What factors are involved in hamstrings injury?
Hamstrings injury is the most frequent muscle injury in sports that require high speeds, such as football, rugby or American football.
Athletes often face adverse weather conditions in their competitions. One of the most worrying factors for coaches and athletes is heat, due to the impact on adaptation responses at central and peripheral levels – such as increased sweating and consequent dehydration, or increased perceived exertion – exercising in hot environments results in markedly decreased performance. In fact, this has led to numerous discussions about the suitability of doing some competitions in the Beijing Olympic Games at the time, and more recently in Tokyo 2020, such as the 50 km walk or the marathon, given that during the summer months, temperatures go up to 30ºC with a humidity level of around 80%.
The heat we produce increases body temperature, which at rest is stable around 36-38ºC and during exercise reaches and remains between 38 and 40ºC in normal conditions. To maintain this “working” temperature, heat must be evacuated, otherwise, the body will overheat and stop functioning. Muscular ability is lost and the brain-system of nerve conduction and cognitive management – stops working with the same ability and speed.
The body loses heat in a perfect way through blood, which collects the heat from the inside and sends it to the skin, by sweat evaporation although it also does it by the radiation of the body in contact with the air as in the case of football. Specifically, in professional football, a loss of 2.5% of body weight (about 2 kg for an 80 kg player) increases the perception of exertion, decreases the ability to dribble and the duration and intensity of sprints. In such decisive moments, as it is the end of a game, this results in skills being substantially reduced due to dehydration.
Different studies have shown how heat affects sports performance. For example, in a study involving 19 elite football players (Mohr and Krustrup, 2013), researchers saw that jumping performance (a marker for fatigue) decreased significantly (6%) during matches played in high temperatures (~30°C), while this decrease was not observed when playing in cooler temperatures (~12°C). In fact, it is known that the best performance in football is found in temperatures between 11 and 15ºC (Link and Weber, 2017; Zhou et al., 2019). In addition, loss of jumping ability correlates with weight loss during the game, reflecting that dehydration plays a major role in performance loss. On the other hand, if humidity is high, sweat does not evaporate as effectively as in a dry environment, and consequently, more sweat is produced in the attempt to balance the temperature, losing more water and compromising hydration and the blood’s regulatory mechanism.
Remember that sweat causes heat loss when it evaporates from the body. If the humidity in the environment is high, sweat does not evaporate, and therefore, its function is diminished and sweating persists. At that point, if the player does not hydrate properly, dehydration limits the body’s ability to lose heat and increases the rate of degradation of muscle glycogen, so nutrient reserves are diminished and fatigue comes prematurely (due to dehydration and lack of fuel), causing physical ability and decision making to decrease (Maughan et al., 2012).
We know that performance decreases as temperature increases, but we can carry out some strategies to mitigate these effects. For example, regular training in hot conditions induces a number of adaptations that prevent, or at least greatly reduce, the impact of heat on performance. In a study published in the Journal of Applied Physiology (Lorenzo et al., 2010), researchers analysed a group of cyclists who were assigned to perform 10 days of training at a temperature of 40ºC as ‘acclimatisation’ or the same training at a less hot temperature (13ºC). After this period, an increase in plasma volume (6.5%) was observed in the group that had acclimatised to the heat, which is a key adaptation since the decrease in plasma volume is a marker of dehydration, and this increase induced by training avoids the negative effects of heat. In addition, the heat-acclimated group improved their performance in heat tests by 5-8%.
It has been proposed that, due to the increase in plasma volume that occurs with heat acclimatisation and in accordance with the so-called ‘Frank-Starling law’, the heart could increase the amount of blood sent to the tissues (known as systolic volume). This increase in systolic volume could result in better performance not only in hot environments but at any temperature. In fact, one of the more curious findings in the above-mentioned study was that not only did heat acclimatisation increase performance in hot environments but so did oxygen consumption (5%) and physical performance (6%) in a time trial when testing in cold temperatures. Other authors conducted a study on 15 football players who trained for one week in Qatar (34.6°C WBGT) and found that, in addition to increased plasma volume, performance in the intermittent Yo-Yo exercise test again at a lower temperature (20°C) improved by 7% (Buchheit et al., 2011).
