The 11 articles included in this review comprised of studies conducted only in the Northern Hemisphere, 8 in North America, 1 in Germany, and 2 in the UK [27, 30, 33, 48,49,50,51,52,53,54,55]. One study used non-human neck model simulations . The human participants of the remaining included studies varied significantly in age. One study included only adult males (men aged 25.5 ± 5.6) . Three studies included collegiate participants, (men aged 19.21 ± 0.918; women aged 19.16 ± 0.898 years)  (males aged 20.3 ± 3.6)  (men aged 21.6 ± 2.8) , three studies included high school and collegiate participants (high school 16.6 ± 0.9, collegiate 20.5 ± 1.4 years) ; (16.3 ± 5.0 years for males and 15.0 ± 4.4 years for females)  (males and females 17.1 ± 3.5) , and three studies included only high school participants (intervention 16.0 ± 1.2 years; control 15.9 ± 1.1) ; (15.0 ± 1.0 years) ; (girls and boys with no age descriptions) .
With regard to sports codes discussed in the selected articles [27, 30, 33, 48,49,50,51,52,53,54,55], two studies reported exclusively on Rugby Union (men; sex unspecified) [48, 53], another exclusively ice hockey (sex unspecified) , two studies on American Football (sex unspecified)  (males only) , two studies focused on men’s and women’s soccer [50, 55] and one study on men’s soccer only . Three studies reported on multiple sports codes, namely, boys’ and girls’ soccer, basketball, and lacrosse , soccer, ice hockey, American Football, wrestling, lacrosse, and martial arts  and baseball and American Football .
Mansell et al. (2005)  reported on the effect of an 8-week isotonic resistance training program on head-neck segment dynamic stabilization in a small sample of men’s and women’s soccer players . The training program consisted of 3 sets of 10 repetitions of neck flexion and extension at 55% to 70% of their 10-repetition maximum, two times a week. Measurements included anthropometric assessments, head-neck segment kinematics and stiffness, electromyographic activity of the upper trapezius and sternocleidomastoid muscles during force application to the head, and neck flexor and extensor isometric strength. Although the intervention group showed a 15% improvement in isometric neck flexor strength, no kinematic, electromyographic, or stiffness training effects were seen. In female intervention group participants, isometric neck extensor strength increased by 22.5%, and neck girth increased by 3.4%. Female soccer players demonstrated less head-neck segment length (7%) and less head-neck segment mass (26%) than men. The researchers concluded that regardless of the improvements in neck isometric strength and increases in neck girth, the resistance training protocol used in this study did not increase head-neck segment dynamic stabilization during force application in soccer players .
Schmidt et al. (2014) reported on the incidence and nature of head impact biomechanics using the Head Impact Telemetry System in a small sample of American Football players who completed preseason cervical testing of isometric neck strength, electromyography, muscle size, and response to cervical perturbation . The reported findings showed the likelihood of sustaining higher magnitude head impacts was reduced in players with greater cervical stiffness and who experienced a smaller amount of angular displacement after impact. The results of this study showed that players with stronger lateral flexors and composite cervical strength had increased likelihood (1.75×) of receiving moderate head impacts rather than mild impacts, compared with weaker players. Similarly, players who developed faster torque in cervical extension had twice the likelihood of receiving severe head impacts (odds ratio [OR], 2.10; 95% CI, 1.08–4.05) rather than mild head impacts. However, players with greater cervical stiffness had reduced likelihood of sustaining both moderate (OR, 0.77; 95% CI, 0.61–0.96) and severe (OR, 0.64; 95% CI, 0.46–0.89) head impacts compared with players who demonstrated less cervical stiffness. The authors conclude that the study’s findings showed that greater cervical stiffness reduced the likelihood of sustaining higher degree head impacts. Further, the results of this study do not support that stronger and larger neck muscles reduce the severity of head impacts .
Lisman et al. (2012) examined the effects of an 8-week isoinertial cervical resistance training program and the electromyographic (EMG) activity of neck muscles and the kinematics of the head and neck in response to a American Football tackle . The results of the study showed modest increases in isometric strength in cervical extension (7%) and left lateral flexion (10%), but the program had no influence on the EMG responses of neck muscles, peak linear, or angular head accelerations during tackling. This authors conclude that this 8-week isoinertial cervical resistance training program did not increase dynamic stabilization of the head and neck during a American Football tackle .
