Author: Suegnet Meyer
This literature review looks at Cervical Injuries that occur during participation in sport. It looks at injuries that happen in sport with frequent impact, e.g. Rugby and American Football, and also injuries due to continuous strain induced by high G-forces, e.g. Motorsport and Bob-Skeleton.
The aims of this literature review are to establish: 1) Which muscles stabilise the neck? 2) Which Cervical Strengthening (CS) exercises are most effective? 3) Whether cervical strengthening is effective for rehabilitation and injury prevention?
For this study, literature from 1990 to October 2013 found via the databases of Google Scholar, Pedro, Pubmed, SportsDiscus and Web of Knowledge, using keyword examples ‘cervical injury’, ’stinger’ ,’g-force’ and ‘neck strengthening’, was reviewed. Furthermore, mainly high quality original studies as well as studies identified from the reference lists were used. ‘Grey literature’ (e.g. reputable websites, online discussions and dissertations) were included.
Spine Injuries in Rugby and American Football
Serious cervical injuries were reduced, since rule modifications in high collision sport of Rugby and Football. Unfortunately minor traumatic cervical injuries still occur. Tackling is a major injury cause, especially during match-play and makes up 34-56% of all rugby injuries (Brooks et al., 2005; McIntosh et al., 2010). The American National Football League reported during 1997 to 2006, 300 players were injured with 350 games missed due to neural traction injuries (Qureshi & Hecht, 2010). Minor injuries, e.g. neural traction injuries, transient brachial plexopathy or ‘stingers’, occur frequently during tackling (Weinberg, 2003). The injury mechanism is uncertain. It is suggested that: a brachial plexus traction injury occurs when the neck is forced into lateral flexion together with shoulder depression (Clancy et al., 1977); or off-centre axial force causing cervical nerve root compression during cervical hyperextension and ipsilateral lateral flexion trauma (Watkins 1986, Levitz et al., 1997); or a direct supraclavicular blow resulting in a brachial plexus compression injury (Di Benedetto & Markey 1984). The frequent minor compression injuries during tackling cause cervical sprain injuries, inflammation and secondary degeneration (Meyer et al., 1994), which can result in cervical stenosis (Torg et al., 1986) and may cause altered cervical spine mechanics (Levitz et al., 1997).
Spine Injuries in Motorsport and Bob-Skeleton
Cervical spines of racing drivers and bob-skeleton athletes are affected by gravity force (g-force) related stressors causing axial force, vibration, mechanical shock, whiplash injuries and concussion (Mansfield & Marshall, 2001; Minoyama & Tsuchida, 2004; Engebretsen et al., 2010). These forces cause sheering, buckling and strain on the cervical region together with accidents that may have impact of more than 25G’s (http://www.bbc.co.uk/sport/0/formula1/24976578). A study of single seated car races found 1.2 injuries per 1000 competitors per race and 0.9 injuries per 1000 competitors per single saloon car race. Saloon drivers suffered 53.2% cervical sprain injuries (Minoyama & Tsuchida (2004).
The bob-skeleton is a small sled without brakes. The athlete drops onto the sled in prone and reaches speeds of 120-135km/hr while g-forces can exceed 5G’s at various angulations including labyrinth track or hairpin bends. Helmets are the only head protection against high g-forces and accidents (http://www.olympic.org/skeleton). During the Winter Olympics 2010, 6% of Bob-skeleton athletes suffered cervical injuries and 5% suffered concussion (Engebretsen et al., 2010). Cervical injuries occur in collision and g-force related sports resulting in injury.
Strength training is suggested as rehabilitation and injury prevention in these sports. However the effectiveness of cervical strengthening (CS) is questionable.
1. The Cervical stabilisers:
The deep cervical flexors (DCF) and extensors (DCE) create a stabilising sleeve around the spine and become dysfunctional during cervical disorders (Falla et al., 2004c; O’Leary et al., 2013). If a specific stabilising muscle group is targeted and trained accurately, an increase muscle activity would cause directional stability and reduce the spinal neutral zone (Panjabi, 1992a & Panjabi, 1992b).
