Poor isometric neck extension strength as a risk factor for concussion in male professional Rugby Union players

Theo Farley; Ed Barry; Richard Sylvester; Akbar De Medici; Mathew G Wilson

doi:10.1136/bjsports-2021-104414

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Poor isometric neck extension strength as a risk factor for concussion in male professional Rugby Union players

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Data Capture
Don Gatherer
Published on: 23 March 2022
Method used in this paper for assessing neck strength- is it valid in rugby players?
Kerry Peek, Don Gatherer and Petrus du Plessis
Published on: 18 March 2022

Published on: 23 March 2022
Data Capture
- Don Gatherer, Chartered Physiotherapist None
I would like to commend the authors on highlighting the risk factors for concussion in Rugby Football Union. This type of research is essential for current and future guidance and therefore to be referenced it must be of the highest academic standard.
Don Gatherer and David Hamilton have published several papers on cervical assessment in rugby and have huge experience in the biomechanical function, action, rehabilitation, and measurement of the cervical spine especially at International Rugby Football Union level.
It is with regret that we are writing to express our great concerns regarding the recent study published in BJSM in particular the prudence of the Testing Protocols and how the findings may be misleading and the results mis-interpreted.
There are a number of methodological issues with this study which will have contributed to the misinterpretation of their results and subsequent conclusions.
The scientific methodology construct of isometric testing of the Head, Neck, and Upper Shoulder Girdle (HNS) must be based upon the correct application of the principles defined in Newton’s Laws of Motion.

The Aim of this study is to produce and measure a validated ISOMETRIC FORCE MAXIMA
To clarity, the test action can be precisely stated as ‘the measurement of a one isometric voluntary muscle contraction repetition maxima’ - 1IVMCmax

Principles and Forces related to this study.

INTRINSIC FORCE
• Head, Neck, and Upper S...
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I would like to commend the authors on highlighting the risk factors for concussion in Rugby Football Union. This type of research is essential for current and future guidance and therefore to be referenced it must be of the highest academic standard.
Don Gatherer and David Hamilton have published several papers on cervical assessment in rugby and have huge experience in the biomechanical function, action, rehabilitation, and measurement of the cervical spine especially at International Rugby Football Union level.
It is with regret that we are writing to express our great concerns regarding the recent study published in BJSM in particular the prudence of the Testing Protocols and how the findings may be misleading and the results mis-interpreted.
There are a number of methodological issues with this study which will have contributed to the misinterpretation of their results and subsequent conclusions.
The scientific methodology construct of isometric testing of the Head, Neck, and Upper Shoulder Girdle (HNS) must be based upon the correct application of the principles defined in Newton’s Laws of Motion.

The Aim of this study is to produce and measure a validated ISOMETRIC FORCE MAXIMA
To clarity, the test action can be precisely stated as ‘the measurement of a one isometric voluntary muscle contraction repetition maxima’ - 1IVMCmax

Principles and Forces related to this study.

INTRINSIC FORCE
• Head, Neck, and Upper Shoulder Girdle are held in the neutral test position by Intrinsic Forces
• Intrinsic Force is defined as the Biomechanical properties of the NMSK system of the HNS
• Intrinsic Force – In this experiment is UNKNOWN and is the force to be measured and defined.
• HNS held in test position isometrically in a defined position for each action
• Intrinsic Force MUST be LESS than the Extrinsic Force for test completion and validity.

EXTRINSIC FORCE
• Extrinsic Force is defined as an external force applied to the Head affecting the HNS.
• Extrinsic Force application is via auto means of manual hand compression through a load cell.
• Extrinsic Force – In this experiment IS KNOWN as the force Maxima must be defined by compression of the load cell in each test action.
• Extrinsic Force test position, once defined, will set the Test Maxima of that test.
• Extrinsic Force application is via single or double action arm actions and thus of different magnitude.
• Extrinsic Force application must be directed at right angles to the point of contact with a controlled rate of force development that allows maximal motor unit recruitment whilst avoiding jerking.
• Extrinsic Force MUST be GREATER than Intrinsic Force for test completion and validity.

