ACL injuries and prevention strategies in youth footballers

Football involves a variety of complex and high intensity movements such as running, twisting, turning, sudden deceleration and landing, all of which can result in several injuries and the anterior cruciate ligament (ACL) is one at high risk of injury (Agel, Arendt, & Bershadsky, 2005; Boden, Dean, & Feagin, & Garrett, 2000). Youth athletes have higher incidents of non-contact injuries due to age and skill factors and of which the knee and ankle have an increased chance of injury (Gall et al., 2006; Peterson, Junge, Chomiak, Graf-Baumann, & Dvorak, 2000; Philippaerts et al., 2006; Price, Hawkins, Hulse, & Hodson, 2004). These types of injuries can occur due to poor movement mechanics and may be prevented through appropriate training or interventions (Dai et al., 2012; Hewett, Lindenfield, Riccobene & Noyes, 1999; Pfeiffer, Shea, Roberts, Grandstrand, & Bond, 2006).

Injuries tend to increase from 9 to 15 years of age in male football players (Price et al., 2004; Shea et al., 2004) however this increases further during adolescence and adulthood (Klenerman, 1994; Granan et al., 2008). The highest ACL injuries occur in age groups U15s, U16s and U17s, with U15s and U17s groups having more ACL injuries in males (Shea et al., 2004). Shea et al. (2004) found 22% of all injuries in youth football players (U5s-U18s) to occur at the knee, with 7% of total injuries being an ACL injury and 31% of knee injuries being an ACL. Injuries in youths have been reported to occur more in competition than matches (50.4% vs. 48.7%) with the lower extremities (75% of injuries) being the most common site (Gall et al., 2006; Price et al., 2004). On average youth players experience 0.4 injuries per player per season, missing 21.9 days of football (Price et al., 2004). Ligament sprains consisted of 20% of all injuries reported and 18% of these occurred at the knee (Price et al., 2004).

ACL injuries have been linked to athletic movement patterns such as landing, cutting and deceleration being performed with small knee flexion angles, increased medial knee valgus displacement, increased hip internal rotation angles, increased quadriceps activation and reduced hamstring activation (Chappell, Creighton, Giuliani, Yu & Garrett, 2007; Malinzak, Colby, Kirkendall, Yu, Garrett, 2001; Ford, Myer & Hewett, 2003; Pollard, Sigward, Powers, 2010). Cochrane, Lloyd, Buttfield, Seward and McGivern (2007) results found ACL injuries to occur the most in sidestepping (37%) and landing (32%) tasks with decreased knee flexion (<30°). This increases ACL strain and reduces hamstring muscle activation and stabilisation of the tibia, which increases anterior tibia shear forces (Shultz & Perrin, 1999). The hamstrings and gluteus muscles play a key role in knee joint stabilisation, especially during landing and deceleration tasks with high eccentric forces, therefore, the strength and ability to recruit these muscles are important for injury prevention (Baratta et al., 1988; Myer, Ford, Brent, & Hewett, 2012; Read et al., 2015; Renström, Arms, Stanwyck, Johnson, & Pope, 1986). Furthermore, the ability to land and turn in the right positions is just as important in terms of injury reduction.

In addition, ACL injuries commonly occur when the knee is in valgus (47.4%) and during internal rotation of the knee (42.1%) (Cochrane et al., 2007; Koga et al., 2010). Youth athletes have been shown to lack knee flexion during landing and jumping tasks altering motor control and stability around the knee (Chappell et al., 2005). Chapell et al. (2005) results also found previously injured athletes to have increased tibial anterior shear forces, increased valgus moments and decreased knee flexion values. These factors increase the risk of ACL injuries (Chapell et al. 2005; Hewett, Myer, & Ford, 2006; Padua et al., 2009). The combination of these factors leads to increased loading of the knee and the ACL during subsequent athletic tasks (Dai et al., 2012). Anterior tibial shear forces have been shown to contribute the most to ACL strain and potential injury (Berns, Hull & Patterson, 1992; Dai et al., 2012). Knee valgus and internal tibial rotation forces also have been shown to increase loading of the ACL (Markolf et al., 1995).

