When we consider endurance performance there appears to be three key parameters that will determine successful performance. These include 1) a high maximal aerobic power, 2) a high fractional utilisation of aerobic power and 3) running/cycling economy. This blog will explore these three key elements and how cardiorespiratory fitness testing can be used to optimise the training experience by identifying the key limiting factor in an individual’s performance.
High Maximal Aerobic Power
This relates to the maximum amount of oxygen an individual can take in and use during heavy exercise also known as VO2max. This will establish a person’s capacity to perform work. The trend is that the more oxygen you can take in, the more work you can perform therefore, the higher your VO2max.
High Fractional Utilisation of Aerobic Power
This refers to the ability to sustain a high percentage of VO2max for extended periods of time and relates to the common thresholds often used such as the lactate threshold and the ventilatory threshold. Steady state exercise (the ability to exercise at a fixed intensity without the onset of fatigue) can only occur up until a certain physiological point – when an individual exceeds this intensity point (or threshold) the accumulation of bi-products produced from energy synthesis will begin to cause fatigue causing the individual to slow down or stop.
Running Economy
This is the energy expenditure for a given running speed or cycling power output, and can be measured through oxygen uptake. The less energy an individual can expend for a given speed, the more economical they will be and this relates more to neuromuscular function in relation to rate of force development (the amount of force that can be exerted into the ground/pedal) and ground contact times. This is primarily developed through strength and power training.
Current State of the Problem
It is well accepted that the use of heart rate zones to control intensity of training is a popular way to ensure that we can safely periodise and plan training across the duration of a training block. This allows us to evoke a significant stimulus within an individual’s physiology in order to prevent injury, fatigue and ensure continual adaptation and progression. However, when we look at how these heart rate training zones are most commonly demarcated – using a percentage of a predicted maximal heart rate value – these training zones can actually end up being high inaccurate.
For example, in order for heart rate training zones to be accurate it is necessary to have an accurate measure of maximal heart rate. However, all equations used to predict maximal heart rate show poor agreement with actual measured maximal heart rate value (Shookstar et al, 2020).
Furthermore a common mistake made by many individuals is that they will simply repeat the same run/cycle session over and over again. The issue here is that there is a decreasing response of a biological system to a constant stimulus (Grissom, 2009) highlighting the importance of including a variation of training intensities to ensure optimal and continuous adaptation.
Therefore, by accurately establishing true heart rate training zones the individual can not only be certain that they are training at the correct intensity for a given session, but also allow for a structured plan to be assembled tightly targeting an optimal variation of intensities resulting in positive adaptation and reducing the risk of overtraining and injury.
Cardiorespiratory Fitness Testing
A cardiorespiratory fitness test involves running on a treadmill or cycling on a fixed bike at a low intensity and then progressively increasing the speed/power until exhaustion is reached. Throughout the test the participant will wear a special mask which will measure how much oxygen (VO2) is inhaled and how much cardio dioxide (VCO2) is exhaled. As exercise intensity increases, there is a concurrent increase oxygen uptake until exhaustion is reached. This maximal amount of oxygen that is taken up and consumed is termed the VO2max and is the gold standard measure of cardiorespiratory fitness.
Using these two values (VO2 and VCO2), a metric called the respiratory exchange ratio (RER) can also be calculated in response to different exercise intensities which will inform on the contribution of energy (fat to carbohydrate) utilisation. The rationale behind this is that there are predominately two main energy sources that the body will use to meet energy demands – fat and carbohydrate. At rest and during low to moderate intensity exercise the body will prefer to use fat as a fuel source as this is more efficient. As exercise intensity increases and the need for energy at a faster rate is required, fuel utilisation will switch from fat into carbohydrate to meet these increased demands. This results in and increase of carbon dioxide production.
Furthermore, by understanding how much fat and carbohydrate are being utilised, we can then calculate the energy expenditure in kcal/min for any given exercise intensity which can allow for the assessment of exercise economy.
What Do We Really Want to Know?
This testing technique allows for a thorough assessment of an individual’s physiology and response to exercise. Importantly it allows for the identification of any potential weakness’ within three key elements of aerobic performance namely respiratory function, metabolic efficiency and cardiac function.
