Screening to stave off hamstring injuries: hype or hope?

Hamstring injuries frequently plague soccer players. Andrew Hamilton looks at potential screening tests to identify athletes at an increased risk of hamstring failure and determines their worth to those seeking to avoid this debilitating injury.

Hamstring strain injuries (HSIs) are one of the most common soccer injuries⁽¹⁾. Research suggests that HSIs account for around one in seven of all sporting injuries, and 47% of all muscle injuries^(1,3)! Furthermore, a study of professional European soccer players found that, on average, a player will miss 18 days and three matches per season due to an HIS. Hamstring strain injuries also account for around 25% of all player absences from games⁽⁴⁾.

Interventions to prevent hamstring injury

Given the high rates of muscle injury and re-injury among soccer players and the modern game’s financial pressures, the treatment of injuries in players is a frustrating and never-ending challenge. Unsurprisingly, there’s been an increasing drive in soccer circles to identify the key risk factors and injury mechanisms related to HSIs. This knowledge would enable the detection of injury-prone players and help trainers design intervention measures to prevent injuries successfully.

One of the most widely-adopted general injury prevention programs deployed in soccer was FIFA’s 11+, a series of running and proprioceptive drills used as a warm-up routine for amateur players with progressions for more advanced players (see figure 1). Professionals and amateurs worldwide now use this program (for a free and fully downloadable copy of FIFA’s 11+ program in PDF form, visit the FIFA medical site). While the 11+ program is beneficial, it primarily impacts the incidence of mild, overuse, and training injuries in teams with low-skill (i.e., amateur) players⁽⁵⁾. By contrast, this intervention doesn’t produce significant injury-reduction benefits in professional players when assessed on injury risk per hour played, with no noticeable reduction in hamstring injuries^(6,7).

Figure 1: Overview of the FIFA 11+ program*

The 11+ consists of three sections (running/strength+plyometrics/running) with a total of 15 exercises performed in a specified sequence at the start of each training session.

*Image courtesy of FIFA. Visit www.fifamedicalnetwork.com/wp-content/uploads/cdn/11plus_workbook_e.pdf for a free PDF download of a detailed description of the program.

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Specific hamstring injury prevention

The mixed results observed in soccer players following FIFA’s generalized 11+ program are perhaps unsurprising. To specifically target hamstring injury risk reduction requires a more specific approach that addresses the movement/loading patterns likely to precipitate an injury. The typical movements and loading patterns associated with a subsequent hamstring injury are:

• High-speed running activities that strain the long head of the biceps femoris⁽⁸⁾.
• High-power activities such as acceleration, deceleration, kicking, and change of direction movements⁽⁹⁾.
• The initial and late swing-phases of high-speed running when large passive torques affect the hamstring muscles. The mechanisms of injury may relate to the timing of these peak torques^(10-12).
• A previous history of an HSI⁽¹³⁾ and older age⁽¹⁴⁾ are additional risk factors for injury, which can occur at significantly lower loading levels than younger players with no previous HSIs.

Any screening interventions should challenge the hamstring muscles to safely identify their capacity to tolerate these kinds of loading forces. This exam should also determine whether the supporting musculature and relevant neural input helps hamstring resilience or hampers it. Other factors are associated with an increased risk of an HSI and are also relevant to a screening process. These include^(15-18):

• Lower limb eccentric muscle strength
• Hamstring/quadriceps (H/Q) ratio
• Lower-limb muscle flexibility
• Balance, agility, and proprioception

However, when researchers carefully examined these factors in the context of actual hamstring injury rates, the results were equivocal^(19-21).

New research

In an attempt to provide some definitive answers to this perennial problem, a European research team monitored elite soccer players in the Kosovo National Premier League over a whole season to answer the question of how to identify players at an increased risk of an HSI. They used a battery of screening tests and correlated them with the subsequent hamstring injury rate per hour of play during the season⁽²²⁾.

They recruited 143 soccer players from 11 teams in Kosovo and administered a widespread health and fitness assessment before and during the season. The study excluded players suffering from acute lower limb injuries or recovering from recent surgical interventions (within the previous 12 months). In addition to testing, all the players completed comprehensive questionnaires to identify other musculoskeletal or medical conditions, any prior hamstring strains, or performance impairments that could increase the risk of an HSI.

