Category: Articles

FMP Article: Rehab & Prevention of Hamstring Injuries

Prevention and rehab of hamstring injuries

Introduction

Hamstring injuries are a major problem in sports. There is a especially a high prevalence in sports that require a lot of sprints, jumps and kicks such as football, rugby, track and field and basketball. Hamstring injuries also have a high propensity for re-injury. One-third of the injuries will recur with the greatest risk during the initial two weeks following return to sport (Orchard et al. 2002). The risk for re-injury remains elevated for at least a year and the subsequent injury is often more severe than the original strain (Warren et al. 2010). In some cases a strained hamstring can cause long-term problems and have a severe impact on a player’s career. The very high risk to recur in the early phase after returning to sport suggests that most rehabilitation plans are inadequate. To design effective rehabilitation programs several questions need to be posed and answered. What are the risk factors that increase the hamstring injury rate and how the training or rehab plan can have an impact on these risk factors? A previous hamstring strain is probably the most important risk factor for future injury. Which muscle properties are altered following a hamstring strain and why do they put the athlete at a higher risk for recurrence when he returns to play? Which exercise selection and training parameters can help prevent injury or are important in the rehabilitation in order to avoid re-injury?

Risk factors

Strength imbalances, bilateral asymmetries and fatigue:

During sprinting and kicking the hamstrings have to brake the knee extension generated by the quadriceps muscles (Chumanov et al. 2012). Because the quads are stronger than the hamstrings, the hamstrings will become fatigued faster. Strength imbalances between both muscle groups will result in a faster decrease of the eccentric knee flexor torque and increase the risk for hamstring strain (Croisier et al. 2008).

Fatigue: Hamstring injuries are more likely to occur at the latter stages of a game (Woods et al. 2004, Ekstrand et al. 2011a, Greig et al. 2009). Eccentric hamstring strength decreases with playing time. The fatigue effect is also speed dependent. Faster running speeds result in a greater decrease of the peak eccentric hamstring torque (Greig et al. 2009). The decreased ability of the hamstring muscles to generate force reduces the energy absorption capacity and predisposes them to strain-type injuries (Garrett 1990).

Core instability: The force and stretch of the iliopsoas during the late stance phase and early swing phase induces an increased anterior tilt of the pelvis. This anterior pelvic tilt results in a greater hamstring stretch of the opposite limb, which is simultaneously in the late swing phase. Increased pelvic tilting when sprinting, due to core instability and compromised pelvic control, results in greater musculotendon stretch and strain of the hamstring muscles during the terminal swing phase (Chumanov et al. 2007).

Weak or inhibited gluteal muscles: Shirley Sahrmann says that anytime you see an injured muscle you need to look for a weak synergist. A synergist is a muscle that performs the same movement or function. Due to delayed gluteus maximus activity, the hamstring muscles become dominant during hip extension, which can cause hamstring strains (Sahrmann 2002).

The gluteus maximus and long head of the biceps femoris play an important role in stabilizing the pelvis. Pelvic instability can alter the muscle activation timing and the load transfer through the sacro-iliacal joint (Leinonen et al. 2000, Nelson-Wong et al. 2012, Janda 1985).

Function of the hamstring muscles during sprinting

The majority of hamstring injuries occur during the late swing phase of running and eighty percent of the hamstring strains affect the long head of the biceps femoris (Koulouris et al. 2003, Chumanov et al. 2007).

During this phase the knee extends while the hip is in flexion, lengthening the bi-articular hamstrings over both joints they cross. The hamstring muscles lengthen and contract eccentrically to brake the knee extension of the swing leg (Chumanov et al. 2012).

Just before foot-strike the hamstrings reach peak force and peak lengths (Simonsen et al. 1985, Thelen et al. 2005a, Thelen et al. 2005b, Wood 1987, Yamaguchi et al. 1989, Schache et al. 2009). At high speeds the EMG activity of the hamstring muscles during the terminal swing phase have been shown to exceed the activity of a maximal voluntary contraction (Kyrolainen et al. 1999).

