🌟Hamstring strain injuries in running based sports are said to be the most common cause for lost training and playing time (REF?). Approximately one in ﬁve professional soccer players will suffer a hamstring injury in any given season and up to 20% of these will re-occur (REF?). It costs an elite soccer club approximately €280,000 per injury with an average loss of 17 days from matches and training (REF?). Hamstring strengthening is an important factor of injury prevention practices. Prevention practices that include the Nordic hamstring exercise with compliance have shown greater results in injury prevention. An improved understanding of this empirical work and the importance of nordics may enable practitioners to make more informed decisions regarding exercise selection for the prevention or treatment of hamstring injury. Research papers that have focused on the testing and intervention of hamstring exercises for hamstring injuries have been depicted form the databases to analyze their methods effectiveness. To make the reader more informed areas of the hamstring anatomy, injuries and methods of nordic curls will be explained below.
🌟The Hamstrings are made up of 3 muscles, semimembranosus, semitendinosus and bicep femoris. The bicep femoris is separated further regarding the attachment site of the origin of the muscle which are the bicep femoris long head and bicep femoris short head (Linklater et al., 2010). The bicep femoris long head (BFLH), originates at the lower and inner impression on the posterior part of the tuberosity of the ischium which is a section of the pelvic girdle and inserts at the lateral aspect of the fibular head (Linklater et al., 2010)The bicep femoris short head (BFSH), originates at the linea aspera and the lateral supracondylar line of the femur and insertion is at the lateral collateral ligament and lateral tibial condyle (Linklater et al., 2010).Semimembranosus originates at the ischial tuberosity of coxa, inserts at the posterior medial condyle of tibia (Linklater et al., 2010). Semitendinosus origin site is at the ischial tuberosity of coxa and inserts at the proximal tibia, medial to tibial tuberosity (Linklater et al., 2010).
🌟From the twelve studies found there was a range of participant numbers from 16 in Sconce et al. 2015 to 283 in Seagrave et al. 2014. The study by Petersen et al (2011) has a mix of both amateur and pro athletes. Other studies such as that by Daneshjoo et al (2013), (Opar et al., 2015a, 2015b)Timmins et al (2016) and Coswig et al (2018),all involve athletes of an elite or professional level in their sport which is mainly focused on Soccer and Australian Rules Football (AFL). One study which investigated the validity of the nordic hamstring curl for a field-based assessment of eccentric hamstring strength by (Seagrave et al., 2014). Blackburn et al (2011) was the only study to involve both male and female participants with men being stronger and therefore being less likely to receive a hamstring injury.
Ages of most of the participants were not always mentioned, which may be a shortcoming. When ages were taken into consideration the average age was 24 years of age with older adults being 30 and the youngest being 18 years of age. One study seemed to be an outlier regarding age (Sanfilippo et al (2013). Sanfilippo et al (2013) had 25 subjects of both male and female whose ages ranged from 16 – 50 years of age. This study did not list how many were over the average of 24 years. Younger athletes are more susceptible to hamstring strains than older athletes but the reason for this is unknown (Seagrave et al., 2014).
Studies that looked at this topic from the view of comparing injury likelihood to strength, or injury rehabilitation and return to sport had mixed ways in which they selected the participants. Some had participants that only had minor hamstring strain that was less than 6 month old but other studies chose anyone that had an injury as long as it was only classified as a strain and that did not require surgery (Sanfilippo et al., 2013), (Seagrave et al., 2014). For future studies, it would be best to clearly use the same definition of what a hamstring injury is and use participants with both strains and serious injuries, such as post-surgery and compare them.
13 out of 13 studies made sure that the participants signed an informed consent form prior to taking part in any study.
🌟Methods of testing concentric hamstring strength were very similar in most cases apart from a few variables. For example, Askling et al, (2003), Sanfilippo et al (2013), (Worrell, 1994), Seagrave et al (2014) Timmins et al (2016a) and Opar et al (2015a) used an isokinetic test for the hamstrings but tested the hamstrings at different degrees of the range of motion (ROM). Sconce et al (2015) used the isokinetic machine to test concentric contraction of the hamstrings at the knee from 30, 60 and 90 degrees whereas a studies carried out by Sanfilippo et al (2013), used the same machine but tested maximum effort knee flexion/extension testing through full range of motion during two conditions: concentric at 60 degrees for 5 repetitions and concentric at 240 degrees with 15 repetitions, Eccentric knee flexion testing at 30 degrees. Peak torque measurements were very standardized throughout six of the studies. Peak torque measured at 60, 240 and 30 degrees in most studies (Askling, 2003) (Timmins et al., 2016a) (Opar et al., 2015b) (Sconce et al., 2015) (Seagrave et al., 2014).