Some researchers have questioned these benefits (Nybo and Lundby, 2016), as the evidence for them is controversial. However, in recent months, new evidence has emerged that may support the ergogenic potential of heat acclimatisation. In a recent study of cyclists, researchers observed that 5 weeks of training in a hot environment (40°C, 1 hour a day, 5 days a week) increased not only plasma volume but also the haemoglobin mass to a greater extent than the same training performed in cold temperatures (15°C) (Oberholzer et al., 2019). Thus, this data suggests that the increase in plasma volume could also induce a compensatory increase in erythropoiesis (i.e., an increase in the production of red blood cells, responsible for transporting oxygen), which could potentially improve performance. Additionally, another study with a similar methodology conducted on elite cyclists showed that 5 weeks of heat training (37.8ºC) improved the haemoglobin mass of elite cyclists more than the same training protocol conducted at 15.5ºC (Rønnestad et al., 2020). However, it is important to note that no significant benefits in sports performance were seen in these studies despite these haematological improvements.
As we can see, before a competition in a hot environment, it is advisable, in addition to learning to hydrate properly, to carry out a period of prior acclimatisation to mitigate the loss of physical and mental performance.
Buchheit, M., Voss, S.C., Nybo, L., Mohr, M., Racinais, S., 2011. Physiological and performance adaptations to an in-season soccer camp in the heat: Associations with heart rate and heart rate variability. Scand. J. Med. Sci. Sport. 21, 1–9. https://doi.org/10.1111/j.1600-0838.2011.01378.x
Link, D., Weber, H., 2017. Effect of Ambient Temperature on Pacing in Soccer Depends on Skill Level. J. Strength Cond. Res. 31, 1766–1770. https://doi.org/10.1519/JSC.0000000000001013
Lorenzo, S., Halliwill, J.R., Sawka, M.N., Minson, C.T., 2010. Heat acclimation improves exercise performance. J. Appl. Physiol. 109, 1140–1147. https://doi.org/10.1152/japplphysiol.00495.2010.
Maughan, R.J., Otani, H., Watson, P., 2012. Influence of relative humidity on prolonged exercise capacity in a warm environment. Eur. J. Appl. Physiol. 112, 2313–2321. https://doi.org/10.1007/s00421-011-2206-7
Mohr, M., Krustrup, P., 2013. Heat stress impairs repeated jump ability after competitive elite soccer games. J. strength Cond. Res. 27, 683–689.
Nybo, L., Lundby, C., 2016. CrossTalk opposing view: Heat acclimatization does not improve exercise performance in a cool condition. J. Physiol. 594, 245–247. https://doi.org/10.1113/JP270880
Oberholzer, L., Siebenmann, C., Mikkelsen, C.J., Junge, N., Piil, J.F., Morris, N.B., Goetze, J.P., Meinild Lundby, A.K., Nybo, L., Lundby, C., 2019. Hematological Adaptations to Prolonged Heat Acclimation in Endurance-Trained Males. Front. Physiol. 10, 1–8. https://doi.org/10.3389/fphys.2019.01379
Rønnestad, B.R., Hamarsland, H., Hansen, J., Holen, E., Montero, D., Whist, J.E., Lundby, C., 2020. Five weeks of heat training increases hemoglobin mass in elite cyclists. Exp. Physiol. In press. https://doi.org/10.1113/ep088544
Zhou, C., Hopkins, W.G., Mao, W., Calvo, A.L., Liu, H., 2019. Match performance of soccer teams in the Chinese super league—effects of situational and environmental factors. Int. J. Environ. Res. Public Health 16. https://doi.org/10.3390/ijerph16214238
An article published in The Orthopaedic Journal of Sports Medicine —in which members of the club’s medical services participated— now suggests to consider the detailed structure of the area affected, and treating the extracellular matrix as an essential player in the prognosis of the injury.