Expert Speaker Series: Anthony Shield on Hamstring Strain Injuries
Updated: Aug 23, 2021
The second post in this ‘Expert Speaker’ series is on the discussion with Dr Anthony Shield from the Queensland University of Technology and the QUT-ACU Hamstring Injury Group. Dr Shield discussed his current research interests, most notably investigating why there is such a high incidence of hamstring recurrence as well as developing their device, the Nordbord, to assess eccentric Hamstring Strength.
Hamstring Strain Injury (HSI) & Recurrence
Neuromuscular inhibition may be a key factor in the high incidence of hamstring injury recurrence. Dr Shield presented evidence (Opar et al, 2012) that showed muscle EMG activity is lowered in the Biceps Femoris on the previously injured side during eccentric contraction. Given that 80% of hamstring strains in the AFL occurred in the Biceps Femoris (specifically the Long Head) this has implications for the design of prevention and rehabilitation. Therefore in the High Performance environment it is important to diagnose the specific location of the muscle strain and structure the rehabilitation and exercise selection specifically to this.
Dr Shield discussed the concept of ‘damage resistance’; how progressive exposure to damaging eccentric conditioning may help to prevent microtrauma building to macrotrauma. While players may be strong, poorly managed training load can make players vulnerable to damage and cause microtrauma – therefore it is important to build eccentric work into the High Performance programme where possible. Relating this to a soccer season for instance, avoiding DOMS during the competition phase is generally essential, especially when you play two games a week. However, if an eccentric programme is introduced into the preseason phase when soreness is acceptable, players can build up damage resistance and this programme could be adapted and continued in season. From my own experiences I think this approach also helps with buy in and compliance from players who would accept soreness in preseason in the knowledge that it will make them feel better in season.
This is a piece of technology the research group have developed to provide objective data for the level of strength an athlete has when performing a Nordic exercise. Their research in AFL has found athletes who are weak at Nordics have a significantly higher risk of hamstring injury, specifically 3-4x greater risk below certain scores. This piece of technology has potentially massive applications in the High Performance setting as it can provide practitioners and athletes with specific targets and the objective assessment of individual progression for both prevention and rehabilitation. The ongoing research also suggests they will be able to establish consensus on thresholds for injury risk in specific elite athlete groups across different sports.
HSI Risk Factors
Understanding risk factors are important for the High Performance setting, both in terms of preventing injury and reinjury. A key point was that muscle strength, ‘damage resistance’ and fatigue all seem to be important modifiable risk factors, with a potential link between the level of fatigue and the extent of muscle inhibition in the Biceps Femoris, which the group are currently researching. This reinforces the important of a well organised High Performance programme, with strong systems in place to monitor and feedback on training load, strength and fatigue.
Prior HSI is the biggest predictor of future injury, which reinforces the focus in the High Performance setting to avoid the initial injury. Given these may cause recurrence via modifiable factors such as decreased strength and flexibility, these must become a focus in rehabilitation.
Age may be seen as an unmodifiable risk factor however, research suggests this can be counteracted via strength gains. I thought this was very interesting as so often we dismiss this as a risk factor out of our control, when clearly this interaction reinforces our responsibility in the High Performance environment to design individual programmes that directly reduce injury risk.
Prevention & Rehabilitation
Dr Shield emphasised three key points in terms of prevention strategies; to warm up well, to be strong eccentrically at long muscle lengths and (whilst it is important to consider the whole chain) do not move focus too far away from the specific site. It was interesting to hear about the functional v structural debate in terms of strength training – in Dr Shield’s opinion ‘to pursue the functional and abandon the structural is the biggest risk’.
Having worked in professional soccer environments that embrace strength training in completing contrasting ways (including none whatsoever) this is a particularly interesting area for me and one in which my own beliefs have changed over time. One of the considerations for such a High Performance environment is the culture and how structural work can be introduced and established when functional work is king.
Dr Shield discussed the importance of minimising pain and long muscle lengths in the early days post injury, but then the emphasis of progressively exposing the hamstring to longer eccentric contractions throughout rehabilitation. Whilst progressive eccentric conditioning provides the benefits of increasing activation, hypertrophy, increasing fascicle length, shifting the peak torque angle and increasing muscle damage resistance, if the Biceps Femoris is inhibited it will still not be exposed during eccentric contraction and may lead to recurrence when placed under true strain in competition. This then has implications for rehabilitation in the High Performance environment, which needs to be designed specifically to the individual muscle – this process may be supported by Carl Askling’s research assessing EMG activity across a variety of exercises.
One of the key take home messages of this interview was the importance of being able to critique; to be able to identify the limitations of academic research and the philosophies of the so-called ‘experts’. Despite being advocators of the Nordic exercise, Dr Shield was also open about the limitations of this exercise, notably that the traditional exercise does not reach longer muscle lengths. Other exercises such as dead lifts and stiff legs may well achieve longer lengths but are also limited for hamstring activation by the increased use of the glute muscles during the exercise. I think developing a critical eye is a very important skill for the High Performance environment, especially because there are so many messages, voices and technologies around. We must also consider the limitations for our specific environment which may differ from the Academic environment. As Dr Shield concluded ‘try to disprove everything and if you cannot, begrudgingly accept it… for now!’