Interpreting a stress test

The first thing a person who engages in constant sports practice should do is a stress test, but what really is a stress test?

It is about evaluating the capacity of an organism to perform a physical activity, combining the respiratory system, cardiovascular and blood systems. Why perform a stress test?

Mainly to rule out and ensure our fitness for sports practice and to know what our tolerance thresholds are for a certain activity, also serving as a starting point to compare the data extracted in subsequent tests, and thus be able to check if I have worsened or improved with the effects of training. It is worth noting that, as the name itself indicates, the “stress test” is a maximum tolerance test, where the test ends when the participant cannot cope with the external load to which they are exposed, or the conditions they are in are not ideal. How are they performed?

This type of test, to be as specific as possible, should be carried out according to the athlete’s sport modality, simulating their preferential activity in which they want to know their state or performance(2).

The collection of values is mainly done in a laboratory, although technological advances have allowed protocols to be brought closer to field tests through portable gas analyzers and wireless data transmitters.

The protocols to follow can basically be of two types: continuous load and discontinuous load, logically this will again depend on the athlete’s modality and the skills that are decisive for the performance of their modality (3).

Main parameters of a stress test

The main data obtained in any stress test, regardless of the modality to be evaluated, are the following:

1. Oxygen consumption (VO2MAX): It is defined as the organism’s ability to absorb, transport, and consume oxygen per unit of time(1). Today it is considered one of the most crucial parameters in endurance and ultra-endurance sports. In reports, it is expressed in ml/kg/min.
2. Heart rate: It provides the number of beats per minute the heart uses to supply blood to our entire body, having a directly proportional relationship with energy expenditure and the previous parameter. Thus, cardiac output is the product of frequency by stroke volume. It is a very variable parameter.
3. Power: In almost all tests, it is expressed in watts (W), for example, in a cycle ergometer or rowing ergometer.
4. Speed: It is usually expressed in km/h and allows us to make a conditional assessment by contrasting speed with the previous parameters.

The relationships established between these four indicators are of vital importance because with them we can optimize improvement: knowing the heart rate at different speeds and the force generated with a number of beats per minute, we know what capacity our organism has to tolerate that effort and be more specific in the application of loads. The more specific we are as professionals, the better results we will obtain, because it is necessary to keep in mind that a key concept is the “dose,” which refers to the minimum amount of exercise not accustomed to.

Secondary parameters of a stress test

1. Lactate concentration: It is expressed in mmol/l. We classify it as a secondary parameter because it is not performed in all tests, but its knowledge is key because the accumulation of this substance in the blood is considered a limiting factor as it triggers fatigue and the inability to continue performing that activity due to acidosis(4).
2. Carbon dioxide production: It dictates the organism’s ability to produce carbon dioxide from the oxygen consumed, and it is also expressed in l/m.
3. Respiratory quotient: It relates the capacity to produce carbon dioxide from the oxygen consumed, the gas analyzer indicates the volume of air inhaled and the volume of air exhaled.
4. Oxygen pulse(5): It establishes a relationship between the O2 consumed during a cardiac cycle, therefore the PulO2 will depend on the stroke volume and the arteriovenous oxygen concentration(6).
5. Pulmonary ventilation: Expressed in the document as (VE) and measured in l/min, it refers to the volume ventilated per unit of time. This measurement also associates ventilatory efficiency due to the close relationship it maintains with VO2 and with VCO2.
6. Respiratory rate: It refers to the succession of respiratory movements of inspiration and expiration. When the rate is higher/lower than normal, we are facing tachypnea and bradypnea respectively(7).
7. Blood pressure: Monitoring it during the test mainly allows us to rule out pathologies and that the athlete tolerates the load.

How to interpret a stress test?

This is the most important part regarding training, because once I have such an amount of data, I must try to organize them to be able to outline a plan for my training.

First, what I need to know is what means I have, that is, if I have a power meter or a device with heart rate, to know which parameters to choose in my training plan.

Secondly, once I have decided on my training means/method, I must look for my thresholds in the stress test: aerobic and anaerobic. In the aerobic, fatigue will come when I exhaust my energy reserves, while in the latter, fatigue concentrates when the level of acidosis is so high that it does not allow me to continue.

Therefore, the primary objective of any endurance athlete will be to delay the onset or entry of the anaerobic system to allow for greater power or speed development for as long as possible, delaying the onset of fatigue.

The anaerobic threshold is around 75% of VO2max, depending on the subject and the level of training. To improve VO2max and increase the threshold, we must work on parameters (speed, HR, or watts) that are close to that zone to tolerate that acidosis and be able to climb a step(8).

To be able to perform this type of work, we need prior improvements in the aerobic threshold that allow us to tolerate this type of load and above all, obtain mechanical efficiency, advancing more while spending less energy.

In the following table, the different metabolic pathways with their respective adaptations and the most appropriate methods for their development are shown.

*View image in full size*

It is worth noting that there is a measure that is assumed to be decisive in endurance sports and that associates power and speed, which is body weight.

Weight is a vital data point, because it is not the same to develop the same power with 70kg as with 80kg, the subject with 70kg will have better relative power because they will offer less resistance and lower energy expenditure due to having to oxygenate less muscle mass.

Below you can enjoy a training webinar on stress tests

Bibliography

1. Vaquero, F.(2008).Exercise physiology.Madrid.Editorial:Médica panamericana.
2. American college of sports medicine(2005).ACSM’s guidelines for exercise testing and prescription.Ed Paidotribo.
3. Wilmore, J & Costill, D(2007).Physiology of sport and exercise.Ed Paidotribo.
4. López Chicharro, J & Fernández Vaquero, A.(2013).Exercise physiology.Madrid.Ed Médica Panamericana.
5. Astrand,P-O.Experimental studies of physical working capacity in relation to sex and age.Kopenhagen.Munksgaard,1952.
6. Whipp, BJ, Higgenbotham, MB, Cobb, FC. Estimation exercise stroke volume from asymptotic oxygen pulse in humans. J appl physiol  1996;81:2674-2679.
7. Gazitúa,R.(2014).Respiration. Manual of semiology.Universidad católica de Chile.
8. Pallarés, JG.Morán-Navarro, R.(2012).Methodological proposal for cardiorespiratory endurance training.Journal of sport and health research.4(2):119-136.