Effects of home confinement due to COVID‐19 pandemic on eccentric hamstring muscle strength in football players

Abstract

At the beginning of March 2020, the World Health Organization declared the coronavirus disease (COVID-19), an infection produced by the virus SARS-CoV-2, a global pandemic. COVID-19 causes an acute respiratory syndrome that can produce severe life-threatening complications in a relatively high proportion of infected individuals.1 The virus that causes COVID-19 spreads mainly from person to person through respiratory droplets, so governments of most countries have declared confinement measures to reduce the propagation. Although the degree of confinement differs among countries, it has often entailed staying at home for several weeks to months. Home isolation has negatively affected all layers of society and has had potentially severe impact on athletes because training and competition in indoor and outdoor sport facilities were suspended. Athletes have been using multiple forms of training to mitigate the detraining effect of confinement on physical conditioning. However, the success of these routines may differ depending on the sports discipline. In professional football, most players have trained at their homes following the programs provided by the teams’ strength and conditioning staff. The training programs have primarily included strength-based activities with body loads, proprioception activities, and high-intensity and intermittent routines performed with low range displacements. However, the critical movements and actions of football, including accelerations, decelerations, sprints, changes of direction, and kicking the ball to pass or shot to goal, are difficult to replicate at home. This particularity may indicate that confinement has produced harmful effects of greater magnitude on the specific physical conditioning of football players than on athletes from other types of sports. Moreover, the COVID-19 outbreak occurred when most of the professional football leagues were unfinished. The tremendous economic revenues of these leagues have increased the pressure to restart the competition as soon as possible.2 However, the confinement may have harmed professional football players to the extent that it may negatively affect both performance and health of players returning to the game. Despite the efforts of both staff and football players to maintain football-specific physical condition, many players may show detraining signs,3 resulting in an increased risk of injury on return to play.4 However, to date, there are no data to assess the detraining effect of confinement on football players. There is significant concern regarding the potential higher incidence of some injuries, such has a hamstring muscle strain injury. This concern is based on the high incidence of hamstring muscle injury in professional football,5 together with the association between high prevalence of hamstring strain injury and reduced eccentric hamstring strength.6, 7 In fact, the most common mechanism of hamstring muscle strain injury is related to the tissues' failure to tolerate the forces during the sprint.8, 9 To the best of our knowledge, no previous investigation has offered data about the effect on confinement on muscle strength, and thus, it is difficult to anticipate the detraining magnitude experienced by football players and its potential effect on injury risk. This letter aims to provide information about changes in eccentric hamstring muscle strength at three time-points during confinement in a group of 30 semi-professional football players. Players had competed in 27 matches at the Spanish Soccer League before the pandemic outbreak and were confined on 14 March 2020. Confinement lasted for 7 weeks. Players were prospectively followed to assess eccentric hamstring muscle strength 14, 28, and 49 days after the onset of home isolation. Hamstring muscle strength was measured using the Nordic hamstring test and a reliable and valid video-based method.10 For this measurement, players performed a 15-minute warm-up consisting of 10 minutes of pedaling on a stationary bike and 5 minutes of dynamic stretching on lower limb muscles. Afterward, each player adopted a kneeling position on a yoga mat while an assistant stabilized the player by pressing on his ankles. On command, the player leaned forward his upper body with full extension of the back and hips to perform the Nordic hamstring exercise, as previously depicted.11 Players were encouraged to perform this leaning exercise in a slow, smooth, and controlled manner, with his hands crossed over the chest. When reaching the maximum point where the player could no longer withstand the force of the fall, the player used the arms to land in a safe manner. This exercise was performed twice bilaterally and twice with each limb, with 2 minutes of recovery between each attempt. The test was videotaped by a researcher with a smartphone (iPhone 11, Apple Inc) placed 3 m away on the right side of the player, 0.4 m over the floor and with the high-speed camera set at 240 frames per second. Afterward, the Nordic break-point angle and the hamstring muscle strength were calculated with a smartphone application (Nordics App). The break-point moment was identified as the first frame in which the hands separated from the chest to buffer the fall. The Nordic break-point angle was calculated as the angle between the initial vertical position of the player and the line joining the right femoral condyle and the right greater trochanter at the break-point moment.12 Participants had been familiarized with this measurement during the football season, and they performed the test with minimal clothing to facilitate the ubication of anatomical locations. Nordic hamstring measurements were obtained at the same time of the day and after a rest day. This test had a high test-retest reliability (coefficient of variation = 1.2 ± 3.2% for the Nordic break-point angle, measured in a subset of 20 football players). Figure 1 depicts the variation in Nordic break-point angle and eccentric hamstring muscle strength through confinement. When compared to the values after 14 days of confinement, bilateral Nordic break-point angle was reduced by 1.9% after 28 days, and by 4.4% after 49 days (P < 0.05). This produced that eccentric hamstring strength decreased by 8.0% after 28 days and by 14.1% after 49 days (P < 0.01). Similar patterns of reduced Nordic break-point angle and eccentric hamstring muscle strength were observed in the dominant and non-dominant limb (Figure 1). It is noteworthy to mention that these reductions were present even though these players had been training at home with specific exercise to maintain muscle strength in all structures of lower limbs, including the hamstring muscles. This magnitude of hamstring muscle weakness might indicate a higher risk for a hamstring injury.13 Thus, the current data might be the first evidence to suggest that football players may be subjected to a higher likelihood of sustaining hamstring muscle strain injury when returning to football training and competition. As we expect to find a similar muscular weakness in other muscles, the authors of this letter call for appropriate period between the end of confinement and the onset of official football competitions, which should be based on the duration of confinement in each country. The period should permit re-training and regaining pre-confinement muscle strength values that allow the execution of football-specific actions such sprints, changes of direction, and kicks without the burden of an increased injury risk. The authors of this letter declare that they have no competing interests. All the authors have equally contributed to the conception and preparation of this letter. The data collection included in this letter has been approved by an Institutional Review Board (code: 16/2019/CEICEGC). Readers interested in the data may obtain a copy of them upon reasonable request to the corresponding author.

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