Because of the differences in hip extension (origin on the pelvis) and knee flexion moment arms (insertion on the tibia), peak lengths are significantly larger in the long head of the biceps femoris, than the semitendinosus and semimembranosus (Thelen et al. 2005a). The greater incurred musculotendon stretch by the biceps femoris may contribute to its tendency to be more often injured than the other 2 hamstring muscles (Thelen et al. 2005a).

Peak lengths do not increase significantly with faster sprinting speeds, while hamstring muscle force and power steadily increase with speed (Chumanov et al. 2007, Chumanov et al. 2011, Thelen et al. 2005a, Schache et al. 2010).

Altered muscle properties and higher risk for recurrence

Previous hamstring injury has been associated with a shifted length-tension curve towards shorter muscle lengths and reduced eccentric hamstring strength towards full knee extension (Brockett et al. 2004). This indicates that after a hamstring strain the hamstring muscles can produce their greatest force at shorter muscle lengths compared to the pre-injury state. This also means that the end-range strength of the hamstrings is reduced. The presence of scar tissue at the site of the injury might be responsible for the shift of peak torque towards shorter muscle lengths (Kaariainen et al. 2000). Scar tissue is less compliant than contractile tissue and can therefore alter the mechanical properties of the muscle (Butler et al. 2004).

Because the peak force during sprinting occurs at longer muscle lengths, a shifted peak torque towards shorter lengths and reduced end-range strength place the muscle at a higher risk for re-injury (Brockett et al. 2004). This is probably a major cause of the very high recurrence rate during the first month after returning to play. Because the scar tissue is less compliant than muscle tissue, extensive scarring requires the muscle fibers in proximity of the scar tissue to lengthen a greater amount to reach the same overall muscle length (Butler et al. 2004). Because the muscle region near the scar tissue is subjected to higher strain, re-injuries mostly occur near the site of prior injury.

Another reason for the reduced end range eccentric hamstring strength is a decreased activation of the biceps femoris towards full knee extension (Sole et al. 2011). A lot of athletes return to sport with inhibition and selective atrophy of the long head of the biceps (Silder et al. 2008, Croisier et al. 2002). the strength abnormalities and scar tissue remodeling and hence an elevated risk of re-injury can persist a lot longer than 6 months after the initial muscle strain (Silder et al. 2008, Croisier et al. 2002). This emphasizes the importance of functional loading and progressive rehabilitation programs.

Eccentric hamstring strengthening

Strengthening the hamstring muscles eccentrically in an elongated range of motion should therefore form an important part of rehab or training (Brockett et al. 2004, Arnason et al. 2008, Askling et al. 2003, Gabbe et al. 2006, Petersen et al. 2011). Eccentric training has been shown to shift the force-length curve to longer muscle lengths (Schmitt et al. 2012, Brockett et al. 2001, Brughelli et al. 2010, Brughelli et al. 2009, Kilgallon et al. 2007). Eccentric training has been shown to shift the force-length curve to longer muscle lengths, so that the optimal muscle length gradually shifts to the zone in which the hamstrings are primarily operating. (Schmitt et al. 2012, Brockett et al. 2001, Brughelli et al. 2010, Brughelli et al. 2009, Kilgallon et al. 2007). Not only eccentric training, but also regular strength training using exercises that are more challenging at lengthened ranges of motion can shift peak torque towards greater muscle lengths (Goldspink et al. 1999, Seynnes et al. 2007).

An eccentric training program has been shown to substantially reduce the incidence of new (60%) and recurrent (85%) hamstring injuries of soccer players (Thorborg 2012). After only 10 days of eccentric hamstring training a shift of the peak torque towards greater muscle lengths has been detected (Brockett et al. 2004, Brockett et al. 2001, Brughelli et al. 2010, Brughelli et al. 2009, Seynnes et al. 2007). Static flexibility programs have been shown unable to influence the length-tension relationship and are therefore ineffective to prevent hamstring strains (Arnason et al. 2008).