APPLICATION
• If an Extrinsic force is applied, the HNS will remain in the neutral test position unless the Intrinsic Force is exceeded by the Extrinsic Force.
• An Extrinsic Force overload will result in Loss of the neutral test position, and this defines the test End Point.
• The End Point represents the HNS Isometric Force Maxima of the Intrinsic Force relative to that test set up and position which can be measured and recorded.
• If the HNS Isometric Force Maxima End Point is not achieved by an applied extrinsic force, the HNS will not move. The resultant Extrinsic force measurement therefore does NOT represent a Force Maxima but an Isometric Force Sub Maxima of the Intrinsic Force.
• Timing out a held Sub Maxima Force also does NOT define or represent a Force Maxima.

Chosen Methodology
• Auto Testing
• Compression load cell
• Timed End Point i.e. Load cell bleep when the input does not increase for 3 seconds.

The TEST must have three basic reproducible elements:
1. A defined Start Point
2. An appropriate application of an Extrinsic force Maxima
3. A defined End Point
Failure to adhere to any one of these basic elements will invalidate the test data

This paper fails on points 2 and 3.
It is important to note that the extrinsic force maxima of all test positions cannot be shown to be GREATER than the Intrinsic force maxima on any of the tests shown in this paper and therefore the input data is flawed and invalid.

Please try the Side Flexion test with your hand and see if you can exert sufficient force to overcome the cervical side flexors by moving the head out of the test position.
You simply cannot because our research shows that the extrinsic force maxima of this test set up is at least 25% lower than the intrinsic force maxima.
Therefore, all tests will register a sub maxima intrinsic score that is equates to the extrinsic force maxima.
Don Gatherer MCSP
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Conflict of Interest:
Founder Gatherer Systems Ltd
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Published on: 18 March 2022
Method used in this paper for assessing neck strength- is it valid in rugby players?
- Kerry Peek, Physiotherapist The University of Sydney
- Other Contributors:
  Don Gatherer, Physiotherapist
  
  Petrus du Plessis, Physiotherapist and Scrum Coach
We are writing to express our concerns regarding this recent study published in BJSM and how the findings may be misleading and the results misinterpreted.

Farley, T., Barry, E., Sylvester, R., De Medici, A., & Wilson, M. G. (2022). Poor isometric neck extension strength as a risk factor for concussion in male professional Rugby Union players. British Journal of Sports Medicine.

There are a number of methodological issues with this study which will have contributed to the interpretation of their results and subsequently how these might be used in practice. Most of these issues were not mentioned or addressed in the limitations of the study and should be highlighted as potential confounders:

Method for assessing neck strength- is it valid for use in rugby players?

Neck extensors

Although the authors use a method of assessing neck strength which has documented reliability in an earlier study,(1) this method has not been validated in rugby players (the published reliability was in healthy adults). The reason this method might not be the most appropriate method for assessing neck strength in rugby players is that these athletes have particularly strong necks, much stronger than the average population. The method used in the Farley et al. study requires the player to self-assess their own neck extensors with the player’s shoulders placed in an anatomically weak position where they may not be able to generate enough strength to counter >...
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We are writing to express our concerns regarding this recent study published in BJSM and how the findings may be misleading and the results misinterpreted.

Farley, T., Barry, E., Sylvester, R., De Medici, A., & Wilson, M. G. (2022). Poor isometric neck extension strength as a risk factor for concussion in male professional Rugby Union players. British Journal of Sports Medicine.

There are a number of methodological issues with this study which will have contributed to the interpretation of their results and subsequently how these might be used in practice. Most of these issues were not mentioned or addressed in the limitations of the study and should be highlighted as potential confounders:

Method for assessing neck strength- is it valid for use in rugby players?

Neck extensors

Although the authors use a method of assessing neck strength which has documented reliability in an earlier study,(1) this method has not been validated in rugby players (the published reliability was in healthy adults). The reason this method might not be the most appropriate method for assessing neck strength in rugby players is that these athletes have particularly strong necks, much stronger than the average population. The method used in the Farley et al. study requires the player to self-assess their own neck extensors with the player’s shoulders placed in an anatomically weak position where they may not be able to generate enough strength to counter >60kg of force to measure isometric neck extensors as reported in earlier studies.(2 3) It is appreciated that the authors did attempt to test shoulder strength, however, they did not replicate the position that the shoulders would be in when resisting the neck extensors (or in fact any of the neck muscle groups). The authors tested shoulder strength in horizontal adduction in front of their chest (figure 1A), whereas for the neck extensor test, the shoulders are in approximately 100 degrees of forward flexion (figure 1C). Therefore, it is disputed that all or some of the rugby player’s will have had the required shoulder strength to self-assess a maximal isometric contraction of their neck extensors (the strongest muscle group of the neck). The results might be more reflective of the player’s shoulder strength to resist the neck extensors in the documented position rather than the neck extensors themselves.