🌟Studies in this area require protocols in place to test or asses the subjects to gather data on baseline scores that are then measured after an intervention, be it a rehabilitation program or a periodized strength program. The Nordic was a very popular and well mentioned exercise throughout the literature and for some protocols it was used as both the strength measurement and the exercise used in the intervention (Timmins et al., 2016a) (Opar et al., 2015b). In a paper published by Opar et al ( 2015b) on the effect of previous hamstring strain injuries on the change in eccentric hamstring strength, there was a protocol used that was very misleading at the start. Injured players and non-injured players were given the same intervention of using both the nordic curl and the Romanian Deadlift (RDL) for two to four repetitions for six to 10 sets. But reading further on, the previously injured athletes were prescribed extra eccentric exercises to further reduce the risk of injury, but these exercises were not laid out or described in the paper in any manner. This is a flaw of this study and shows that the research was not their priority and that their main goal was to have professional players available and did not want any reoccurring injuries. Their reasons for doing this would make sense if the groups that done more work (the injured group) would have improved at a faster rate than the uninjured group, but they did not. Not mentioning the extra exercises that were given to the injured group does not make this study transparent. The results from the study could have had a greater inequality of levels of improvements to no improvements if they all stuck to the one program.
A very interesting and innovative protocol to test hamstring isometric strength was used by Schache et al (2011). Strength was testing using a blood pressure cuff. The subject lay supine on the floor with a 90-degree bend at both knee and hip. The ankle of the tested leg was placed on top of an inflated cuff on a table and the participant was told to drive the heel into the cuff and a measurement was given. This way of measuring the strength of the hamstring weekly was later found to be beneficial in predicting hamstring injuries, but the study did not have a big numbers of participants so further studies in this area would help validate it. The protocol for measuring isometric strength was innovative and could possibly be a quick protocol to use in a team setting (Schache et al., 2011).
Daneshjoo et al (2013) studied the effectiveness of two separate warm-up protocols in pro soccer players. These protocols were named 11+ and Harnoknee. The two protocols have separate exercises but were both introduced to see the effect they have on hamstring strength to prevent injuries. The Harmoknee group used the concentric moment of the nordic curl and the eccentric moment of the nordic curl was used in the 11+ group. Bigger strength improvements were seen in the 11+ group and it was noted that the nordic curl used in the 11+ was superior to that done in the Harmoknee group. This is similar to studies by (Seagrave et al., 2014), (Opar et al., 2015a), (Timmins et al., 2016b), (Sconce et al., 2015)(Seagrave et al., 2014) (Alonso-Fernandez, Docampo-Blanco and Martinez-Fernandez, 2018) that showed that the eccentric portion of the hamstring can be well developed using a nordic curl.
🌟Hamstring injuries were only defined in three studies. Seagrave et al., (2014) defined that an injury was any injury to the hamstring that caused the player to be removed from the team line-up for at least one day. Worrell, (1994) detailed that a hamstring strain is a non-contact injury that presents in two forms: (1) sudden onset and pain (2) slow onset with muscle tightness. A strain was deﬁned as any acute hamstring pain that resulted in ceasing training and was later diagnosed by the club medical staff. The injury diagnosis also included the presence of pain during an isometric contraction and during any knee ﬂexor muscle length test (Timmins et al., 2016b). Timmins et al (2016a) and Blanco (2018) were the only ones who used ultra sound and MRI imaging to diagnose injuries. The main causes of hamstring injuries is a short fascicle length, higher angle of peak torque and eccentric weakness in the hamstring (Timmins et al., 2016b) (Sconce et al., 2015) (Aquino et al., 2010) (Seagrave et al., 2014)( Blanco et al, 2018). This would suggest that increasing fascicle length and strength would be a good protocol to add to training sessions (Daneshjoo et al., 2013), (Timmins et al., 2016a) (Blanco et al, 2018). Opar et al (2015b) and Blanco et al (2018) found that most hamstring injuries occurred in the BFLH during the terminal swing phase of high speed running. During this phase the hamstrings are eccentrically decelerating the knee and hip before they bear weight ( Blanco et al, 2018). Each hamstring injury can cost a pro football team up to €280,000 and the average days away as a result of an injury is 25 days (Daneshjoo et al., 2013).
