Introduction
Concussions are a major concern in contact sports, and research is currently underway to explore preventative measures to reduce the risk of exposure (2,10,13,18 9 5,8,15,24,29 19
Alsalaheen et al. (1 7,8,15,29 8 15 5 11 plyometric -type training programs focused more on optimizing neck muscle reactivity. They theorized that the ability to rapidly dissipate anticipated or unanticipated impact energy could be improved through more efficient and shorter latency excitation-contraction coupling. In other words, the shorter amount of time it takes the neck to respond to a blow, the more efficiently it can absorb that force. The analogy of viewing the neck muscles as damping springs appears appropriate here, where more rapid recruitment of motor units of the neck muscle complex (including trapezius, sternocleidomastoid, scalenes, splenius capitis, and the deep segmental muscles) should, in theory, be more resilient to sudden load, reducing the energy transfer to the head and the brain. A focus on dynamic plyometric training of the neck muscles is also supported by Hrysomallis (14 Plyometric training has shown some potential for injury prevention in other body regions (23
Toward this, a new neck training protocol focused on dynamic, multiplanar plyometric training has been designed. Akin to a “hula hoop” for the neck, the protocol leverages self-generated centripetal force using the neck to keep a weighted arm rotating about the top of a custom-fitted helmet. In theory, the use of self-generated force should present less risk of overexertion and injury than that created by an external machine, although several questions need to be answered before a fully powered clinical trial can be appropriately undertaken. These include recruitment and retention of participants, estimates of effect size following the training, and collection of any adverse events. As such, the purpose of this pilot study was to explore the feasibility of a planned future trial to explore the effectiveness of training on the novel neuromuscular strengthening device shown in Figure 1 . This pilot study involved a 7-week training protocol in a sample drawn from a highly trained athletic population of university football players.
Figure 1.: Neuromuscular training device. The small weighted arm swings freely about the centrally mounted axis such that the athlete wearing the helmet and using coordinated head movements can get the weighted arm spinning. The faster the individual can get the weighted arm spinning, the greater the centripetal force generated and the stronger and faster the neck muscles need to respond and contract to maintain the weight spinning.
Methods
Experimental Approach to the Problem
This pilot study was designed as a quasiexperimental clinical trial with an intervention group and a control group, with a planned subanalysis of the change in performance metrics on the neuromuscular training device in the intervention group. All consenting participants continued to engage in team-mandated off-season training sessions. The intervention group also participated in a 7-week neuromuscular neck-training program (exposure variable) involving 2 sessions per week of approximately 10 minutes each. The outcome variables in the study were change in multiplanar isometric neck strength and performance on the neuromuscular training device after training. Strength changes in the transverse plane (axial rotation ) were of particular interest as strengthening in this direction is clinically challenging.
Subjects
Participants for this pilot study were selected from a list of current players who had participated in spring training and performed baseline strength testing. There were 38 eligible players for this study in total. For feasibility, selection was based on players who were locally available to train during the 7 weeks leading up to the start of the fall football season. The control group was selected to match the intervention group on height (threshold ± 5 cm), body mass (±8 kg), age (±2 years), and neck girth (±3 cm) (Table 1 ). All subjects were between 18 and 23 years of age. Subjects were excluded if, at the time of recruitment, team medical staff indicated that there were any concussion symptoms or musculoskeletal issues that prevented them from participation in their team prescribed preseason training. Figure 2 presents a flow diagram of the participants through each stage of the study. Formal written informed consent was obtained from all subjects before participation in the study. The Western University Human Research Ethics Board approved the project before recruitment.
Table 1.: Subject demographics (mean ± SD ).
Figure 2.: Flow diagram of subject recruitment and training.