The tests and screening

The screening tests were comprehensive, multi-faceted, and included the following:

*Anthropometric and body composition measurements (BMI).

*A battery of fitness tests, which included:

• Sit and reach testing to assess hamstring flexibility.
• The Nordic hamstring strength test (subjects either ‘failed’ or ‘passed’ depending on whether they could hold the position beyond 30 degrees from the vertical starting position – see figure 2).
• Dynamic measurements of eccentric and concentric hamstring and quadriceps strength using isokinetic testing. Subjects performed over a 100-degree range using a Biodex System 3 dynamometer to calculate hamstring to quadriceps (H/Q) strength ratios.
• Countermovement jump performance.
• Speed, acceleration, and agility/change-of-direction tests (the latter using the Illinois Agility Test, a reliable and valid test in team sports)⁽²³⁾.

Using a relatively large sample size for the duration of a season allowed the researchers to collect a large amount of data. Over the entire study period, players registered a total of 36,833 hours of exposure time, consisting of 31,998 hours of training and 4,834 hours of match play. On average, each player participated in 25 matches and attended 149 training sessions, resulting in a mean exposure time of 258 hours per player - 33.8 hours (13.1%) of match play and 223.8 hours (86.9%).

Figure 2: Nordic hamstring strength test

A pass is awarded if subjects can maintain the position at or beyond 30 degrees from the vertical starting position.

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The findings

The key injury incidence findings from this study were as follows:

• There were 43 HSIs in total (16 during training and 27 during matches (equating to an 11-fold higher injury rate per hour in matches).
• The total injury incidence for the hamstring muscles was 1.17 injuries/1000 hours.
• The players’ positions on the soccer field had no bearing on injury incidence.
• On average, 27.9% of players sustained at least one hamstring injury.
• Of the HSIs recorded, 23% involved a hamstring re-injury.

However, the key question is whether any screening tests determined which players were at particular risk of an HSI (see table 1). Essentially, older players with a previous hamstring injury and a higher BMI had an increased injury risk. Poorer quadriceps torque outputs (when assessed on the dynamometer at 60 degrees per second) and a failed Nordic strength test were also associated with higher injury risk. None of the other factors seemed to impact injury incidence.

Table 1: Statistically significant factors associated with an increased risk of HSI⁽²²⁾

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The implications

On the face of it, these results seem to provide useful data for clinicians seeking to screen for and minimize the risk of a future hamstring injury. However, the screened factors that significantly impacted injury rates were few. When researchers removed unmodifiable factors such as age and previous injury history, only body mass/BMI, Nordic strength test performance and quadriceps torque were associated with an HSI risk. In addition, subjects weren’t fatigued (as they might be toward the end of the match) when they participated in the screening tests. Therefore, practitioners should exercise caution when applying these conclusions to real-life scenarios.

These findings are perhaps a little disappointing. While players can improve body composition and BMI appropriate nutritional interventions, they are not trainable in the same way as other parameters such as strength, flexibility, and agility. In terms of trainable factors, only a failed Nordic strength test predicted the increased risk of injury with a high degree of significance. The reduced quadriceps torque test also predicted increased injury risk, albeit with much lower confidence.

What do these findings mean for clinicians and trainers with soccer players in their care? Firstly, it is important to understand that there is still no scientifically validated screening protocol to identify players at risk of an HSI accurately. In the above study, a failed Nordic hamstring strength test, a previous injury history, and reduced quadriceps torque did collectively predict increased risk but only by around 26%. In other words, in a random sample of players, many who performed well on these tests would still suffer a future injury. In short, there is still no bulletproof screening procedure for HSI risk.

Despite that, this study does suggest that specific strength training of the hamstrings - using the Nordic hamstring curl exercise - is likely to add some additional resilience against hamstring injury⁽²⁴⁾. The increased incidence of HSI in the older players, those with higher BMI and with a previous history of injury in this study is in line with findings from other studies⁽²⁵⁾; therefore, clinicians might wish to preferentially target older players with a previous history of HSIs for specific hamstring training. These players may also benefit from a less intense match schedule with longer recoveries and guidance on nutritional strategies to optimize BMI and body composition.

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