Biomechanical Factors Contributing to Hamstring Injuries in Football: Strength and Conditioning Coach’s Perspective On Preventing This Common Injury
Football is a dynamic sport that requires the athlete to perform many different movements and skills such as rapid acceleration and deceleration, quick change of direction, jumping, kicking and sliding (Ivan, 2012; Woods et al., 2004). Hamstring injuries involving strains that cause structural damage to the muscle fibers (Mueller-Wohlfahrt et al., 2013) are common in various sports including football and mostly occur in non contact actions during running and sprinting and have a high risk of re-injury (Schache, Wrigley, Baker, & Pandy, 2009; Ahmad et al., 2013; Petersen, Thorborg, Nielsen, & Hölmich, 2010). Hamstring injuries in elite English football players contributed to over 12% of all injuries and lead to substantial time off for the athletes (Woods et al., 2004). In English and Australian professional football leagues, clubs can have five to six injured players per club every season, therefore it is in the interest of both the clubs and athletes to find solutions to reduce and prevent this injury as well as optimise rehabilitation (Woods et al., 2004). Whilst there are different factors that can contribute to hamstring injury, the focus of this essay is on the biomechanical factors. Understanding the biomechanical conditions that result in hamstring injury is fundamental to prevention and can help optimise rehabilitation (Schache et al., 2009; Cross, Gurka, Saliba, Conaway, & Hertel, 2013). This essay will attempt to provide strength and conditioning (S&C) coaches with a better understanding of the injury, provide practical tools to prevent or decrease risk of injury and identify recommendations for the sport of football.
The hamstrings are a group of three bi-articular muscles located at the back of the thigh that play an important role in the movement of two joints, the extension of the hip and the flexion of the knee in the gait cycle (Ivan, 2012; Beltran, Ghazikhanian, Padron, & Beltran, 2012). The muscles include the lateral long and short heads of the bicep femoris and the medial semitendinosus and semimembranosus that originate from the ischial tuberosity (Ahmad et al., 2013; Ropiak & Bosco, 2012). This group of muscles works as an antagonist to the muscles of the quadriceps that extend the knee joint and flex the hip joint (Ivan, 2012). The hamstrings also contribute to posture stabilisation and the control of the pelvis region (Ivan, 2012). All these factors highlight the important role these muscles play in all major musculoskeletal movements and emphasise their importance in sports such as football (Woods et al., 2004).
Rapid muscle contraction generating explosive forces is important for success in many sports and the hamstring muscles have the ability to generate high forces rapidly through their eccentric and concentric contractions due to their high composition of type II muscle fibres (Ahmad et al., 2013; Ropiak & Bosco, 2012). A hamstring muscle’s length can change by up to one third as a result of eccentric or concentric contraction and is subject to high forces in closed and open kinetic chain activities (Ivan, 2012; Bennell et al., 1998). In football, hamstring injuries mainly occur whilst running or sprinting in the biceps femoris with the muscle-tendon junction being the most common injury site (Cross et al., 2013), therefore it is important to look at running and sprint mechanics (Petersen & Hölmich, 2005). Out of the three hamstring muscles, the bicep femoris has the greatest muscle tendon length and is stretched the most during sprinting, hence being the most frequently injured muscle (Cissik, 2012). Football also requires quick change of direction and speed (Woods et al., 2004). This rotational demand may also be a factor to the higher rate of bicep femoris injuries as they act as lateral rotators when the knee is semi flexed and the hip is extended (Woods et al., 2004). In the first half of the stance phase the hamstrings remain active through concentric contraction, resisting knee extension whilst extending the hip (Petersen & Hölmich, 2005; Ropiak & Bosco, 2012). The hamstrings act to decelerate knee extension distally whilst proximally assisting hip extension in the latter stage of the swing phase when running (Ahmad et al., 2013; Ropiak & Bosco, 2012). During the eccentric contraction of the hamstrings at the end of the swing phase, the muscles reach maximal length and it is suggested this is when strain injury is most likely to occur just before heel strike (Ahmad et al., 2013). Although at this point there is no contact with the ground by either limb, kinetic energy generated by the swing limb is still been absorbed with this energy increasing with running speed (Chumanov, Schache, Heiderscheit, & Thelen, 2012). This suggests that hamstring injuries in athletes are due to excessive strain in eccentric contractions and not to force (Liu, Garrett, Moorman, & Yu, 2012). This has been shown to be the cause of muscle damage in animal studies as well, where elongation speed and activation length were the cause of injury and of its severity, and not muscle force (Lieber & Friden, 1993). This also coincides with findings from Heiderscheit et al. (2005) in which the bicep femoris was found to reach peak muscle tendon length during the late swing phase and that this length surpasses peak length seen in the medial hamstrings and upright posture position. In relation to football, repetitive bouts of short and long sprints require a high range of limb motion and maximum forces due to quick acceleration of limb segments and angular velocities which may contribute to the high rate of hamstring injuries seen in the sport (Ropiak & Bosco, 2012).