Neck flexors

In contrast, the mean neck flexor strength of 31.3-34.2 kg in study by Farley et al.(4) compares more favourably to earlier studies of between 25-36kg,(2 5 6) further raising the question whether the neck strength assessment method used in the study by Farley et al.(4) was valid for assessing neck extensor strength, with the true neck extensor score likely being under-estimated.(7)

Flexor/Extensor ratio

In the Farley et al. study the Flexor/Extensor ratio (F/E) is 0.75 (pre), 0.77 (mid) and 0.82 (post).(4) which is much higher than the F/E ratio of between 0.65 to 0.70 reported in an earlier systematic review.(3) A high F/E is strongly influenced by weak neck extensors (or a sub-maximal extensor strength score) and/or strong neck flexors.

Lateral flexors

In the Farley et al.(4) study the lateral flexors are lower than the mean flexor strength score by 20-25%. This is most unusual. Normative data from healthy adults has shown the lateral flexors to be similar or stronger than the flexors,(8) including the study of healthy adults by Versteegh et al.(1) on which the neck strength testing method is based. In rugby players, this strength profile is more pronounced, with every study (2 5 6 9-13) in a recent systematic review (3) of neck strength in rugby players which tested players in a seated position reporting the lateral flexors to be higher than the neck flexors which is particularly pronounced in forward players.(3) It is possible that a sub-maximal result for lateral flexors was also achieved in the Farley et al. study which might be due to the testing method requiring only one arm to self-resist the lateral flexors (left and right), whereas two arms are used to self-resist the extensors and flexors. Even if we accept that a make technique will lead to lower isometric neck strength values than a break technique, it does not explain why the lateral flexors are the weakest group by some margin in this study (in conflict with virtually every other study in rugby).

Testing sequence

It is possible that in the study by Farley et al. because every player was tested using the exact same sequence of neck strength testing, with the flexors tested before the extensors and lateral flexors, that this may also have contributed to the findings. Something which is also not discussed as a potential confounder.

Player demographics

Playing positions

The breakdown of the playing positions of the included participants was not reported.(4) Studies in rugby which have assessed neck strength by position (forwards and backs) have shown that forwards are considerably stronger than backs particularly in their neck extensors.(3) This is also not presented as a limitation and potential confounding factor.

Neck strengthening

It is unclear from the Farley et al. study whether any of the players already participated in neck strengthening exercises, or some other activity which may have contributed to the neck flexors reporting higher mean values than the lateral flexors as well as contributing to higher starting mean F/E ratio (Table 1)4 than the F/E ratio reported in earlier rugby studies,(3)Thus, potentially confounding the results.

Other methodological concerns

High dropout rate

Another issue is the high dropout rate. Only 33% of players completed post-season testing including almost all of the international players (likely the strongest players in the study). It was unclear how missing data were handled and how this may have influenced the findings. Also, the number of concussions reported is a bit confusing. From what we could tell, the data are presented for 30 concussions in 29 players, but there were 39 players noted as not attending one of the test sessions due to a concussion, cervical or shoulder injury across the 2 follow-up periods, it is unclear if that’s in addition to the 29 reported and how this influences the results.

Measure of Variance

In Tables 1 and 3 the strength means have very small standard deviations which seems to imply that either the players showed roughly the same assessed strength or there were issues with the testing method. In Table 3,4 the unadjusted Incidence Rate Ratio (IRR) for the extensors is 0.87 with 95% CI of 0.75-1.00 with the upper 95% CI of 1. These results are not dissimilar to IRR for the lateral flexors (except for having a non-significant p value). Given the concerns with the testing method, caution is advised in presenting the neck extensors as being the most important muscle group for reducing concussion risk.

Interpretation of the results

The concern is that if the testing method does not accurately reflect the true strength values of the neck extensors or the lateral flexors, the study’s main finding that poor neck extensor strength is a risk factor for concussion(4) is compromised and should be interpreted with extreme caution.