The DCE: Semispinalis cervicis, Multifidus, Rotatores muscles and the cranio-cervical extensors function as the dorsal inter-segmental neck stabilisers, producing extension and located close to the vertebrae. Multifidus consist of 77% Type I and 23% Type II fibre composition with small moment arms to create only capacity of 1Nm but fibre type can be transient to type II (Uhlig et al., 1995; Boyed-Clarke et al., 2002; Anderson et al., 2005). Due to changes on Cross-sectional area (CSA), DCE muscle dysfunction is implied in whiplash and cervical pain (Elliott et al., 2006; Elliott et al., 2008). In addition, reduced muscle activation has been shown on muscle functional MRI (mfMRI) (O’Leary et al., 2012). Future research in dynamic situation is required to assess muscle activation in various dynamic exercise via mfMRI.
Longus Colli & Capitis (DCF)
The DCF is a spinal stabiliser controlling the cervical lordosis and intervertebral motion (Mayoux-Benhamou et al., 1994). A CSA of Longus Colli demonstrated 53% type I and 47% type II fibre composition indicating a phasic as well as a postural function (Boyed-Clarke et al., 2001). The DCF fibres attach to the cervical vertebral laminae and maintain the cervical posture against gravity and have an antagonistic relationship with Multifidus (Mayoux-Benhamou et al., 1994). The superficial flexors Anterior Scaleni (AS) & Sternocleidomastoid (SCM) are unable to control the segmental motion and result in uncontrolled motion if no DCF activation occurs (Falla et al., 2007). The AS & SCM EMG measurements will increase due to over activity while the DCF will have reduced muscle activation. Fatigue sets in if overtraining occurs at low loading and is more apparent at 25% of the maximal voluntary contraction (MVC). When planning a training programme excessive loading and fatigue must be avoided to avoid since it will result in deactivation of the DCF (Falla et al., 2004e).
Initially, the DCF was described as one functional unit (Falla et al., 2004a; Falla et al., 2004d). Recently, it was confirmed by mfMRI that the DCF is activated during cranio-cervical flexion (CCF). It showed that Longus Colli functions optimally to control the cervical lordosis during CCF while Longus Capitis controls CCF with cervical flexion (CF) (Cagnie et al., 2008).
2. The Best CS exercises.
Acute cervical pain
During acute cervical pain and dysfunction distorted neuromuscular function occurs between the superficial and DCF and needs to be relearnt by selective CCF training using a Pressure Biofeedback (Stabilizer Chattanooga South Pacific Australia) (Falla et al., 2003a; Falla et al., 2003b; Jull et al., 2008; Jull et al., 2009). Promoting good posture by lifting the sternum and nodding ‘yes’ will increase DCF activation, reduce cervical lordosis and improve the muscular force-length relationship (Falla et al., 2007). Others, however, promote good posture by advising to ‘stick the chest out and avoid shoulder protraction’ (Weinberg, 2003; Charbonneau et al., 2012).
Various cervical strengthening (CS) methods exist, e.g. manual isometrics, buddy- assisted- and swissball CS, wrestling forwards- and back-bridging (www.boksmart.co.za, Frounfelter, 2008), theraband (www.thera-bandacademy.com) and isokinetic training.
Cervical extensors are strengthened with higher loads with good results (Ylinen et al., 2003; Falla et al., 2013). An increased CSA and noted neuromuscular adaptations showed improvement of the Splenius Capitus, Semispinalis Capitis & Cervicis and Multifidus after dynamic high resistance exercise in healthy subjects (Conley et al., 1997a). Furthermore after 12 weeks of high load training, T2 MRI investigations showed hypertrophic changes of 25% for the DCE (Conley et al., 1997b). The optimum position where Semispinalis muscle works is at 15 degrees CF in extension (Elliott et al., 2010).
Using low cost Theraband for isometric, concentric and eccentric CS, which can be performed in any direction. Initially, starting with isometric training in the neutral position reduced risk of injury to the cervical structures exist (Qureshi & Hecht, 2010). However, research has shown that painful necks of females can be strengthened with a high load of 1x15reps of 80% of maximal isometric contraction 5x.week-1, and then reduced to 3x.week-1. Theraband has significantly strengthened isometric cervical flexion (+110%), Rotation (+76%), and extension (+69%) in combination with aerobic exercises (Ylinen et al., 2003). In the 2 year – follow-up muscle strength had improved by 44% flexion and rotation together with 27% in extension (Ylinen et al., 2006). Cervicogenic headache in females improved by using high loading strengthening and endurance Theraband exercises 5x.week-1 (Ylinen et al., 2010). Theraband can safely be used during initial rehabilitation to reduce pain and headache by doing relatively high isometric resistance 3-5x.week-1.
