There is no denying that the most common injuries in sport are knee related, particularly in those involved in high speed running such as field sports, those on the track, gymnastics and sports with high demand for change of direction such as tennis.
Exact statistics in the area are impossible to obtain as individuals competing at the top-level rarely take part in scientific studies, nor do they share every detail of their journey in the public domain for the risk of exposing compromising information that could remove their competitive edge. That being said, having worked with a large variety of individuals across various levels and sports, as well as networking with coaches that have done the same, it is clear that the knee itself can be a problematic area for everyone should it not be trained appropriately.
By everyone, I am referring to general population who attend the gym or run as little as once per week, right up to the elite who might be putting as much as 28 hours per week into their training.
I believe that it is now common knowledge that the strength of the hamstrings is of great functional importance in order to maintain the stability of the knee, particularly through the anterior plane. Whether it be during deep squats or higher velocity movements such as acceleration, deceleration and change of direction.
Static stabilisers play a role in the stability of the knee joint, which include non-contractile structures (i.e. ligaments) as well as dynamic stabilisers, which are contractile units (i.e. muscles and tendons).
In elite football (soccer) alone, current research highlights that 31% of all injuries are associated to the hamstring muscle group. That's an alarming figure.
Recently, Rhodes, et al (2020) attempted to compare eccentric hamstring strength and dynamic stability of the knee amongst elite soccer players to assess whether there was an association with hamstring weakness and the likelihood of knee injury.
Fifty-nine soccer players from two English premier clubs completed eccentric hamstring strength testing with the NordBord, (a device which I am very lucky to have used, which measures eccentric and isometric hamstring strength, see below).
Athletes performed 1 set x 3 maximal reps of the bilateral Nordic hamstring exercise. Players were instructed to gradually lean forward at the slowest possible speed while resisting the movement with both limbs, while the trunk and hips remained neutral and hands held across the chest. Each participant’s breakpoint angle was calculated utilising motion analysis.
Dynamic stability was assessed via the Y-Balance Test (YBT). Participants performed 3 reps and pushed the reach indicator block as far as possible with the contralateral limb in the anterior (ANT), posteromedial (PM) and posterolateral (PL) directions. Eccentric hamstring strength data along with YBT scores were collected and analysed.
Takeaways & Considerations
S&C coaches and physical therapists should certainly prescribe eccentric training interventions within individuals strength programmes due to the relationship between posteromedial and posertolateral directional stability at the knee joint, as shown in the results from the aforementioned study. Effective examples of this are the Nordic hamstring curl and Romanian deadlift), due to the positive relationship between posteromedial (back and in) and posterolateral (back and out) directional stability at the knee joint, as shown in the current study.
Above: The Romanian deadlift
In my opinion, the Romanian Deadlift should be loaded with a strength focus, in that repetitions of <8 are used with loads >80% 1RM approx 1-2 times per week alongside other lower limb strength movements. Whereas with Nordics, I believe less is more, whilst ensuring high frequency. So, as with this study, I am a fan of just 3 reps for around 2-4 sets performed at the end of strength sessions at least 4-6 times per week.
Surprisingly, this study demonstrated a lack of relationship between anterior stability and eccentric hamstring strength. As I said earlier, it was previously being thought that poor anterior stability was associated with poor hamstring strength. Whilst one study is not enough to eradicate this long term thought, it certainly makes it important for health and fitness professionals to understand that other risk factors, like previous injury and psychosocial aspects, also contribute to injury occurrence in addition to poor strength measures. As mentioned in a previous blog post, these factors can include:
psychological stress (external to the sport)
poor sleep hygiene
poor technique during high force movements such as deceleration, landing and change of direction
cyclic hormonal changes throughout menstrual cycle in women
poor relative lower limb strength
Incorporating eccentric hamstring strength exercises into an athlete’s programme may help reduce the risk of muscular injury, however, the effect on joint injury risk is not that conclusive. As is the case with this study, eccentric hamstring strength only addresses one area of dynamic stability for the knee joint.
Eccentric hamstring strength exercises such as the Nordic Hamstring Curl and the Romanian Deadlift are critical in helping reduce the risk of knee injury, however, performing these alone placed within a well structured programme will not meet the demands of increasing robustness in dynamic knee stability across all planes.
Surprisingly, the study mentioned within this blog suggests that there is no correlation between eccentric hamstring strength and anterior knee stability, instead, it appears to have a better relationship with posteromedial and posterolateral stability.
This study suggests a further need to investigate the relationship between eccentric hamstring strength and anterior tibial translation.
To be on the safe side, it is critical that S&C coaches adopt a well rounded multi-factorial approach in helping prevent knee injury by putting provisions in place to educate the athlete in other key areas, not just hamstring strength.
These other key areas could include:
Improving sleep quality
Adopting a sound individualised hydration and nutrition plan
Improving landing, deceleration and change of direction technique
Incorporating effective recovery strategies as part of the daily/weekly programme
Increasing relative lower limb strength (in addition to eccentric hamstring strength)
Helping the individual to learn how to manage stress and understand its impact on injury risk