Re-activating the long head of the biceps femoris

Rehabilitation programs also need to focus on the re-activation of the long head of the biceps femoris muscle to counter the inhibition and atrophy associated with hamstring injury. The long head of the biceps femoris is a thick muscle with a large cross-sectional area and short, pennate fibers, especially suited for high force contractions over a shorter distance (Lieber et al. 1993, Kellis et al. 2012, Makihara et al. 2006). During the stance phase of running the hamstring muscles have to contract forcefully while there is less change in muscle length because of the simultaneous hip and knee extension. This is in accordance with research that revealed the forward lunge, which involves simultaneous knee and hip extension, especially loads the long head of the biceps femoris (Mendiguchia et al. 2013). Exercises which leg action is similar to the stance phase of running, like a resisted slide-board back lunge, a step-up or a walking lunge can counter the inhibition and atrophy associated with hamstring injury. It is therefore recommended to integrate hip-dominant exercises, such as the lunge, in which the length of the hamstrings remains more or less constant into every athlete’s rehabilitation and training schedule.

Horizontal force production

Acceleration speed is an important characteristic of performance in team sports. Recent literature shows that the ability to generate large amounts of horizontal force is more important for acceleration than vertical force production (Rabita 2015).On return to sport after hamstring injury, athletes are slower and demonstrate substantially lower horizontal force and power outputs during sprinting (Mendiguchia et al. 2014). This inability to produce a high level of horizontal force is probably related to the inhibition of the long head of the biceps femoris and stresses the importance of hamstring exercises that mimic the muscle actions during a sprint. Clear decreases in horizontal force production capacity during sprint acceleration have been demonstrated after hamstring injuries in football players (Roksund 2017). Research also indicates that a reduced ability to produce horizontal force during the acceleration phase of sprinting is indicative of an increased risk of hamstring injuries (Edouard 2021). Athletes should integrate exercises that train horizontal force production from both a performance and injury prevention point of view.However, many traditional lower body strength exercises, such as squats, Olympic lifts, and deadlifts, primarily train vertical force production.

Re-activating the gluteus maximus and enhancing intermuscular coordination

The gluteus maximus is a very powerful hip extensor and also plays an important role in the stabilization of the lumbo-pelvic region. Pelvic instability, back pain or other lower body injuries can alter the muscle activation timing (Leinonen et al. 2000, Nelson-Wong et al. 2012, Janda 1985). The hamstring muscles then become dominant during hip extension as a result of gluteal inhibition or weakness (Sahrmann 2002). Hip extension is initiated by the hamstrings and erector spinae while the activation of the gluteus maximus is delayed (Leinonen et al. 2000, Nelson-Wong et al. 2012, Janda 1985). The gluteus maximus should be the primary hip extensor. Diminished gluteal function will place a higher load on the hamstrings and increases the risk of hamstring injury. Training the hamstrings in isolation only increases the load on the (in many cases already tired) hamstrings without promoting the correct coordination patterns between glutes and hamstrings. Rehabilitation programs for hamstring injury should focus on restoring proper coordination patterns, consist of exercises that (re-)activate the glutes and enhance the intermuscular coordination between the glutes and hamstrings. An example of such an exercise is the Resisted back lunge (fig. 9) against resistance. The gluteus maximus is especially active during activities that involve a vigorous hip extension such as sprinting or climbing stairs (Zimmermann et al. 1994). Stabilizing the trunk and pelvis against gravity also requires a strong glute contraction (Marzke et al. 1988). The pull of the cable during the Resisted back lunge (fig. 9) creates a hip flexion force against which the gluteus maximus has to stabilize. The movement also mimics the hip action during running. As in sprinting, the body should be pulled over the foot through a powerful hip extension. The one-legged stance also enhances the activation of the gluteus medius and maximus.  Explosive posterior pelvic tilt The pelvis is an essential segment in the proximal-to-distal sequencing of explosive movements (Shan 2005). An explosive backward tilt of the pelvis allows greater force production at the hip level and facilitates an efficient power transfer during sprinting (Sado 2019). The inability to maintain a stable posterior pelvic tilt during sprinting causes premature hamstring fatigue, increases injury susceptibility, and impairs sprint biomechanics (Small 2009).The differences in efficiency between a neutral and anterior pelvic tilt are related to the technique. An anterior pelvic tilt during sprinting results in too high a heel lift, a foot contact too far in front of the body’s center of gravity at the end of the swing phase, and longer contact times. This is also referred to as back side running mechanics. When the pelvis is in a more neutral position, the high knee action results in more active ground contact, closer to the body’s center of gravity, resulting in shorter contact times and higher ground reaction forces; the so-called front side running mechanics.  The final stage of hamstring rehabilitation  The last phase of hamstring rehabilitation determines the success and the chance of recurrence. Plyometric and ballistic exercises with a horizontal force vector, resisted sprinting and stair sprints are essential to make the transition to full sprint speed.Resisted sprinting and horizontal-vector plyometrics, such as the alternate leg bound and speed hop, approximate the joint angular velocity of sprinting, but with less hamstring extension (Osterwald 2021). These movements train correct intersegmental control, proximal-to-distal sequencing of rapid movements, and intermuscular coordination. The biceps femoris consists of a higher percentage of fast fibers that are preferentially recruited during explosive movements (Evangelidis 2017). Plyometric and ballistic exercises with a horizontal force vector, resisted sprinting and stair sprints maximally activate the biceps femoris and develop horizontal force and sprint speed. These exercises, together with a progressive partial integration in sports training, will facilitate a successful return-to-play.  Selection of most efficient hamstring exercises from a biomechanical perspective The single-leg RDL (fig.1), Roman chair hamstring curl (fig.2) and the Nordics (fig.3) are the best exercises to improve eccentric hamstring strength at an elongated ROM (McAllister 2014). These exercises change the optimal length of the hamstrings so that they can produce greater forces with longer muscle lengths (Opar et al. 2012). This is important because the hamstrings function at greater muscle lengths during sprinting (Chumanov et al. 2007).