In a study of fighter pilots during flight, EMG muscle activation was recorded to determine how much training load is required to avoid injury at various Gz-levels. At low Gz-levels (+1Gz) training load should be performed at 15% of a maximal voluntary contraction (MVC). Training with Theraband is indicated for athletes who participate at very low G-forces or during initial rehabilitation (Netto et al., 2007).
It has been shown that Theraband causes lower EMG peak muscle activation in comparison to Cybex and provides less resistance during strengthening (Burnett et al., 2008).
Dynamic strengthening and progressive loading with isokinetics are used effectively in final rehabilitation or injury prevention in healthy subjects. It has been shown that training at 3x.week-1 for 12 weeks with progressive loading increases muscle strength. Male aviators followed a dynamic resistance programme for flexion and extension at 40% and 75% of 10RM, but were tested isometrically and dynamically. Isometric improvement shown after 4 weeks was 32.7% and at 8 weeks it was 32.7%. Extension improved in all the weeks significantly. Dynamic strength improved 39.2% after 8 weeks and 49.3% at 12 weeks (Taylor et al., 2006). Strength training duration should be 10-12 weeks for neural adaptation and hypertrophic changes to take place in the muscle (Taylor et al., 2006). It is evident that dynamic testing values are much higher which may question differences of assessment methods. Isokinetic strength training is measured with ease, however due to differing equipment used during studies and different populations it is challenging to make a reliable comparison between studies. Additionally isokinetic units are expensive and training is not performed in a sport specific position (Burnett et al., 2008).
Multi-Cervical Unit (MVU)
The MVU is used in the aviation industry. A study was performed using dynamic concentric and eccentric loading at 24-114% of MVC for 2x.week-1 for a duration of 10 weeks. It demonstrated MVC significantly improved isometric neck strength (64.4% Flexion; 62.9% Extension; and 62.9% Left lateral flexion) in comparison to Control. Theraband improved 42% for Flexion only (Burnett et. al, 2005).
Interestingly, good results have been shown in current rugby trends (iCSP-discussion), using a dynamic force equalising system (patent 2012-E80664(56) that permits active rotation and movement while functional training takes place to enhance the head being stationary and integrating reflex activity of the senses and vestibulum (www.gatheresystems.com, Peek & Gatherer, 2005; Brooks & Kemp, 2011). The system causes strengthening in an active movement and is controlled. It excludes harmful axial and shear forces during training in sport specific positioning. It can be used for rehabilitation and injury prevention. Further investigation is required.
3. Is strength training effective for rehabilitation and injury prevention?
Pain conditions: CS and reduced pain
Low load craniocervical flexion and higher load strength and endurance exercises in females (Randløv et al., 1998; Ylinen et al., 2003; Falla et al., 2007 & 2013) together with both genders (Highland et al., 1992; Jordan et al., 1998; Chiu et al., 2005) and longer term training (Ylinen et al., 2006; Ylinen et al., 2010) have been shown to reduce cervical pain and improve strength. In long term studies, patient compliance influenced results (Hakkinen et al., 2008). Pain can influence the strength training measurements. By measuring the effect on the sympathetic nervous system it has been shown that exercise can provide immediate pain relief response (O’Leary et al., 2007). CS in combination with other passive modalities can also reduce pain (Jordan et al., 1998; Bronfort et al., 2001). Furthermore, improvements observed in the control groups is explained by fear reduction, a biological variant or due to frequent testing that reduces pain (Ylinen et al., 2003; Chiu et al., 2005). This shows the importance of using randomised controlled studies.