Another important activity in football is kicking, which requires the lengthening of the hamstrings across both the knee and hip joints (Bennell et al., 1998). Hamstring injuries linked to kicking action have been reported to be more severe than those sustained from running and lead to longer recovery time (Brooks, Fuller, Kemp, & Reddin, 2006). When striking the ball during the acceleration phase, the forces driving the hip into flexion and the demands of eccentric braking action of the hamstrings are high, which may predispose the hamstrings to injury (Carlson, 2008). Bennell et al. (1998) suggest that a lack of hamstring muscle strength and endurance or imbalance between limbs can be a possible factor to injury during kicking, however Hoskins and Pollard (2005) dispute this as they found no significant difference in the rate of injury between the dominant kicking leg and the non dominant non-kicking leg. According to Small, McNaughton, Greig, Lohkamp, & Lovell (2009) another possible cause of injury is over developed psoas muscles usually associated with lumbar lordosis resulting from the kicking action required in football. Decreased hip flexor and quadriceps flexibility combined with fatigue can lead to an increase in anterior pelvic tilt (APT) angle. These conditions cause an increase in hamstrings relative length increasing the likelihood of injury (Hoskins & Pollard, 2005). Despite all the aforementioned studies, there is no general consensus on the exact cause of injury during kicking as variables such as the running speed prior to the kick, stride length and the way the ball is struck are difficult to assess (Lees, Asai, Andersen, Nunome, & Sterzing, 2010).
A number of risk factors have been identified to contribute to hamstring injuries that can also impact the mechanics of running and kicking (Liu et al., 2012). These risk factors have been described as modifiable factors such as muscle strength and imbalance, fatigue, flexibility, core stability and non-modifiable factors such as injury history (Liu et al., 2012; Hoskins & Pollard, 2005; Ahmad et al., 2013; Petersen & Hölmich, 2005). These different factors can contribute to the mechanical causes of the injury in different ways with the possibility of one or more factors leading to injury (Hoskins & Pollard, 2005). The S&C coach can address a number of the modifiable risk factors to prevent hamstring injury through a better understanding of how they affect the mechanics of the hamstrings. Linking the modifiable risk factors to how they affect the mechanics of the hamstrings through existing literature is important in order to set the ground for recommendations and procedures that S&C coaches can follow.