Procedures
Self-reported age, height, and body mass were collected from each player at the start of the study. Neck girth was measured using a flexible measuring tape at the level just below the thyroid cartilage. Isometric neck strength (recorded in newtons) was measured using a handheld dynamometer according to a previously described assessment protocol (28 axial rotation . This protocol has shown good to excellent test-retest reliability (intersession intraclass correlation coefficient range from 0.87 to 0.95 and SEM range from 12.7 to 20 N for all tested directions). The strength values for the 3 cardinal planes of motion were used for analysis (sagittal plane: flexion/extension, frontal plane: right and left side flexion, and transverse plane: right and left axial rotation ). The average between the right and left sides for side-flexion and rotation was used for their respective single plane isometric strength analyses. A single metric termed “composite neck strength” was also calculated as the arithmetic mean of all 6 movements. This allowed easier comparison of findings against those reported by Collins et al. (5
Both the intervention and control groups participated in their team prescribed off-season training program that included 2 neck-training days per week. One day would involve training on a 4-way uniplanar (flexion/extension and right/left side-flexion) isotonic neck-strengthening machine, 2 sets of 8–12 repetitions in each direction. The second day involved “manual neck” strengthening with a workout partner, wherein the partner applied manual resistance to neck movement in each of the same 4 directions, 1 set of 5–8 repetitions. The 2 different protocols were separated by 3–4 days and were administered under the guidance of the team's strength and conditioning coach.
Pretest
The intervention group players were fitted with a secure football helmet with flange-mounted bearing attached to the top (Figure 1 ). A 25-cm rod extended from this bearing such that the rod was perpendicular to the bearing and parallel to the floor. A small mass (125 g) was located at the distal end of the rod (weighted arm). With the helmet tightly secured on the head, the players were seated on a bench with their back unsupported and feet flat on the ground. Players then created coordinated circumduction movements of the head using the neck muscles to start the weighted arm spinning about its axis while keeping the rest of the trunk as motionless as possible. As spin speed increased, the small weight provided increased resistance to the neck muscles through centripetal force. Once the subject felt comfortable with the movement pattern, they completed 3 sets of 50 revolutions in each direction (clockwise and counterclockwise), for a total of 6 sets. The weight selection and protocol of 50 revolutions in each direction was informed by prior in-laboratory testing with select members of the target population, feasibility of training time, and as best as possible matched to the theory of intensity needed for neuromuscular adaptations and coordination of neck muscle activity. Each set was timed with a stopwatch and recorded (time clockwise [T cw50 ] and time counterclockwise [T ccw50 ]). A portable cycling computer was used to count the revolutions and calculate the velocity of each revolution of the set. The peak velocity achieved in kilometers per hour was then stored on the cycling computer and recorded for analysis. Given speed (km·h−1 ) and the preset distance per revolution (200 cm) from the cycling computer, the peak revolutions per minute (RPMpeak) was calculated. The best T cw50 , T ccw50 , and the best RPMpeak of all 6 sets were used as outcomes.
Because this type of training has not been previously examined, an effective training protocol has not yet been established. Therefore, the meta-analysis of Peterson et al. (22
Posttest
On the final training session, the players again completed 3 timed sets of 50 revolutions in each direction. The best T cw50 and T ccw50 were recorded along with the best RPMpeak of the 6 sets. After completing the neuromuscular evaluation and a short rest (approximately 3–5 minutes), the isometric neck strength protocol was repeated using the handheld dynamometer. The control group also performed the follow-up isometric neck strength testing.