Healthy subjects: CS improvements, gender and age
CS has improved in healthy subjects but usually at higher loading (Pollock et al., 1993; Conley 1997a & 1997b; Jordan et al,1999; Burnett et al., 2005; Mansell et al., 2005; Taylor et al., 2006). Some of these results would be influenced by including both genders in a study. Males have higher isometric neck strength compared to females during flexion (20%) and extension (25%) (Jordan et al., 1999). Females sustain higher acceleration forces post-strength training (Mansell et al., 2005), hence during whiplash injuries, acceleration speeds sustainable in different genders vary for males and females: 5g and 3.5g-force respectively (Vasavada et al., 2001). In addition females have smaller heads and reduced anterior-posterior dimension of C3-C7 vertebrae influencing the acceleration force and explains why women are more prone to whiplash (Jordan et al., 1999; Vasavada et al., 2008). Studies indicate that age or gender difference does not change the fibre composition (Boyed-Clarke et al., 2001). Nevertheless, due to these facts mixed gender studies should be assessed with caution.
Protective equipment and improved skill help avoid neck injuries. In a comparative neck strength study, no differences were found between asymptomatic aviators exposed to G-forces and non-aviators (Seng et al., 2003). It was suggested that it was due to the use of protective equipment. Technique modification is also suggested in rugby to prevent injuries (Brooks & Kemp, 2010). Training positions needs to be taken in account when strengthening and enhanced tackling skills according to player position will prevent injuries (McIntosh et al., 2010).
Correct Training mode
A certain training mode would produce specific results (O’Leary et al., 2012). A study that used slow isoinertial CS in football, resulted in minimal improvements with no EMG muscle activation changes whilst performing a tackle. In another study, involving footballers, an 8 week isotonic resistance programme was performed and head-neck dynamic stability was then tested by unanticipated weights that were dropped from a small height. Although the isometric strength did improve by 15%, no dynamic head-neck segment stability, muscle stiffness or EMG differences, were found (Mansell et al., 2005). Plyometric training was suggested. It is possible that insufficient loading was used in training and that exercises were not in a dynamic mode or perhaps an insufficient motor learning pattern (Lisman, 2009; Falla et al., 2013). Specific strength training requires a specific training mode to achieve the end goal.
It is suggested that during an anticipation action, muscle tone would increase however if the impact is unanticipated an injury would occur (Mihalik et al., 2011). During tackling, the axial loading causes loss of the cervical lordosis and subsequent buckling. Injuries occur in 2-31ms (measured at direct vertical loading while the player will be in an oblique position) while muscle reflex activating only happens at 60ms, however a certain amount of muscle stiffness would be present (Nightingale et al., 1996; Nightingale et al., 2000). Hence, if the player is able to anticipate the impact and by increasing the tone and stiffness, the impact may be reduced with lower risk of injury, indicating CS as a means of injury prevention.
Proprioception resistance training improves muscle strength. A recent study demonstrated that resistance together with proprioception improved cervical strength by 29.3% while the CSA increased by 18.9%-32.3% while the resistance only group improved by 8.9-9.9% (Kramer et al., 2013). Proprioception facilitates a neuromuscular response where the brain stimulates the cervical muscles causing hypertrophic changes in the proprioceptive group. The cervical spine needs to balance the head and stabilise the senses. There is an interaction between the vision stimuli and vestibular sense organ of the head together with the proprioceptors of the neck. In addition vision has a huge effect on the cervical spine muscle activation to perform at an optimum level (Keshner et al., 1995).
4. Further studies
Further high quality studies should aim to standardise muscle strengthening measurements in sporting populations. Further research should explore the integration of proprioceptive training in strengthening programmes to enhance neuromuscular response for improvement of rehabilitation and injury prevention.
Cervical strengthening is used in the rehabilitation of cervical injuries and injury prevention of collision and G-force related sports. Strength training that integrates correcting the neuromuscular performance of DCE and DCF and then progressing with loading has shown to be successful in rehabilitation of cervical injuries. The programme needs to take factors like fatigue and pain and gender into account to optimise results. CS for injuries has not been refuted but may not be a single modality to achieve optimum response. Other protection, for example safety equipment and skill may provide additional benefit to prevent injuries. Further studies needs to consider the implementation of more neuromuscular training in combination with strength training to deliver optimum results in rehabilitation and for injury prevention in sport.
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