Terrestrial Locomotion

What are the key factors limiting endurance capacity and how does one improve them?

If you were a runner or a coach, and someone were to ask you what a training program would look like – if it were not to be politically correct, if it were not to entertain the runner, keeping him or her from being bored or mentally exhausted by doing the same painful activities day after day, but if it were simply aimed at maximizing a runners endurance capacity for a half marathon – what would such a training program look like?

For the impatient reader, the (very) short answer is: High intensity interval training or HIIT.

The slightly longer answer will give you an understanding why:

A. Key factors that limit endurance capacity
In short, endurance capacity is limited by four key factor groups:

1. Oxygen uptake (VO2max)
2. Oxygen consumption
3. Lactate threshold
4. Running economy

In order to improve endurance performance, all four key factor groups needs to be improved.

Oxygen uptake (VO2max)

The ability of the cardiovascular system to transport oxygen from the atmosphere and to the muscles where it is utilized during mitochondrial respiration to produce ATP is the central component of VO2max (Roberts & Robergs 1997). The three main central limitations to oxygen delivery are pulmonary diffusion, cardiac output, and blood volume and flow (Bassett & Howley 2000).

In general, current research tends to support the oxygen uptake – as opposed to oxygen consumption – of being the main limitation to VO2max (Wilmore and Costill 1999), but the importance of peripheral components should not be discarded completely.

Oxygen uptake (VO2max) – Cardiac output

Cardiac output, i.e. the blood volume in liters per minute that is pumped from the heart is commonly identified as one of the main limiting factors to oxygen delivery and VO2max (Bassett & Howley 2000). In order to increase the cardiac output, either the number of heart beats per minute, or the stroke volume, i.e. the amount of blood pumped per heart beat, must be increased. The maximal heart rate is quite stable and remains pretty much unchanged with endurance training, although it might increase marginally with weight loss. The stroke volume, one the other hand, increases substantially from endurance training, typically because of increased heart chamber size and heart muscle strength and stronger elastic recoil for ejecting the blood to the body tissue. Actually, the variation in individual maximal stroke volume explains most of the range observed in VO2max when comparing trained and untrained individuals: Untrained individuals experience maximum stroke volume typically at an intensity around 50% VO2max, whereas the stroke volume in highly trained individuals increase up to 100 % of VO2max (Robergs & Roberts 2000).

In short, to improve cardiac output, you need to improve your stroke volume by strengthening your heart.

Oxygen uptake (VO2max) – Pulmonary diffusion

The lungs transfer oxygen from the atmosphere to the blood, and carbon dioxide from the blood to the atmosphere. Pulmonary diffusion is the exchange of oxygen and carbon dioxide between the lungs and the blood. In normal individuals, pulmonary diffusion is not a limiting factor to VO2max. However, in highly trained endurance athletes, with much higher cardiac outputs, pulmonary diffusion might become a limiting factor to VO2max. However, pulmonary diffusion is thought to play a minor role in the overall limitation of oxygen delivery for endurance performance.

Oxygen uptake (VO2max) – Blood volume and flow

Working muscles demand oxygen and nutrients, and this demand increase with the level of intensity. To meet these needs, either more blood must be transported to the muscles, or the blood’s transport capacity must be improved. Regular training will increase blood volume by increasing the level of antidiuretic and aldosterone hormone, causing the kidneys to retain water, and by increasing blood plasma, i.e. the fluid portion of blood. The increase in fluid will improve the blood flow through blood vessels, by reducing the viscosity, thereby increasing the amount of blood being transported to the muscles. The blood’s ability to transport oxygen and nutrients can be improved by increasing the density of red blood cell through endurance training – although the potential increase varies greatly between individuals.

Oxygen consumption

The peripheral component of V02max is the ability of exercising muscles to extract and utilize oxygen (Robergs & Roberts 1997). The key factors limiting this ability include muscle diffusion capacity, mitochondrial enzyme levels, and capillary density (Bassett & Howley 2000). Mitochondrial enzymes levels can be improved through endurance training, which basically allows the working muscles to use more oxygen, giving a higher VO2max (Honig, Connett, & Gayeski 1992). In addition, endurance training can also give a 20% increase in capillary density, i.e. the location where oxygen is exchanged between the blood and muscle (Robergs & Roberts 1999). The potential for increasing muscle diffusion capacity is not expected to be significant.

Lactate threshold

The lactate threshold refers to the intensity of exercise at which there is an abrupt increase in blood lactate levels (Roberts & Robergs 1997). When this threshold is reached, lactate begins aggregate within the muscles, leading to decreased muscle performance, and many believe the lactate threshold to be the most responsive physiological parameter to endurance training (McKardle, Katch, & Katch 1996), as well as the primary indicator of endurance performance (Wilmore & Costill 1999).

In untrained endurance individuals, the lactate threshold occurs at approximately 50-60% of VO2max. Following endurance training, individuals generally improve the lactate threshold to 75% of VO2max; with values at 80-90% of VO2max having been reported in elite, world-class endurance athletes (McKardle, Katch, & Katch 1996). The performance benefit of this training adaptation is that an individual is capable of maintaining a higher steady state exercise intensity during the endurance exercise, i.e. higher speed, before lactate starts to aggregate.

The physiological explanations for lactate threshold improvements following endurance training are related to increased mitochondria size, numbers, and enzyme levels. Following endurance training, mitochondria size and numbers have been reported to increase by 50-100%, thus increasing the mitochondrial respiration capacity (Holloszy & Coyle 1984). Additionally, increase in mitochondrial enzymes will also enhance mitochondrial respiration capacity. The combined result of these adaptations is a delayed timeline to lactate threshold and greater capacity to perform endurance exercise.

Running economy

The term running economy is used to express the oxygen consumption required to perform a given running workload. Differences in oxygen consumption between individuals at similar exercise workloads illustrate the individual variation found in running economy. Consequently, individuals with similar VO2max values can have much different endurance performances depending on their economy of movement. In fact, high correlations between 10-km running performance and economy have been reported between runners with comparable VO2max values (Conley & Krahenbuhl 1980). Individual exercise economy is enhanced with endurance training and has been explained by improvements in biomechanical techniques in performing the specific physical activity (Roberts & Robergs 1997).

B. How to improve key factors limiting endurance capacity

 As we could read about in the previous section, the most important key limiting factors are basically oxygen uptake (VO2max) and lactate threshold.

French exercise physiologist Veronique Billat found that the fastest way to reach a high oxygen uptake is to run high intensity intervals at a speed that elicits your VO2 max, basically the fastest effort you can maintain for about eight minutes. There are various programs for high intensity interval training, both with very short intervals and those with considerably longer intervals. A compromise that is also easy to remember, is the 4 x 4, i.e. four intervals of four minute each with two minutes of resting in between.

When it comes to improving the lactate threshold, research has indicated that training programs that are a combination of high volume, maximal steady-state, and interval workouts have the most pronounced effect on lactate threshold improvement (Roberts & Robergs 1997, Weltman 1995). However, the highest improvement in lactate threshold per minute seems to be obtained through high intensity interval training.

In short, high intensity interval training seems to be the most efficient training method in terms of endurance performance versus training time invested. However, it should be noted that the likelihood of a person giving up training because of lack of desire to perform nothing else but high intensity interval, also is high …

For more information, I strongly suggest reading these great articles. Actually, most of the information gathered for this article comes from article about the physiological factors limiting endurance exercise capacity:

Metabolic Factors Limiting Performance in Marathon Runners

The Physiological Factors Limiting Endurance Exercise Capacity

Lactate Theshold Training