Based on their results, the authors of the Farley et al. study state in their discussion “that the extensor muscles may have a larger role to play than previously thought in attenuating forces of impact” and that it is possible that extensors provide a “defensive mechanism at reducing the forces of impacts during sagittal plane impact.”(4) However, the authors do not present any rationale on how the extensors alone would do this. It is certainly possible that there is a strength threshold for all neck muscle groups, however, strong extensors alone are unlikely to reduce the risk of injury without also having strong flexors and strong lateral flexors. Even if we assume that the method for testing neck strength achieved maximal isometric results for all directions of movement, the results of this study are almost certainly confounded by a high starting mean F/E ratio, which is not discussed as a limitation. The real concern is that readers will implement “a team wide neck extension program”(4) (as stated in their discussion) at the expense of neglecting to strengthen the neck flexors and lateral flexors. By selectively strengthening one muscle group, there is a potential to increase injury risk by adversely altering a normal neck strength profile.

In summary, we believe that there are important methodological issues within this paper which compromise the data and interpretation of the results.

We welcome clarification from the authors.

We would like to point out that at the IOC conference in Monaco in 2021, Kerry did voice her concerns with the lead author of this paper about their findings before this study was published, which is why we feel compelled to raise them again here as none of the concerns raised have been addressed in the published paper.

References:

1. Versteegh T, Beaudet D, Greenbaum M, et al. Evaluating the reliability of a novel neck-strength assessment protocol for healthy adults using self-generated resistance with a hand-held dynamometer. Physiotherapy Canada 2015;67(1):58-64.

2. Geary K, Green BS, Delahunt E. Effects of neck strength training on isometric neck strength in rugby union players. Clinical Journal of Sport Medicine 2014;24(6):502-08.

3. Chavarro-Nieto C, Beaven M, Gill N, et al. Neck strength in Rugby Union players: a systematic review of the literature. The Physician and Sportsmedicine 2021;49(4):392-409.

4. Farley T, Barry E, Sylvester R, et al. Poor isometric neck extension strength as a risk factor for concussion in male professional Rugby Union players. British journal of sports medicine 2022

5. Geary KBBa, Green BSPb, Delahunt EPBcd. Intrarater Reliability of Neck Strength Measurement of Rugby Union Players Using a Handheld Dynamometer. Journal of Manipulative & Physiological Therapeutics 2013;36(7):444-49.

6. Hamilton D, Gatherer D, Robson J, et al. Comparative cervical profiles of adult and under-18 front-row rugby players: implications for playing policy. BMJ open 2014;4(5):e004975.

7. Ashall A, Dobbin N, Thorpe C. The concurrent validity and intrarater reliability of a hand-held dynamometer for the assessment of neck strength in semi-professional rugby union players. Physical Therapy in Sport 2021;49:229-35.

8. Seng K-Y, Peter V-SL, Lam P-M. Neck muscle strength across the sagittal and coronal planes: an isometric study. Clinical Biomechanics 2002;17(7):545-47.

9. Maconi F, Venturelli M, Limonta E, et al. Effects of a 12-week neck muscles training on muscle function and perceived level of muscle soreness in amateur rugby players. Sport Sciences for Health 2016;12(3):443-52.

10. Naish R, Burnett A, Burrows S, et al. Can a Specific Neck Strengthening Program Decrease Cervical Spine Injuries in a Men's Professional Rugby Union Team? A Retrospective Analysis. Journal of Sports Science & Medicine 2013;12(3):542-50.

11. Olivier PE, Du Toit DE. Isokinetic neck strength profile of senior elite rugby union players. Journal of Science & Medicine in Sport 2008;11(2):96-105.

12. Hamilton DF, Gatherer D. Cervical isometric strength and range of motion of elite rugby union players: a cohort study. BMC Sports Science, Medicine and Rehabilitation 2014;6(1):32.

13. Barrett MD, McLoughlin TF, Gallagher KR, et al. Effectiveness of a tailored neck training program on neck strength, movement, and fatigue in under-19 male rugby players: a randomized controlled pilot study. Open access Journal of Sports Medicine 2015;6:137.
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Conflict of Interest:
Don Gatherer is the founder and director of Gatherer Systems.
Kerry Peek and Petrus Du Plessis declare that they have no competing interests.
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