There is overwhelming evidence that repetitive concussive and subconcussive impacts in contact sports such as rugby, American football and boxing are a real cause for concern. The acute and chronic health implications are now well documented and thus numerous risk management strategies have been proposed by those associated with contact sports (Benson et al., 2013; Tator, 2012). However due to the nature of these sports there are inherent limitations when trying to minimise risk. Therefore, preventative approaches such as enhancing athlete preparation is considered a primary pathway to increase safety (Gilchrist, Storr, Chapman, & Pelland, 2015). One approach that has been at the forefront of discussion is neck strength. Strengthening the neck musculature has been advocated as a means to reduce concussion incidences and is now considered an accessory component in many athletic preparation programmes across a variety of sports (Gilchrist et al., 2015; Hrysomallis, 2016). This critical review will look to explore the validity of this advocacy and highlight the need to adequately represent the demands of the neck musculature during impacts.
It is widely accepted the primary mechanism of a concussion, medically referred to as a mild traumatic brain injury (MTBI), is the accelerations imposed on the brain caused by external forces which result in deformation of brain tissue inside the skull (Meaney & Smith, 2011). The acceleration is dependent on the forcefulness of the impact and the direction of travel, typically defined as either translational or rotational. The greater the impact the greater accelerations imposed on the brain and thus the increased chance of brain trauma. Rotational acceleration is particularly concerning, causing shear induced tears often resulting in greater tissue damage within the brain (Gennarelli, Pintar, & Yoganandan, 2003). In sporting contexts, this is most notably witnessed in combat sports with strikes to the chin or side of the jaw causing the head to rotate (Cantu, 2006).Walilko (2005) found, using the Head Injury Criterion (HIC) scale, that boxers were more at risk of a MTBI from rotational accelerations whereas alternatively in American football, translational accelerations showed a higher correlation to MTBI. Instinctively, due to the importance of the neck musculature in supporting the weight of the head and controlling head position, developing the athletes neck strength utilising a variety of multi-planar exercises will help resist displacement of the head during impact. This would reduce the accelerations imposed on the brain and minimise the chances of sustaining a MTBI (Gilchrist et al., 2015; Hrysomallis, 2016).
However, whilst this is perhaps a theoretically intuitive hypothesis it is yet to be substantiated and evidence is largely inconclusive. Collins et al. (2014) found neck strength and size reduced MTBI incidences in high school sports and stated, “for every 1lb increase in neck strength the odds of concussion are decreased by 5%”. Conversely, Mihalik et al. (2011) and Schmidt et al. (2014) found peak isometric neck strength did not reduce MTBI incidences amongst ice hockey and American football players over a competitive season. Additionally, Lisman et al. (2010) found that although American football players increased peak maximal isometric neck strength, this did not reduce head and neck kinematics after impact. This seems to vary across different sports, with evidence suggesting increases in neck strength reduces head and neck kinematics in soccer players after performing a header (Peek, Elliott, & Orr, 2020). Furthermore, Schmidt et al. (2014) observed an increase in neck strength had actually increased the chances of MTBI incidences. This could be considered as an example of risk compensation (Gilchrist et al., 2015), a theory often commonly discussed with the use of helmets when cycling and headguards when sparring. That is, athletes enter into more reckless impactful collisions under the assumption they had increased their immunity to injury (Hagel & Meeuwisse, 2004). However, more research is necessary to determine if this phenomenon is consistent with neck strength training in contact sport. It can be argued this shouldn’t detract from potential benefits, and often safety precautions vastly outweigh any negative results from compensating behaviour (Hedlund, 2000; Thompson, 2001).
Due to the underlying mechanisms of MTBI and inconclusive prevention strategies, it is perhaps prudent to consider that not all strength training interventions are equally effective in building the specific qualities to reduce head and neck kinematics post impact. Additionally, and unfortunately, more uncertainty is realised when the literature finds it difficult to quantify what a weak and strong neck actually is. However, Newtons second law of motion can be a guiding principle for coaches. That is, in this instance bigger does mean better. Increasing the mass of the neck musculature and the supporting structures would help reduce displacement, or certainly mean larger forces would need to act on the head and neck to create displacement. A dedicated neck training programme with the purpose of hypertrophy would therefore be intuitive. This is not only the consideration; it is often said, producing as much force as possible in minimal time is the most important key performance indicator in elite sport performance. It would also appear the same rings true for resisting head impacts. The total contact duration from a punch has been measured at 13ms in Olympic amateur boxing (Walilko, 2005) and in college football head contact collisions were measured at 14ms (Rowson, Brolinson, Goforth, Dietter, & Duma, 2009). Additionally, Feng et al. (2010) demonstrated in vivo the displacement of the brain had terminated after numerous accelerations and decelerations at 56ms from a mild subconcussive linear head impact. This is of importance as Almosnino, Pelland and Stevenson (2010) found it took athletes up to 145ms to achieve 50% of maximal isometric neck strength in 5 different directions. There becomes an evident problem with this data and the demands placed on the neck musculature during contact. Athletes simply do not have sufficient time after impact to produce maximal force in order to overcome and resist displacement of the head. It is then postulated enhancing rate of force development (RFD) of the neck musculature to stabilise the head, in addition to developing maximal strength and size, is vital to reduce head accelerations and thus should be reflected in training programmes (Eckner, Oh, Joshi, Richardson, & Ashton-Miller, 2014; Gilchrist et al., 2015). Furthermore, this notion is strengthened by research that demonstrates athletes who anticipate impact by pre-tensing are able reduce head and neck kinematics (Eckner et al., 2015; Kumar, Narayan & Amell, 2000).
Exercise selection should then adequately represent these findings in training programmes. Perturbation training performed by drop loading, quick release, anticipated and unanticipated direct contact with the head resulting in a rapid isometric contraction of the neck musculature, is considered one optimal method to develop neck stiffness and reduce head and neck kinematics post impact (Eckner et al., 2015; Le Flao, Brughelli, Hume, & King, 2018; Schmidt et al., 2014; Simoneau, Denninger, & Hain, 2008). However, often neck training programmes are rudimentary in their design, missing this key piece to the puzzle and thus failing to adequately prepare the athlete for impacts. That is, neck strength training is often shielded from methods which are required to develop RFD and build sufficient strength (Cidzik, 2015; Maffiuletti et al., 2016). This may be due to the accessory nature of these exercises, lack of experience of the coach or athlete or perceived fragility of the cervical spine. Therefore, programmes are typically skewed towards simplistic dynamic exercises performed for high repetitions which don’t adequately create a large enough stimulus for adaptation and certainly don't meet the demands of contact sport. Table 1. demonstrates the common training interventions utilised.
In conclusion, although promising preliminary research exists in resisting post impact head and neck kinematics, more appropriate methods need to be utilised and tested in the literature for clear direction. With this stated, there is no reason, outside of individual risk factors, that neck training should not be included within contact sports physical preparation. It is important to remember utilising exercises which follow a logical progression with the goal of increasing exposure to higher intensity perturbations and isometrics is paramount. At best the athlete minimises their risk of MTBI, at worst the athlete develops a stronger neck and perhaps helps minimise other potential neck injuries from occurring.
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