In younger athletes ACL injuries are less common (Van der Sluis, Elferink-Gemser, Brink, & Visscher, 2015; Tursz & Crost, 1986) however, this may be due to less bodyweight, slower running velocities and smaller joint levers (Barber-Westin et al., 2005). When the athletes mature however, they may be at increases risk of injury (Adirim & Cheng, 2003). Injuries may increase around an athletes peak height velocity (PHV) as during this period of increased rate of growth and body mass comes with changes in limb lengths which may increase joint loads and increase risk of ligament damage during maturation (Adirim & Cheng, 2003; Rumpf & Cronin, 2012). Therefore, monitoring athletes closely during this period is important to decrease the likelihood of injury. Figure 1 shows the impact growth may play on ACL injury risk.

Figure 1 – Influence of growth on ACL injury risk (Cited from Myer, Chu, Brent, & Hewett, 2008).

Previous injuries to athletes who have sustained knee sprains have been shown to be at increased risk of being reinjured at the same site (Arnason et al., 2004). Kucera, Marshall, Kirkendall, Marchak and Garrett (2005) found youth athletes (U12s-U18s) who injured their knee previously were 6 times more likely to reinjure than athletes with no previous injury. In addition, athletes who have sustained a ACL injury have a over 50% chance of experiencing knee osteoarthritis later on compared to uninjured players who have a decreased risk (Lohmander, Englund, Dahl, & Roos, 2007; Neyret, Donell, Dejour, & Dejour, 1993). Furthermore, injured athletes may display decreased levels of neuromuscular control during fundamental movement skills (FMS) and stabilisation tasks (Myer, Chu, Brent, & Hewett, 2008). Neuromuscular deficiencies in such tasks may include having muscle imbalances, reduced strength and reduced stability, or using excessive trunk lean during FMS (Hewett, Paterno, & Myer, 2002; Ford, Myer, Hewett, 2003; Myer, Brent, Ford & Hewett, 2011; Myer, Ford & Hewett, 2004). Therefore, previous injuries may be a risk factor for youth athletes. Examples of neuromuscular deficiencies can been seen in figure 2.

Figure 2 – Neuromuscular deficiencies upon landing (Cited from Myer, Brent, Ford & Hewett, 2011).

Neuromuscular deficiencies will be discussed below, such as differences in hamstring to quadriceps (H:Q) strength. The H:Q ratio may influence force production and absorption during FMS (Myer et al., 2011). Movements in football require high amounts of eccentric loading that involve both the hamstring and quadriceps muscles. Therefore, if a youth athlete displays asymmetry differences then this may increase the risk of non-contact ligament injuries (Noyes, Barber, & Mangine, 1991; Paterno, Ford, Myer, Heyl, & Hewett, 2007). The H:Q ratio has been shown to increase with age with less mature boys and girls have weaker hamstrings (Ahmad et al., 2006). The hamstrings play an important role in reducing anterior shear forces, which load the ACL (De Ste Croix, 2007; Hewett et al., 2005). The hamstrings tendons stiffen and stabilise the knee joint and increase knee flexion available at the joint (Myer et al., 2010). Therefore, the hamstrings need to be stronger or have small asymmetry differences in comparison to the quadriceps as they play an important role in injury prevention. Holcomb, Rubley, Lee, and Guadagnoli (2007) results found strength training (ST) targeting the hamstrings to improve the H:Q ratio (>1). A H:Q ratio lower than 60% has been shown to be a risk factor for previous ACL injured athletes therefore, targeting the hamstrings would be beneficial (Hewett, Lindenfeld, Riccobene, & Noyes, 1999; Myer et al., 2009). In addition, Zebis, Andersen, Bencke, Kjaer, and Aagaard (2009) results found hamstring EMG preactivity to be lower than quadriceps EMG preactivity in ACL injured athletes. Huston and Wojtys (1996) results similarly found male athletes to recruit hamstring muscles first whereas, female athletes recruited quadriceps first then hamstrings during contractions and isokinetic strength tests. Therefore, ST targeting the posterior chain will be beneficial for reducing injury risk in youth footballers.

Leg dominance during FMS occurs from muscle imbalances or asymmetries during loading, co-ordination and stability or balance tasks (Myer et al., 2004). In sport these actions are common for example, single leg loading occurs during landing, sprinting, jumping and changing direction. Therefore, if strength and asymmetries differences occur then an athlete may be at a higher risk of injury (Hewett, Ford, Hoogenboom & Myer, 2010). Imbalances greater than 15% have been found as a key injury risk for athletes (Knapik, Bauman, Jones, Harris, & Vaughan, 1991). Results from Zebis et al. (2009) found ACL injuries to occur on their dominant leg during side cutting tasks. Similar findings from Brophy, Silvers, Gonzales, and Mandelbaum (2010) found injured male players to injury their more dominant kicking limb more during non-contact ACL injuries (74.07%). In contrast, female players only had 33.36% of non-contact injuries to their dominant leg.

