish your FTP and then see what you can do. Have fun and let me know how it goes…

References:

1. Abbiss, Chris, Laursen, Paul – Models to Explain Fatigue During Prolonged Endurance Cycling. School of Exercise, Biomedical and Health Sciences, Edith Cowan University, Australia. 2005
2. Coggan, Andy – Explaining Lactate Threshold. Webinar Presentation. 2010
3. Robergs, Robert A., Ghiasvand, Farzenah, Parker, Daryl – Biochemistry of exercise-induced metabolic acidosis. Am J Physiol Regul Integr Comp Physiol 287: R502–R516, 2004