🌟Compliance of the programs, assessors and participants was not always maintained. The studies by Seagrave et al (2014), Daneshjoo et al (2013) finished the intervention with less participants due to players being transferred away from the clubs that they were with prior to the study. This can be a downfall of including professional athletes during a transfer window in a study. Participants in the study by Opar et al (2015b) performed extra exercises that was not part of the research which can be a flaw in that study as they did not comply with the guidelines of the program. Programs that were to be followed outside the observation of assessors relied on the participants complying with the guidelines of the intervention either by themselves or a coach. Petersen et al (2011) prescribed the nordic curl for the intervention group after every training session. This could either be carried out under supervision by the soccer coach or by themselves. Those that complied with the ten-week study found a 60% reduction in recurring injuries at the end of the ten weeks. There were no reports of muscle soreness during this study when performing the nordic curl after week one (Petersen et al., 2011). It was found that nordic curls do not reguire a lot of volume for them to be beneficial (Daneshjoo et al., 2013) (Petersen et al., 2011)(Timmins et al., 2016a). In fact, Seagrave et al (2014) found that just an average of 3.5 repitions a week can lower injury risk by 60%.
🌟The nordic curl has been found to be beneficial for both a measurement of strength and an exercise to increase eccentric strength (Timmins et al., 2016a) (Daneshjoo et al., 2013)(Sconce et al., 2015)(Seagrave et al., 2014). It has been proven that the nordic hamstring curl lengthens fascicles and strengthens the hamstring (Opar et al., 2015a)(Timmins et al., 2016a). What has been made evident by Seymore et al (2017) and (Daneshjoo et al., 2013) is that nordics curls should be used when warm and as apart as the potentiation part of a warm -up and not something to do at the start of any training session (Blanco et al 2018). Best results have been found using the nordic curl (knee dominant exercise) along with a hip dominant exercise such as the RDL (Opar et al., 2015b). This works the hamstrings at both the origin and insertion point of the muscle (Seymore et al., 2017). Low volume has been found to be beneficial and easy to comply to for athletes that are returning to sport post injury or a preventative measure to prevent hamstring strains (Aquino et al., 2010). In studies bone by Sanfilippo et al (2013) inadequate strength was seen to be the biggest factor of hamstring strain injuries however, increase in strength and an increase in fascicle length has been found to be more beneficial in prevent new and recurring hamstring injuries
🌟Eccentric strength reduces hamstring injury risks by keeping it “Long and Strong”. Different exercises provide different hamstring muscle activation. Hypertrophy of the hamstring can be caused by acute activation. Knowledge of the different hip vs knee dominated exercise can provide more exercises for selection (Muscle and Motion 2020). The Nordic hamstring exercise has been seen to almost halve the rate of hamstring injuries and re-injuries and can reduce return to play time (Bourne et al. 2018). From this evidence of the benefits of strengthening and lengthening the hamstrings, it may enable coaches to make better informed decisions around exercise selection for the prevention and treatment of hamstring strain injury, especially that including Nordics. To make the found data undeviating, more studies should test using both acute and chronic hamstring injuries, done all year round and not only during game seasons and using a larger population mix.
🌟Other things to note:
- Hamstrings that are exposed to high speed running for field based sports also are at less of risk of injuries
- Other pieces of equipment such as GHD, Belt squat, reverse hyper and sled dragging are great for improving strength and volume in hamstrings but studies on these pieces equipment are few and far between so further research with them would be great. Also try them out for yourself and see your hamstring strength improve fast.
- Monitor your volume and see what sort of volume you can handle to allow for recovery.
- Never underestimate the importance of hamstring strength. Hamstrings are vital in all sports and aspects of strength so work them from knee and hip.
- CUT OUT THE LONG DRAGGED OUT RUNS THEY ARE NOT SPORT SPECIFIC DURING THE SEASON. If you are not exposed to high speed running and speed development, you are simply just leaving the door open to non contact injuries in important muscle groups like, the hamstrings. Hamstrings exposer to high speed running should be gradual to allow for adaptation. Also have the confidence to take rest days and have them incorporated into the periodized training schedule and follow it. Find someone that will monitor running distance and acute and chronic training load. They are great predictors of injuries. Don’t be stupid and think more is better, be smart and train optimally.
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