Football is a sport characterized by continuous changes in activity, alternating high intensity actions with short resting periods to recover from them 1. In addition, the game also demands tackles, shoots, jumps, accelerations, decelerations, changes of direction and dribbles 2, making the sport highly physiologically demanding 3. This is why muscle injuries of the lower extremities are frequent in male elite and amateur football players 4, but most of these injuries do not take place because of physical contact, as only 5% of the injuries occur during foul play, which means that it may be possible to prevent at some extent these injuries 5. A congested calendar of matches and training can induce fatigue, as some player can be unable to assimilate these matches and training loads, increasing the risk of injury and underperformance 6. This was specially remarkable in 2020–2021 season, where the post lockdown congestion schedule after a home-based training impacted negatively in some team´s number of injuries and players performance 7. In this sense, coaches and researchers agree about the importance of managing fatigue for optimal muscular adaptations, decrease injury risk and improve performance 8. However, very few studies have investigated actively how to measure fatigue and modify the training accordingly to it 9. Which is especially important in the situation COVID-19 has provoked in football, where clubs need to be prepared for circumstances never experienced before 7.
There are many methods to measure fatigue, but most of them present some disadvantages: heart rate measurements are the most common, but require qualified staff and its precision for high intensity training is under question 10. Blood and bone markers are invasive and very difficult to administer, which makes them unsustainable during regular season training 11, and questionaries are very simple to administer, but very subjective, which make them a measure with a high risk of bias 12.
On the other hand, it has been evidenced that velocity loss is a reliable marker of neuromuscular fatigue 13, which is described as the decreased in the voluntary force in a muscle group 14. This is the reason why most of the scientific literature use the Repeated Sprint Ability test (RSA) to measure fatigue, especially in team sports such as football 15, as RSA and the ability to exercise at high intensity are key capacities for optimal performance 16, and a decreased sprint repetition capacity is a good indicative of fatigue in sport 17. This method with the incorporation of a stretching- shortening movement, like Countermovement Jump (CMJ) can provide very deep information about fatigue 18 as good relationships between sprint ability and CMJ capacity has been verified 18.
Thereby, scientific evidence have found CMJ performance to be an objective marker of fatigue and supercompensation 19,20, as neuromuscular fatigue have been associated with a decrease in the average CMJ height 9, making this method also very usual to asses neuromuscular fatigue 19,21. However, when using RSA to assess neuromuscular fatigue, the number of sprints induces great variability between athletes 22, and when using CMJ it is important to consider that the same fatiguing stimuli can elicit different effects between individuals in the results 23. That is why it can be considered to use other technologies, like Infrared Thermography (IRT) which is a non-radiating, contact-free, safe and non-invasive technology that monitors physiological variables through the control of the skin temperature 24.
This technology emerges from the correlation between muscle activation and skin temperature 25. When someone exercises the muscles can increase or decrease the temperature depending on the activity and the intensity of it, and thermography is a reliable method to asses this temperature 26, as the stress caused by physical efforts can cause changes in blood flow that influence skin temperature 27. Athletes are presumed to keep constant the thermal patron in baseline conditions 28, and thermal asymmetries are linked to factor related to injuries, such as inflammation or secondary trauma 29, IRT can detect these asymmetries comparing bilateral body areas 27, showing potential injury risk due to incorrect work assimilations provoked by factor like excessive training, bad technique or muscle overload 30. The relevance of IRT is that it can detect the temperature asymmetries (and consequent risks) before other markers such as pain, making this method extraordinary effective and applicable to prevent injuries before they happen 30. If IRT provides coaches the ability to asses neuromuscular fatigue before exercises, it can allow coaches to modify the training load proactively, decreasing injury risk and increasing performance 9. This advantage is especially remarkable in sports like football, where high- intensity interval training combined with the weekly competition can lead locomotor system to its anatomical and physiological limit 24, incrementing exponentially the risk of minor injuries, overuse injuries, lower leg injuries and muscle strains 24. In recent years some authors have explained the efficiency of this technology for injury prevention in medicine 31,32, however, research with IRT in athletes, like football players, have not been widely investigated, as only a couple of studies have explore the use of IRT for preventing football injuries 27,33, nevertheless, none of the studies realized with thermography aimed to use it to detect fatigue in football players, to adapt training loads depending of it, so this concept demands research.
Therefore, the purpose of this study was to determine the suitability of thermography to detect fatigue in male football players through bilateral body asymmetries. RSA and CMJ will be used as control variables to check if thermography really detects fatigue, as they are methods more developed and studied in the literature with good relationships between then.