Adherence was measured as the proportion of neuromuscular training sessions that each subject attended over the maximum offered (n = 14). Dropout rate was defined as subjects who completed baseline (pre) testing for the intervention group with the neuromuscular training device but did not complete the final follow-up (post) testing. Questions about adverse events from the previous session were asked at each subsequent session (3 4
Statistical Analyses
Subject characteristics were explored descriptively (mean, 95% confidence interval [CI]), along with recruitment rate, retention rate, adherence rates, dropouts, and adverse events reported as proportions. Raw change along with 95% CIs and percentage change were calculated for pre and post values for isometric strength and for the neuromuscular training device performance metrics. An important function of pilot research is to provide an estimate for the magnitude of effect of the intervention, for purposes of calculating the sample size of a fully powered study. Accordingly, effect sizes (Hedge's d ; 95% CI) (20 T CW50 , T CCW50 , RPMpeak). The independent variables were time (pre and post) and group (intervention and control). Hedge's d was used for effect size calculations because this value is considered unbiased and more accurate than Cohen's d when the sample size is less than 20 (20). Effect sizes and 95% CIs were calculated in Microsoft Excel (Microsoft; Redmond, WA, USA) using the formulas described by Nakagawa and Cuthill (20
Results
Of the 24 eligible participants, 21 consented to participate for a recruitment rate of 88%. Of those, 18 completed the pre- and posttest measures with a retention rate of 86%. Of those entered into the intervention arm, 8 of 12 (67%) completed at least 11 (79%) of the 14 training sessions over 7 weeks. Reasons for noncompletion were that 3 participants declined to participate in the study and 1 participant was involved in an unrelated motor vehicle collision before the initiation of the training protocol. Although some did report the experience of delayed onset muscle soreness for up to 24 hours after the training, none rated it as any more intense than that experienced routinely after other types of training sessions. No participant in the intervention group reported any other adverse events at any time during the protocol. Participants in the intervention group attended an average of 85% of the 14 training sessions (mean = 11.9; range = 11–14). Of the 12 enrolled into the control group, 2 participants were lost to follow-up (1 missed posttesting and 1 was no longer on the team at the end of the study period).
The mean isometric strength values, raw and percent change, and observed effect sizes between the intervention and control groups pre and post testing are presented in Table 2 . The point estimates indicated that composite neck strength improvement favored the intervention group. Mean change in composite strength of the intervention group was 32 N (95% CI, 13–50), whereas in the control group, it was 12 N (95% CI, −10 to 34). Change in axial rotation strength, the direction of most interest, demonstrated the largest mean difference between the control and intervention cohorts of 46 N (95% CI, 9–83) and the largest effect size with 95% CIs that do not include zero (Hedge's d Δrotation = 1.24; 95% CI, 0.23–2.26). As retention was not consistent between groups, a sensitivity analysis in which only 8 control subjects were selected as matched to the intervention group revealed nearly identical findings to the full sample (not shown).
Table 2.: Isometric strength values pre and post testing with mean change in newton (N) and effect sizes of the difference between control (Con) and intervention (Int) groups (95% CI).*†
Pre- and postneuromuscular performance parameters over the 7 weeks of training, along with mean change scores, percent change, and effect sizes, are presented in Table 3 . The RPMpeak over each training session during the 7 weeks of training is displayed in Figure 3 . Consistent with a training effect, all performance parameters showed a qualitative improvement over the course of the 7 weeks of training protocol, evident as early as training session 3.
Table 3.: Pre- and posttraining performance of the intervention group (n = 8) (95% CI).*
Figure 3.: Mean RPMpeak for the intervention group performance on the neuromuscular training device for each of the 14 training sessions with 95% confidence intervals.*Start of significant difference in RPMpeak for training sessions from session 1 indicated by point estimates outside of 95% confidence intervals. All subsequent sessions were also significantly different than session 1. RPMpeak = peak number of revolutions per minute of the spinning weight.