Respiratory Function refers to how well an individual can utilise their actual lung volume when exercising. This achieved by measuring lung function prior to the cardiorespiratory fitness test to measure a value known as FEV1 which is the maximal amount of air that can be exhaled in the first second of a maximal exhalation. If the individual cannot reach at least 80% of their FEV1 value during the test, then this is highlighted as a performance limitation in that the full lung capacity cannot be utilised when exercising.
Metabolic Efficiency measures the ability to use fats and carbohydrates at different exercise intensities. If the body switches from using fat to carbohydrate at very low intensities then this can indicate a limitation in this ability and therefore a metric that should be improved.
Cardiac Function is the ability of the cardiovascular system to deliver oxygen to the working tissue and is key for energy utilisation.
By identifying the characteristic that is limiting aerobic performance, a tailored exercise plan can then be assembled to focus on any of these specific limitations.
Maybe the most important element in demarcating heart rate training zones though, may be the identification of the intensity at which an individual switches fuel sources. This is how the moderate exercise intensity domain is identified - often known as zone 2. This is important because everyone will switch fuel sources at different intensities; therefore this metric is highly individual. Generic percentage based heart rate training zones will not provide this detail of analysis, therefore may be a highly erroneous approach to exercise programming.
These specific demarcation points that exist in everyone’s physiology are often referred to as thresholds. It is these key points that will firstly allow us to accurately determine an individual’s true heart rate training zones, but as an individual increases their fitness, these demarcation points will likely also improve therefore allowing us to objectively track improvements in performance/fitness overtime.
Intensity Drives Adaptation
Many endurance athletes will follow a polarised approach to their training distribution. This means that most (approximately 80%) of their training will be at a moderate intensity below their race pace and the remainder (20%) will be above their race pace. The moderate intensity training allows for a high training volume to be accumulated without significant fatigue. This then allows the hard session (20%) performed with real intent and high quality. This polarised method has been shown to stimulate greater training effects than between threshold training in not only in elite endurance athlete but also recreational athletes too (Munoz et al, 2014).
A key element in adaptation to exercise is the SAID principle. This stands for Specific Adaptation to the Imposed Demands and highlights the fact that the body will adapt to the stress that is exposed. Therefore, to improve a specific characteristic, we must exercise at the correct intensity in order to achieve this adaptation. This means that we can begin to associate specific heart rate training zones with their type of energy contribution, which can not only optimes the training effect but also assist with potential nutritional strategies based on the aim of a given session.
Risk of All-Cause Mortality
Cardiorespiratory fitness testing is not only important from a performance perspective but it is also the strongest tool we have in predicting health and longevity. VO2max is the gold standard measure of cardiorespiratory fitness (CRF) with a VO2max value of below 27ml/min/kg associated with a 40% increase in all-cause mortality.
All-cause mortality is a statistic we can use to establish an individuals risk of death based on their current health status and habits/lifestyle. For example, smoking is associated with a 40% increase in all cause mortality; meaning that an individual that smokes is 40% more likely to die than a non-smoker.
When we look at CRF, risk of all-cause mortality is even more profound. If you are in the bottom 25% for your age and gender for VO2max and you compare to those who are just in the 50th to 75th percentile your talking about a 2x difference in risk of all-cause mortality.
Furthermore, if you compare the bottom 25% with the top 2.5%, so bottom quarter up the superior level for a given age, you’re talking a 5x or 400% difference in risk of all-cause mortality. That’s the single strongest association you can see in any modifiable behaviour.
Additionally, just a 3.5ml/min/kg increase in your vO2max is comparable to:
⬇️ 7cm reduction in waist circumference
⬇️ 5mm/Hg decrease in systolic blood pressure
⬇️ 1mmol/L decrease in blood triglycerides
⬇️ 1mmol/L decrease in blood LDL (bad) cholesterol
⬆️ 0.2mmol/L increase in blood HDL (good) cholesterol
What the Future Holds
Cardiorespiratory fitness testing is a hugely beneficial yet underutilised tool to assist with both endurance performance and health promotion. By providing comprehensive, personalised and actionable insights into an athletes physiological capacities we can empower athletes, coaches and health organisations to make data driven decisions that lead to peak performance, injury prevention, long term athletic success and improved health.