In terms of muscle imbalances in youth athletes (U13s- U17s) differences have been found between limbs during squatting (Atkins, Bentley, Hurst, Sinclair, & Hesketh, 2015). Results found peak ground reaction forces (GRF) during the deep squat to be highest in the non-dominant leg. In addition, the largest differences between limbs were found in the U14s (13%) and U13s (11%) ages. This could be due to athletes going through PHV and experiencing body changes explaining performance differences (Lloyd et al., 2014; Philippaerts et al., 2006). Therefore, muscle imbalances are a risk for ACL injuries in youth athletes and ST needs to involve single leg training and coaches need to closely monitor asymmetries to avoid future injuries.

Reduced knee stability could involve a valgus alignment or medial collapse of the knee during FMS or in training and matches (Barber-Westin, Noyes, & Galloway, 2006; Schmitz, Shultz, & Nguyen, 2009). This mechanism has been found to increase ACL injuries in male athletes due to the high amounts of stress placed on the ACL (Hewett et al., 2005). Stress increases on the ACL may be due to the muscles not fully absorbing GRFs, which places higher forces on the ACL which may lead to injury (Myer et al., 2010). For male and female youth athlete’s knee valgus is common during FMS and this increases athlete’s risk of injury (Bergeron et al., 2015; Noyes et al., 2005; Schmitz et al., 2009). This may occur due to the athlete not fully learning the FMS or not being coached around the skill. The use of feedback around foot and trunk position during change of direction tasks has resulted in reduced valgus moments (Dempsey, Lloyd, Elliot, Steele & Munro, 2009). Peak valgus moments have also been shown to decrease through ST and balance training (Cochrane et al., 2010). Cochrane et al. (2010) results found machine-based resistance training to reduce peak valgus moments by 27% and balance training by 62%. Therefore, youth athletes may benefit from regular exposure and coaching around FMS, and ST (Holcomb et al., 2007; Myer et al., 2011). This could benefit youth athletes and the likelihood of ACL injuries throughout their sporting career. Therefore, these strategies might be used to increase knee stability.

Excessive trunk lean during FMS may occur due to a lack of strength from the core musculature or the inability to position the trunk in line with the athlete’s center of mass (COM). Therefore, preventing them to resist and transfer forces around an athlete’s COM (Hewett et al., 2005; Myer et al., 2011). An athlete’s growth and maturation can influence this as well (Myer et al., 2010). For example, an increase in lever lengths causes an increase in an athlete’s COM challenging trunk stability further (Myer, Chu, Brent, & Hewett, 2008). During maturation if an athlete is not strong enough to deal with an increase in body mass and size then may struggle to absorb forces and be stable which may result in trunk lean and increases in joint loading (Hewett & Johnson, 2010). Findings from Zazulak, Hewett, Reeves, Goldberg and Cholewicki (2007) found females with reduced core stability and core proprioception to predict knee injuries. Therefore, the core musculature should support the pelvis and the spine during stability and FMS tasks to reduce injury risk (Ireland, 2002). Exercises that can improve trunk stability may include training the pelvic and hip stabilisers and inner core musculature such as the transverse abdominus and multifidis (Myer et al., 2010).

In conclusion, the ACL is at increased injury risk due to the game demands of football (Agel, Arendt, & Bershadsky, 2005; Boden, Dean, & Feagin, 2000). Common mechanisms that lead to ACL injuries are reduced knee flexion angles, knee valgus displacement and increased hip internal rotation angles (Chappell, Creighton, Giuliani, Yu & Garrett, 2007; Malinzak, Colby, Kirkendall, Yu, Garrett, 2001; Ford, Myer & Hewett, 2003). Previous injuries and neuromuscular deficiencies can increase youth athlete’s risk of injury (Arnason et al., 2004; Myer, Brent, Ford & Hewett, 2011). Prevention strategies such as targeted ST, single leg training, the use of feedback and trunk and pelvic training may improve neuromuscular deficiencies and aid movement and injury prevention (Dempsey et al., 2009; Holcomb et al., 2007; Myer at al., 2010).

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