Wednesday, September 11, 2013

Optimizing Training Part 2: Lactate and Ventilatory Threshold Training

This article is the Second Part of a Series on Optimizing Run Training

The first part of the series can be found here

Interestingly enough this applies to any exercise that crosses multiple energy systems.  So this is perfectly applicable to many mid range, with respect to time, workouts.

Metabolic Pathways Overview

All energy transformations that occur in the body are referred to as metabolism. A metabolic pathway is a series of chemical reactions that will result in the formation of ATP and waste products (such as carbon dioxide). 

The three energy systems of the body are the:
  1. Anaerobic ATP-PC (often referred to as phosphagen) system - Short term.
  2. Aerobic glycolysis (break down of sugar) - Mid Term also called the Lactate System
  3. Aerobic mitochondrial respiration (cellular production of ATP in the mithochondrion) - Long term.

ATP-PC(Adenosine Tri-Phosphate - Phospho Creatine) is the simplest energy system of the body with the shortest capacity (up to 15 seconds) to maintain ATP production. During intense exercise, such as in weight lifting and sprinting, the ATP-PC is the most rapid and available source of ATP.

Energy for exercise activities requires a blend of all the energy systems. All energy system are active all of the time, but depending on the duration of the work, one energy system will predominate. The determinants of the involvement of the particular energy system are dependent on the intensity of the exercise. 

1) Lactate Removal
Although once considered a negative with respect to athletic performance, increased lactate production which occurs only during high-intensity exercise is natural.  Even at rest a small degree of lactate production takes place, which indicates there is also a lactate removal system or else there would be lactate accumulation occurring at rest. The primary means of lactate removal include its uptake by the heart, liver, and kidneys as a fuel .  Within the liver, lactate functions as a chemical building block for glucose production (known as gluconeogenesis), which is then released back into the blood stream to be used as fuel elsewhere. Additionally, non-exercising or less active muscles are capable of lactate uptake and consumption. At exercise intensities above the lactate threshold, there is a mismatch between production and uptake, with the rate of lactate removal lagging behind the rate of lactate production.

2) Fast-Twitch Motor Unit Recruitment
At low levels of intensity, long slow distance for example, primarily slow-twitch muscles fibers are recruited to support the exercise workload. Slow-twitch muscle has a high aerobic endurance capacity using the mitochondrial respiration energy system as opposed to the ATP system. With increasing exercise intensity there is a shift towards the recruitment of fast-twitch muscles, which have metabolic characteristics that are geared towards glycolysis. The recruitment of these muscles will shift energy metabolism from mitochondrial respiration towards glycolysis, which will eventually lead to increased lactate production.
Unfortunately and somewhat confusingly, the lactate threshold has been described with different terminology by researchers, including maximal steady-state, anaerobic threshold, aerobic threshold, individual anaerobic threshold, lactate threshold,  lactate breaking point, and onset of blood lactate accumulation.  Whenever reading on the topic of lactate threshold it is important to realize that these differing terms are essentially describing the same physiological event.

Traditionally there has been several ways to optimize running training: Ventilatory Threshold Training, Anaerobic Threshold Training, Heart Rate Monitoring, and VO2Max:

What is the Ventilatory Threshold?
As exercise intensity increases breathing increases in the early phases in a roughly linear fashion. As the intensity of exercise continues to increase, there is a  point at which breathing increases e in a non-linear fashion. This point where breathing increases at a much higher rate is called the ventilatory threshold. The ventilatory threshold corresponds (but is not identical) with the development of muscle and blood acidosis. Blood buffers(sodium bicarbonate), which are compounds that help to neutralize acidosis, work to reduce the muscle fibers acidosis. This leads to an increased need to expel carbon dioxide, which the body attempts to eliminate with the increase in ventilation.

Because increased ventilation occurs with increasing blood lactate values and acidosis, scientists originally believed this was an indication that the ventilatory and lactate threshold occur at similar exercise intensities. This interpretation is appealing because measuring the ventilatory threshold is non-invasive compared to the lactate threshold. And while numerous studies have shown a close correlation between the thresholds, separate studies have demonstrated that different conditions, including training status and carbohydrate nutritional supplementation, can cause thresholds in the same individual to differ substantially.

What is the Anaerobic Threshold?
The term anaerobic threshold was introduced based on the concept that at high-intensity levels of exercise, low levels of oxygen (or hypoxia) exists in the muscles. At this point, for exercise to continue, energy supply needed to shift from the aerobic energy system (mitochondrial respiration) to anaerobic energy systems (glycolysis and the phosphagen system).

The use of the term Anaerobic Threshold is misleading. Using the term 'anaerobic threshold' suggests oxygen supply to muscles is limited at specific exercise intensities. However this is not the case muscles do not become deprived of oxygen - even at maximal exercise intensities.  The limiting factor is Carbon Dioxide expulsion caused by the buffering of muscular acidosis not oxygen uptake. 

The second argument against using anaerobic threshold is that it suggests at this point in exercise intensity, metabolism shifts completely from aerobic to anaerobic energy systems.  Anaerobic energy systems (glycolysis and the phosphagen system) do not take over the task of ATP regeneration completely, even at higher intensities of exercise, but augment the energy supply provided from mitochondrial respiration.

What is the Heart Rate Threshold
In the early 1980’s, Italian researchers developed the methodology to detect the lactate threshold through a running test by determining the heart rate deflection point.  This easy and non-invasive approach to indirect lactate threshold measurement has been utilized extensively for training program design and exercise intensity recommendations

The heart rate deflection point is only visible in about half of all individuals and commonly over-estimates lactate threshold.  Because of these findings, and  errors associated with its use, heart rate variability among populations, I do not recommend the heart rate threshold method when designing endurance training programs for clients.  In part 3 I will discuss a simpler and possible more accurate way to monitor this which doesn't require constantly looking at a heart rate monitor.

Summary of Anaerobic, Ventilatory, Lactate and Heart Rate Thresholds
In summary, ventilatory and lactate thresholds, although similar, do not occur at exactly the same exercise workloads. The use of the term anaerobic threshold is not strictly correct although it is common usage and helps make the concept easier to understand (I think). 

One option for optimizing training is where training intensity would be based upon the velocity (speed) that corresponds to the lactate threshold.. Despite this, it is well known that following endurance training, the lactate threshold will occur at a higher relative percentage of an individual’s maximal oxygen uptake (VO2max) than prior to training. This adaptation allows for an individual to maintain higher steady-state running velocities or cycling workloads, while maintaining a balance between lactate production and removal. Endurance training influences both the rate of lactate production and the capability for lactate removal.

The reduced lactate production, at the same given workload, following endurance training can be attributed to increased mitochondria size, mitochondrial numbers, and mitochondrial enzymes.  The combined result of these training adaptations is an enhanced ability to generate energy through mitochondrial respiration, thus lowering the amount of lactate production at a given workload.

In addition, endurance training causes an increase in lactate usage by muscles, leading to a greater capacity for lactate removal from circulation, consequently, despite the heightened lactate production rates occurring at high levels of exercise intensity, blood lactate levels will be lower. 

Endurance training may also improve capillary density around the muscles, especially the slow-twitch muscles. This adaptation improves blood flow to and from exercising muscles, which will enhance the clearance of lactate and acidosis.

Next: Part 3: Lactate Threshold Training Programs and Workouts


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