Wearing American Football helmets increases cervicocephalic kinaesthetic awareness in “elite” American Football players but not controls

The population size for this study was determined from previous studies of active cervical range of motion (ACROM and CKR effect sizes 0.3 to 1.2) [11]. Fifteen AF player volunteers (22.2 SD 1.9 years) were recruited from the British Collegiate American Football League. All players came from one of the national semi-finalist teams (Cardiff University Cobras, Southampton University Stags England), with nine volunteers having represented Great Britain at the collegiate level (equates to playing in the National Collegiate Athletic Association [NCAA] Division III within the USA). For inclusion in this study each player had to have had a minimum of 3 years playing experience of full contact, kitted AF. Further criteria included participants being currently asymptomatic for neck pain or discomfort. A screening questionnaire was completed by all recruited participants to determine presence of current or previous neck trauma, surgery or disorders that may exclude them from participating in the study or influence the results: e.g., dizziness, tinnitus, diabetes mellitus, asthma, hypertension, headaches/migraines.

Initially 15 age and size matched control volunteers were recruited, however only 11 of these fulfilled the inclusion criteria (n?=?11; 22.5 SD 3.6 years). Controls were trained athletes who participated in non-contact amateur competitive sports such as triathlon, swimming, water polo and basketball. All participants volunteered and gave written informed consent after receiving verbal and written information about the study, which was approved by the ethics committee of the School of Applied Sciences, University of Glamorgan, and follows the Helsinki Declaration ethical guidelines.

The method employed here is based on that described previously [11]. The protocol will be described in 3 sections: anthropometrics, assessment with a cervical range of motion (CROM) device, and helmeted assessment. The study presentation order regarding CROM or helmet measurements was randomised between participants to remove potential order effects.

Anthropometric measures: neck girth (Hoechstmass HM-82203 Rollfix Tape Measure; Cranlea, Bourneville UK), body mass (floor scales model 761, Seca GmBH, Germany) and height (stadiometer model 202, Seca GmBH, Germany) were recorded. Participants sat in a chair which was height adjustable, so that their hips, knees and ankle angles were all set to ~90° SD 5°, as assessed by a professional quality JAMAR E-Z Read goniometer (Physiomed, Manchester, UK). The chair was positioned so that the vertex of the participants’ head was 1 m (SD 5 %) from a custom made wall mounted chart (Fig. 1a) that was to receive light from a laser either mounted onto the CROM device (Performance Attainment Associates, Lindstrom MN, USA) or AF helmet with face guard (see below and Fig. 1b c for details). The participants were instructed not to arch their thoraco-lumbar spine during extension, to ensure only neck muscles were engaged in the flexion-extension movements. A biofeedback cuff (Stabilizer™ pressure bio-feedback, Chattanooga group, Encore Medical Texas, USA) was placed between the posterior upper thoracic spine region and the back of the chair and inflated to 40 mm Hg. Measurements were repeated if the pressure deviated by 2 mm Hg during head movements. Each participant was guided through a warm-up and familiarization session (the equivalent of three repeated trials) before assessment began.

To measure ACROM, a cotton bandana was tied around the head (above the brow anteriorly and tied posteriorly above the occiput), to cushion the body of a CROM device and ensure stability regardless of idiosyncrasy associated with head/skull morphology. The CROM was placed onto the head as described by the manufacturers: the magnetic yolk was not used in these experiments, as rotation was not measured in this study. CROM devices have been used extensively in this type of research and have been shown to have sufficient accuracy [17], validity [18, 19], and reliability [20] for studies of ACROM such as this. This CROM device had a custom made laser block (Perspex block containing a pencil laser (class 3a: 650 nm: miraclebeam™ Pacoima, CA) mounted (screwed) on the rotational arm slightly forward of the position which would overly the vertex (Fig. 1b). This was used for assessment of CKR using the CROM or adapted helmet.

The participants’ ACROM in full flexion and full extension was assessed as follows: the participant was asked to maximally flex their head forwards by tucking in their chin to their chest, or extend their head back while maintaining their shoulders and mid-to lower back in a normal upright position (including their normal curvature). There was a 2 s hold at the end of each movement to establish the end point reading (angle). After each head movement, the participants were asked to return to their neutral starting position (looking directly ahead).

To assess ACROM wearing the football helmet a standard mid-sized AF helmet and grill was adapted as follows (Fig. 1c): an attachment for the rotational arm of the CROM was custom made of aluminium and bolted onto anterior midline of the helmet; between the two upper anterior grill anchor points. A further custom-made aluminium block was bolted on the left lateral aspect above the grill attachment point (in line with the vertex in the coronal plane). This was used to affix a gravity goniometer that had been extracted from a spare CROM device. Using the same type and manufacture of goniometer allowed the modified helmet assessment to have comparable reading accuracy to the CROM.

Laser repositioning was used in the assessment of CKR. Participants were asked to close their eyes and find a comfortable neutral head position, at which point the laser was switched on. Once the laser light was visible on the wall mounted chart (Fig. 1a), the chart was moved so that the laser light impacted the centre of the chart (position 0, 0). Following chart alignment, participants were instructed to repeat the flexion and extension movements (returning to their perceived neutral position between movements) keeping their eyes closed. Repositioning was assessed by returning to perceived neutral from both full flexion and full extension, with the order of head movement alternated to reduce any order effect. The CKR was recorded using an adaptation of the procedure reported by Revel [21]. Once the participant affirmed they had returned to neutral, the actual position of the laser on the wall chart was noted (distance from the centre, direction in relation to undershoot or overshoot as well as lateral deviation).

Data was tested for skewness and kurtosis and deemed to be normally distributed for statistical analysis. A repeated measures ANOVA was used to identify main effects, post-hoc analysis using the Paired Student’s T-test for the helmet effect separately in each group (controls and AF). As direction of change was not immediately predictable, 2-tailed analysis was used. Probability values of 0.1 to 0.05 were considered as signifying strong trends and values 0.05 were deemed a significant change. Effect size (ES) was calculated using the method of Cohen’s D [22]. Statistical analysis was performed using SPSS 18.0 for Windows.