Hamstring strains are the most common injury that result in lost training and playing time in running-based sports (Opar et al., 2012). While the cause of hamstring strains is multifactorial, most injuries seem to occur during the late swing phase of high-speed running and nearly 80% affect the long head of the biceps femoris (Bourne et al., 2017). In professional soccer, about one in five players will sustain a hamstring in a given season and above 20% will reoccur, resulting in on average 17 days out of training and competition (Ekstrand et al., 2001).
Hamstring strengthening is an important component in injury prevention and has been extensively researched in recent years. Many interventions using the Nordic hamstring exercise and other long length exercises have proven to be more effective than conventional exercises in decreasing the amount of time missed due to injury (Askling et al., 2013).
Strength Training to Prevent Hamstring Injury
Strength training to prevent hamstring injuries is critical as it is assumed that stronger muscles are more resistant to strain injuries (Burkett, 1970). The largest isokinetic investigation of elite soccer players found that lower levels of eccentric knee flexor strength significantly elevated the risk of hamstring injury (van Dyk et al., 2016). Further, strength imbalances may contribute to hamstring injury, as professional soccer players with isokinetically measured strength imbalances or hamstring-to-quadriceps ratios of <0.66 had a significantly increased risk of injury (Croisier et al., 2008; Cameron et al., 2003).
There are interactions between eccentric knee flexor strength, age, and previous hamstring injury (Bourne et al., 2017). Higher levels of eccentric strength have shown to mitigate some of the risk of injury associated with being older or having a history of hamstring injury.
Does Strength Training Protect Against Injury and Reinjury?
Numerous prospective studies have shown strength training with an eccentric bias or at long muscle length reduces the risk of hamstring injury as long as compliance is high.
Among other studies, a 10-week Nordic hamstring program involving Danish soccer players found that the intervention group experienced 71% fewer first-time and 85% fewer recurrent injuries over the following season compared to the control group (Peterson et al., 2011).
Studies using strengthening exercises at long muscle lengths have also accelerated return to sport and reduced reinjury rates (Bourne et al., 2017). Protocols emphasizing long length hip extension movements have been found to produce faster return to play than protocols consisting of exercises performed at shorter hamstring lengths (contract-relax stretch, supine bridge, etc.) (Askling et al., 2013). Another rehabilitation study including 50 participants and emphasized eccentric exercises at long hamstring lengths found that all 42 athletes who were compliant and completed the study remained injury free for almost two years after returning to sport, while four athletes who were non-compliant were suffered a reinjury within the following year (Tyler et al., 2016).
Adaptations to Different Exercises
Different training interventions may activate the hamstrings differently based on different exercises that are used and may have important implications when aiming to prevent hamstring injury. Given the association with hamstring injury, it is important to consider factors such as biceps femoris long head fascicle length and eccentric knee flexor strength.
Professional soccer players with shorter biceps femoris long head fascicles were over 4 times more likely to sustain a future hamstring strain than those with longer fascicles (Timmins et al., 2015). It is hypothesized that shorter fascicles with fewer sarcomeres in series are more susceptible to damage during the active lengthening of the muscle. Fascicle lengthening is thought to be mediated by the addition of sarcomeres in series which helps prevent the over-lengthening of sarcomeres during eccentric exercise (Bourne et al., 2017). Further, biceps femoris long head fascicle length has been shown to increase with eccentric resistance training. Concentrically biased training has not been shown to produce similar lengthening (Timmins et al., 2015).
Although muscle volume does not seem to be a risk factor for hamstring strain injury, because muscle strength is directly correlated to anatomical cross-sectional area, it may be beneficial to include hypertrophy as a goal when improving hamstring strength (Blazevich et al., 2009).
Does Training Volume Matter?
Both muscle strength and architecture are the most likely contributors to an athletes’ susceptibility to sustaining a hamstring strain injury. Therefore, focusing on hamstring strength and fascicle length are the most likely modifiable factors in reducing injury risk. Eccentrically focused exercises have been shown to increase knee flexor strength and fascicle length and should be a focus when training to prevent hamstring injury. FIFA11+ suggests one set of 3-5 repetitions for beginners, 7-10 repetitions for intermediate, and 12-15 repetitions for advanced athletes (Severo-Silveira et al., 2018).
A study of elite young soccer players comparing the effect of low- vs. high-volume eccentric training on knee flexor strength and fascicle length found that an in-season 6-week low-volume (10 reps/week) eccentrically biased program of Nordic hamstring exercises and straight leg deadlifts produced similar results to those seen a group performing 4 times more repetitions (Lacome et al., 2019). The results of similar studies have also found that within-session volume might not be a key factor for eccentric training-induced adaptations. As the inclusion of high-volume eccentric training in-season is not ideal, these results demonstrate that even low volume eccentric training can be implemented to see adaptations in knee-extensor strength and fascicle length to protect against hamstring strain injuries.
While within-session volume may not be the most important factor in producing eccentric training-induced adaptations, it may be critical to progress the volume or intensity over the course of the training period. When compared to a constant training group, a group following a progressive training plan of Nordic hamstring exercises demonstrated greater changes on fascicle length (Severo-Silveira et al., 2018). While both protocols improved both concentric and eccentric hamstring strength and improved the hamstring to quadriceps ratio, the authors found that progression in intensity may be required to see more significant improvements in fascicle length. The exercise can be progressed by adding an external load (plate, medicine ball, etc.) or by increasing the lever arm by going though a greater range of motion in a controlled motion (the point of failure gets lower to the ground) which will increase the strength needed to execute the movement. Appropriately progressing the movement throughout the season will ensure that volume can be kept relatively low while still being beneficial in the prevention potential hamstring injury.
- Hamstring strains are the most common injury that results in lost training and playing time in running-based sports.
- Both muscle strength and architecture are the most likely contributors to an athletes’ susceptibility to sustaining a hamstring strain injury.
- Higher levels of eccentric strength were able to mitigate some of the risk of injury associated with being older or having a history of hamstring injury.
- Even low-volume (10 reps/week) of eccentrically-biased training can be implemented to see adaptations in knee-extensor strength and fascicle length to protect against hamstring strain injuries.
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