Muscle injury in soccer players: the most recent findings in thermography
Thermography has been shown as a useful tool to prevent, support diagnosis and follow up muscle injuries in soccer. Fernández-Cuevas & del Estal-Martínez (2020) published a muscle injury thermal description with a sample of 18 professional players. This publication is a reference to better guide diagnosis support, injury monitoring and return to play decisions using thermography in professional soccer players.
Muscle injury and thermography
Have you ever wondered what happens when a muscle injury occurs from thermal perspective? Don’t worry if the answer is “no”, it is a rare question. Moreover, it is very hard to find any scientific evidence about the thermal response of this kind of pathology/injury (and from most of them).
Indeed, it is somehow logical since there are other clinical informations that might be more relevant from a medical perspective, for instance: any diagnostic gold standard technique, as ultrasound. Nevertheless, skin temperature and thermography have shown an increased potential from both, scientific and praxis approaches.
But then, if we are finally forced to answer we will probably say “it gets warmer” (yes, even myself -at least some years ago-). Nowadays, we have enough cases and evidence to decide between the two options “Cold or Hot“: it gets colder, with no doubt.
The problem of muscle injuries in soccer
The persistent problem of injuries in high performance sports boosts the search of technologies to reduce injury incidence. Despite the improvement in technology and training methodologies, the injury incidence in soccer (among other disciplines) is even increasing. Lately, infrared thermography has been shown as an useful tool to prevent, support diagnosis and follow up injuries in soccer (Gómez-Carmona et al. 2020).
Thermal description of most common soccer injuries
Fernández-Cuevas, Ismael & del Estal-Martínez, Alejandro (2020) published a Conference Paper at the “I International Congress on Application of Infrared Thermography in Sport Science” carry out in Valencia, Spain. They presented the thermal profile description of most common soccer injuries by Infrared Thermography. A serie of cases studies to describe the response of skin temperature in the most common injuries in professional soccer (you can see the 10 minutes presentation here).
The objective of Fernández-Cuevas & del Estal-Martinez (2020) was to describe the thermal profile before, during and after the most common soccer injuries. In order to establish a reference in order to better guide prevention, diagnosis support, injury monitoring and return to play decisions using infrared thermography with professional soccer players.
For this purpose, they gathered injury cases from 50 soccer male players (27.6 ± 4.6 years, 73.6 ± 6.42 kg, 1.81 ± 0.26m) competing in different European professional teams. They were assessed before, during and after suffering most common soccer injuries: ACL rupture, muscle injuries, ankle sprain, fasciitis, bone fracture, nerve issue, contusion and tendinopathy. In this review, we are going to focus on the results for muscle injuries.
Muscle injuries in soccer players
Most of the muscle injury cases gathered were hamstring injuries. The etiology of this kind of injury indicates that high speeds at high length are the typical injury mechanism for the hamstring injury. This mechanism also involves a high load over the sciatic nerve in it maximal stretch. Kouzaki et al. (2017) assessed sciatic nerve conductivity in athletes with a history of hamstring strain injuries. Their results showed that both latency and velocity of conduction in the injured group was significantly longer and lower, respectively, than that of uninjured limb (p<0.05).
In healthy players, the asymmetry between both body regions before injury does not differ from 0,05ºC. However, during the first 48 hours after the injury occurs a hypothermic response (-0,48ºC) is described. ThermoHuman software asymmetry metrics usually shows a warmer alarm in the oposite region (since the hottest region is prioritized). This can be confusing, but if we have previous data, the coefficient of variation metric will show us a colder alarm on the affected area highlighting that the most significant change is happening in the colder region (Figure 2).
We hypothesize three main reasons that might explain the hypothermic response: firstly, a lack of muscle contraction; secondly, a potential small static bleed; and thirdly and more importantly, a nerve system reaction. Reasonings as the one showed by Kouzaki et al. (2017) reinforce the idea that the most predominant factor for the hypothermia to occur in the region of interest (ROI) of hamstring is the involvement of the nervous system (Figure 3).
The hypothermic asymmetry continues to be present with a mean of -0.45ºC during the first month. However, as the recovery process goes, the asymmetry trends to decrease. It is highly interesting to perform adjunctive evaluations to increase awareness of factors that affect muscle injury (fatigue, strength, RFD, range of motion, etc). Those factors have been described by Huygaerts, Shaun, et al. (2020) in their review “Mechanisms of hamstring strain injury: Interactions between fatigue, muscle activation and function“. The authors described how it might be related to the neuromuscular and the lumbopelvic control. Also decreasing knee stability, leg stifness and muscle-tendon unit energy transfer (Figure 4).
In addition, there is retrospective evidence that prior HSI is associated with strength-endurance deficits (Lord, C. et al. 2018) and prospective evidence that a weaker test score on a hamstring specific strength-endurance test (single leg hamstring bridge) is related together to HSI risk (Freckleton, G, et al. 2014).
Conclusion
Without pain and injury, the tendency is to keep a thermal balance between bilateral areas. Once muscle injury occurs, the thermal profile shows a skin hypothermic response of -0,48ºC on average, which can help to support diagnosis during the first 48 hours. This colder pattern persists during the first month (-0,42ºC on average). However, the tendency is to slowly get back to symmetrical profiles, which might help to quantify recovery and to make return to play decisions. Thermal asymmetries are a solid method to determine healthy status, support diagnosis, monitor injury recovery and make return to play decisions with objective data with soccer players.
ThermoHuman has had the support of the Funds of the European Union and the Community of Madrid through the Operational Programme on Youth Employment. Likewise, ThermoHuman within the framework of the Export Initiation Program of ICEX NEXT, had the support of ICEX and the co-financing of the European Regional Development Fund (ERDF). 