The causes of hamstring injuries have been attributed to a lack of strength in the hamstrings as well as an imbalance or lack of strength between the lower extremities in quadriceps and hamstring muscles and or between left and right hamstrings (Bennell et al., 1998; Ahmad et al., 2013). Woods et al. (2004) state that high angular velocities can cause hamstring injury in eccentrically weak hamstring muscles and lead to the recurrence of injury. Strong hamstring muscles are not only important to preventing hamstring injuries but can also help prevent anterior cruciate ligament (ACL) injuries (Ivan, 2012). The hamstrings provide rotatory and anterior stability preventing hyperextension of the knee that can result in excessive anterior tibial movement in relation to the femur and the tearing of the ligament as a result of overstretching (Ropiak & Bosco, 2012). A lack of strength in the hamstring muscles where they are incapable of withstanding forces during a sprint can result in the large forces loading on the ACL causing a possible injury (Petersen et al., 2010). According to Carlson (2008) extension through the knee causing the hamstrings to over stretch whilst its eccentrically contracting can be a result of a low hamstring to quadriceps strength ratio. This concurs with Beltran et al. (2012) in which they state that imbalance between hamstring and quadriceps muscles strength where the hamstrings are weaker can contribute to injury during sudden movements when the hamstring changes function from acting as a stabiliser to performing contraction.
As football requires repeated bouts of short and long sprints this can lead to hamstring fatigue and change in running mechanics (Hoskins & Pollard, 2005), which is a possible cause to why hamstrings injuries are mainly sustained at the end of each half during football matches (Small et al., 2009; Woods et al., 2004). Fatigue can disturb the mechanics of the hamstrings by reducing their force production during eccentric contractions leading to lower internal forces compared to external forces causing the hamstring muscles to contract whilst elongating (Hoskins & Pollard, 2005). This was also shown in marathon runners who suffered decreased eccentric strength after a race (Carlson, 2008). Fatigue can lead to a decrease in strength and flexibility causing the muscles to gradually and partially relax, increasing the risk of the hamstring muscles overstretching as their antagonist action diminishes against the force of knee extension during the swing phase (Ropiak & Bosco, 2012).
Various studies have looked at the role of hamstring flexibility in relation to hamstring injury but none have been able to establish a clear link (Hoskins & Pollard, 2005). While there is little evidence to support the use of stretching to prevent injury to the hamstring muscles it has been shown that stretching for rehabilitation can be useful (Carlson, 2008). Prospective studies looking at hamstring injury and knee flexor flexibility found no link, however studies looking at pre-season stretching data found association with injuries sustained in the season (Ahmad et al., 2013). Dadebo, White, & George (2004) conducted a study involving 30 English football clubs and found that stretching protocols help reduce the rate of hamstring strains in professional football players. This can be a result of increased muscle temperature allowing better force absorption by the muscle tendon (Ahmad et al., 2013).
Core muscle weakness also plays an important role in the mechanics of the hamstring muscles with research recently pointing to an association with hamstring injury (Ahmad et al., 2013). Increased APT due to weak core muscles can place the hamstrings in a mechanically inefficient position influencing hamstring stiffness and length (Carlson, 2008).
Football players and athletes with a history of hamstring injuries usually have hamstring muscle weakness that increases the risk of their re-injury due to scar tissue at the site of injury (O’Sullivan, O’Ceallaigh, O’Connell, & Shafat, 2008). Therefore when managing hamstring injury it is important to ensure that the athlete returns to action when they have fully recovered as early return can lead to further injuries (Thorborg, 2012). The ACL arc links the knee to the hamstring muscles (Hoskins & Pollard, 2005). It is suggested that the hamstring muscles activation during gait is regulated by proprioceptive feedback via afferent input from the muscles and ACL mechanoreceptors (Hoskins & Pollard, 2005). It is possible that previous injury and resulting scar tissue and weakened muscle can disrupt this process and lead to injury reoccurrence.
Having identified the modifiable and non-modifiable factors contributing to injury, it is important for the S&C coach to use this knowledge and apply it to the athlete program in order to reduce the risk of injury or optimise rehabilitation. Factors such as muscle strength and trunk stability that fall under the realm of the S&C coach can certainly be enhanced. Brukner, Nealon, Morgan, Burgess, & Dunn (2013) recommended a 7-point treatment system that involved a number of areas that an S&C coach can contribute in including eccentric strengthening exercises, improving resistance to fatigue, stretching and core stability.