Discussion
The purpose of this pilot study was to investigate the feasibility and anticipated training effect of a novel neuromuscular training device in a cohort of highly trained and otherwise healthy athletes. Two-thirds of the subjects approached for involvement in the intervention arm of the study completed the training program. Subjects who trained on the device demonstrated an 85% adherence rate with no dropouts, and there were no adverse events reported to the investigators by the subjects in the duration of the study. This is slightly higher than the compliance rates seen in other neuromuscular training programs in athletic populations ranging from 52 to 79% (12,21,25,26 27
This new 7-week training protocol was demonstrated to be potential for improving isometric composite neck strength (increase of 20 N, 95% CI −8 to 48 N, over control) with a moderate to large effect size (Hedge's d = 0.68). Additionally, training on the device in conjunction with traditional neck strengthening may be an effective means of improving isometric neck axial rotation strength above traditional neck strengthening alone (mean increase of 46 N; 95% CI, 9–83 N, over control group). As both the intervention group and control group continued their standard preseason training that involved using the 4-way neck machine, it is not surprising that there was evidence of improvement over time for flexion and side-flexion in both groups. The 4-way neck machine trains the neck isotonically in these directions and is known to improve isometric neck strength in those planes (14 axial rotation strength is not trained with the 4-way neck machine. There are few methods available for training neck rotation strength owing largely to the shape of the head that is not conducive to adding rotational resistance. Yet forces in the horizontal (rotational) plane have previously been associated with more vulnerability to postconcussive syndrome (19
Predictably, the results also indicate that training on the neuromuscular device improves performance on the device; however it is valuable to note the extent of change. RPMpeak more than doubled, and both T cw50 and T ccw50 times were roughly cut in half after the training, suggesting that this change may be to the result of improved neuromuscular control or coordination. It is interesting to the authors that the average RPMpeak after training was more than 250 RPM. This represents more than 4 revolutions per second and would suggest that the neck muscles involved in the training were contracting at a rate of more than 4 contractions per second to achieve this speed (or less than 250 milliseconds per contraction). In trials involving an unanticipated head perturbation in a group of healthy adults, the neck muscles responded with a peak latency of 224 milliseconds (1 11 11,16,17,24
There were several limitations to this study. First, the study population was a group of highly trained male athletes (university football players) and not representative of the general population. Different populations may respond differently to the dynamic training presented here, and other sporting populations that include both male and female subjects who may benefit from dynamic neck strengthening should also be investigated. Second, measurement bias may have occurred because the subjects were not blinded to the training they received and those in the intervention group may have put more effort into their neck strength assessments post training. The use of the 4-way neck machine by control and intervention groups and other training techniques was also not documented or controlled, preventing the exploration of combined training. This, along with other potentially unknown variables, may have been confounding variables. Although potential differences were seen with the 7-week training program, it is likely that the protocol could yet be refined to optimize the training effect. For instance, Conley et al. (6
In summary, this pilot study has demonstrated that the protocol seems to be feasible in that all 8 participants completed at least 79% of the training, and no adverse events were reported. It has also provided important information for conducting a fully powered study. For example, a trial comparing improvements in composite neck strength using the device over and above a control group using traditional neck strengthening could be conducted by recruiting a minimum of 67 football players. With an expected 88% recruitment rate and 86% retention rate, this would provide 25 subjects per arm and would achieve 80% power to detect differences of 20 N or more in composite neck strength (6
Practical Applications
Prior research suggests that neck strength may play a significant protective role in the incidence of concussion. It has also been suggested that research into neck-training programs with a goal of decreasing concussion risk should focus on multiplanar dynamic, plyometric -type contractions that increase the short-latency rate of force development. The results of this pilot study provide a method of neck training that is consistent with the suggested approach and has demonstrated effectiveness at improving dynamic neck strength as measured by performance on this device. For the strength and conditioning coach, physical therapist, or clinician, the type of neuromuscular training described in this study provides a time-efficient method of multiplanar neck training that includes axial rotation . This is a direction of strengthening that is very difficult to achieve through traditional methods. The presented method also offers these professionals a novel means of dynamic neck strengthening for their clients with a more ballistic or plyometric -type of approach to neck muscle training than traditional neck strengthening alone.
Acknowledgments
The authors thank all the players who participated in this study and trained on the device during their off-season. The authors also confirm that no financial support was received to conduct this research. The author T. H. Versteegh is the developer and owner of the patent of the training device used for the intervention group in this research and cofounder in the company that owns the rights to this patent. The author L. Fischer’s spouse is also a cofounder in this company. The other authors have no personal, financial, or institutional interest in the device described in this article. The results of this study do not constitute endorsement by the National Strength and Conditioning Association.
This research received no external funding. J. MacDermid was supported by a Canadian Institutes of Health Research Chair in Gender, Work and Health and the Dr. James Roth Chair in Musculoskeletal Measurement and Knowledge Translation.
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