Figuur 1: Single-leg RDL

Figuur 2: Roman chair hamstring curl

Figuur 3: Nordics

The single-leg RDL & high pull (fig. 4), the Roman chair hamstring curl (fig. 2), the Keiser acceleration (fig. 5) and Horizontal step-up (fig. 6) require an explosive posterior pelvic tilt or the ability to maintain a stable posterior pelvic tilt. An explosive posterior tilt allows large joint forces at the hip and facilitates an efficient power transfer during sprinting (Sado 2019).

Figuur 4: Single-leg RDL & high pull

Figuur 5: Keiser acceleration

Figuur 6: Horizontal step-up

The Horizontal step-up (fig.6) and Keiser acceleration (fig.5) will help improve horizontal force production. An improved ability to produce horizontal force increases acceleration speed and reduces the risk of hamstring injuries (Rabita et al. 2015, Roksund et al. 2017). The eccentric RDL slam (fig.7) and hip extension plyos (fig.8) are plyometric exercises specifically targeting the hamstrings. A hamstring injury is a high speed injury. The hamstrings function at high contraction rates during sprinting (Chumanov et al. 2007). Plyometric hamstring exercises improve the energy-absorbing capacity of the hamstrings at the speed-end of the spectrum (Swinnen 2016).

Figuur 7: Eccentric RDL slam

Figuur 8: Hip extension plyos

The Resisted slideboard back lunge (fig.9) is a very effective exercise to strengthen the hamstrings and gluteal muscles. This exercise improves horizontal force production and inter-muscular coordination between the glutes and hamstrings.

Figuur 9: Resisted slideboard back lunge

Conclusion

In summary, hamstring injuries are a significant concern in sports, and their high recurrence rate necessitates comprehensive prevention and rehabilitation strategies. Addressing risk factors such as strength imbalances, fatigue, core instability, and inhibited gluteal muscles is essential. Understanding the biomechanics of hamstring function during sprinting and the impact of altered muscle properties post-injury is crucial for effective rehabilitation. Incorporating eccentric hamstring strengthening, re-activating the biceps femoris and gluteus maximus, enhancing intermuscular coordination, and focusing on horizontal force production are key components of successful rehabilitation. The final stage involves plyometric and ballistic exercises to transition athletes back to full sprint speed, ultimately promoting a successful return to play. By addressing these factors, athletes can reduce the risk of hamstring injuries, enhance their overall performance, and extend their careers in sports.

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