Strength training programs have been shown to help reduce the risk of injury, decrease rate of re-injury as well as restoring pre-injury muscle strength (Carlson, 2008; Thorborg, 2012). Carlson (2008) recommends beginning rehabilitation training with gentle concentric exercises first followed by eccentric open chain exercises. Chumanov et al. (2012) recommends eccentric contraction exercises that involve active lengthening in order to prevent hamstring injury as they found them to be more effective then concentric based exercises due to their ability to replicate the late swing phase mechanical demands. Schache (2012) carried out an intervention study to investigate if eccentric strengthening exercises can prevent or reduce the rate of hamstring injuries in male football players and found a reduction in both new and recurrent hamstring injuries in the exercise group when compared to the control group. He concluded that hamstring exercises performed in training could reduce the rate of injury. Clark, Bryant, Culgan, & Hartley (2005) recommends using the Nordic hamstring exercises for neuromuscular adaptations to help prevent hamstring injuries. The use of Nordic curls in the prevention of hamstring injury is supported by various literature and has consistently shown positive results (Thorborg, 2012). Including three sets of eight reps during one to three sessions a week of the Nordic curl exercise in a strength program has been shown to reduce the rate of hamstring injury (Schache, 2012). Two programs are shown in Table 1 that have both been used to effective use. Cissik (2012) recommends the use of Olympic lifts and derivative exercises to focus on eccentrically strengthening the hamstring muscles. Exercises such as Romanian deadlifts and good mornings can be used with emphasis paid to the eccentric part of the lifts to increase eccentric hamstring strength. The author does not see the need to directly program for limb deficiencies as that should be part of a balanced S&C program and may be better reserved for advanced athletes.
It is suggested that interval anaerobic sprint training can be used to reduce hamstring injury rates as it can replicate match conditions and improve neuromuscular coordination reducing the risk of disruption to sprint mechanics during game play (Brooks et al., 2006 ; Elliott, Zarins, Powell, & Kenyon, 2011). Cissik (2012) recommends track work involving acceleration, speed endurance and maximum velocity to improve the hamstring muscles ability to resist fatigue. Football players can carry out specific football drills in combination with track drills such as straight leg bounds in order to enhance resistance to fatigue. For injured players, these exercise will help ensure that they return to match play in a fully recovered state allowing them to perform to their potential during game play. Various studies have linked fatigue with the muscles ability to stretch (Small et al., 2009). According to Carlson (2008) including four sets of stretching exercises post injury four times a day are more effective when compared to only four sets once a day. This is to help restore muscle flexibility, which decreases post injury. Dadebo et al. (2004) emphasises the use of static and proprioceptive neuromuscular facilitation (PNF) stretching with 15 to 30 seconds employed as stretching times as they found the number of times a stretch is done is not as effective if they are not held for long enough.
Carlson (2008) suggests that rehabilitation programs should not be limited to hamstring region alone as programs that include core stability perform better than isolation programs due to the mechanics and function of the hamstrings. Altered biomechanics due to increased APT for example suggests that non local hamstring factors are just as important to the treatment and rehabilitation of injury (Hoskins & Pollard, 2005). The author suggests that sports such as football require core strength to stabilise the pelvis as they involve a combination of open and closed kinetic chain exercises. From this essay author’s own experience having experienced two hamstring injuries of which one was serious, it is recommended that strength training be implemented early in the athlete football career and continues throughout. Having a good strength program from an early age that includes Olympic weightlifting may be beneficial in preventing this injury occurring in the long term.
In summary, hamstrings injuries are the most common injury in football players resulting in weeks off match play and have a high risk of injury reoccurrence therefore the S&C coaches should emphasise injury prevention in their programs. A better understanding of the biomechanics of the injury helps identify the factors that contribute to the mechanical failure of the hamstring and how they interact with one another which allows the S&C coach to devise the appropriate preventive or rehabilitation plans. Through adequate strengthening of the hamstring an S&C coach can help reduce the rate of injury and reoccurrence. It is suggested that an S&C coach should have a good understanding of hamstring injury and running mechanics and use this knowledge when devising training plans when working with football teams and previously injured athletes.
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