Mihalik et al. (2011) evaluated the effect of cervical muscle strength on the head after an impact by collecting data from accelerometer instrumented ice hockey helmets throughout a playing season . A small sample of players’ cervical isometric strength measurements were recorded using a hand-held dynamometer (Model: 01163; Lafayette Instrument, Co, Lafayette, IN) in the preseason. Muscle strength testing involved two practice trials performed before three test trials (of 3 s duration) for each direction of motion, with a 30 s rest period between trials. The maximum forces for each of the three test trials were averaged and normalized to the player’s body mass. These data were compared with head biomechanics from the collected helmet data (Head Impact Telemetry System). The authors identified significant differences in cervical muscle strength between the participants. However, no differences were recorded in peak linear (PLin) or peak rotational acceleration (PRot) for the anterior neck flexors (PLin = 0.399; PRot = 0.060), anterolateral neck flexors (PLin = 0.987; PRot = 0.579), cervical rotators (PLin = 0.136; PRot = 0.238), posterolateral neck extensors (PLin = 0.883; PRot = 0.101), or upper trapezius (PLin = 0.892; PRot = 0.689). The authors concluded that the findings of this study do not support that cervical muscle strength is a factor in modifying head impact severity .
Collins et al. (2014) reported on a large sample of adolescent athletes (n = 6662) over a full academic year in multiple contact sports, namely soccer, basketball, and lacrosse . In this study, the researchers developed and validated a cost-effective tool to measure neck strength in these athletes and found a high correlation (0.83 to 0.94 for the four neck strength measurements—all p values < 0.05) between the hand-held dynamometer and tension scale measurements. High inter-tester reliability was observed between different athletic trainers (ATs). In the second part of the study, AT’s recorded athletic anthropometric measurements, exposure, and injury data on the internet-based data collection tool developed for the National High School Sports-Related Injury Surveillance Study. Athletes were prospectively monitored for sustaining a concussion from 2010 to 2011. The results showed that the rate of concussion in the three contact sports was higher in adolescent girls when compared to adolescent boys (4.9 per 10,000 athlete exposures in girls and 2.5 per 10,000 athlete exposures in boys), with soccer having the highest rate of concussion (5.2 per 10,000 athlete exposures) followed by lacrosse (3.7 per 10,000 athlete exposures) and basketball (2.3 per 10,000 athlete exposures). Girls had an increased likelihood of concussion overall (OR = 1.8, 95 % CI 1.36–2.49) and in basketball (OR = 2.7, 95% CI 1.53–4.71) and soccer (OR = 1.8, 95% CI 1.17–2.69). However, no difference was identified between girl and boy lacrosse players (OR = 1.0, 95% CI 0.44–2.10). The researchers reported that a smaller mean neck circumference, smaller mean neck to head circumference ratio, and weaker mean overall neck strength were significantly associated with concussion. Overall, sex (p < 0.001), sport (p = 0.007), and neck strength (p < 0.001) were found to be significant predictors for sustaining a concussion. Specifically for neck strength, the authors reported that for every 1 lb increase in neck strength, the likelihood of concussion decreased by 5% (OR = 0.95, 95% CI 0.92–0.98) .
Hislop et al. (2017) evaluated the effects of a prescribed series of progressive warm-up exercises in a cluster-randomized trial of adolescent rugby players injuries over one season . The specific particulars of the intervention and control group exercises are not described in detail and the reader is directed to a previous study by the author for more information . The intervention group exercises consisted of isometric neck exercises, whole-body resistance training, plyometric training, and landing and cutting running movements. These exercises were to be completed in the initial fifteen minutes of training and before every match, although the authors report that certain exercises were “withdrawn when the program is performed prior to matches” . The control group exercises were structurally indistinct to the intervention program and consisted of exercises that were considered “best practice” in schools’ rugby including a running-based warm-up, dynamic stretching, wrestling, mobility, speed, and agility-related exercises . School coaches recorded training exposure, player injury details, match exposure, and program compliance on paper-based or electronic report forms. School medical staff recorded the injury location and diagnosis. The intention-to-treat analyses indicated unclear effects of the trial arm for overall match injury (incidence rate ratio [RR] = 0.85, burden RR = 0.83) and match contact injury (incidence RR = 0.85; burden RR = 0.88). The researchers conclude that the players in the intervention group reported substantially reduced incidence of upper limb injury and concussion. Further, teams that completed the intervention program three times per week reported substantial reductions (72%) in overall match injury incidence (RR = 0.28, 0.14 to 0.51) and concussion incidence (RR = 0.41, 0.17 to 0.99) compared with the control program .
Similarly, Attwood et al. (2018) investigated the effects of a movement control program to reduce injury risk in rugby union players . The intervention program involved proprioceptive, mobility, and strengthening exercises targeted at the lower limb, shoulder, head and neck over seven 6-week progressive phases. The control program involved dynamic stretching and non-targeted resistance exercises in a similar progressive structure to the intervention program. Participants were blinded to which program they received. Each participating club nominated an individual who was trained to deliver the program to the players, and a representative to record first team match exposure, exercise program compliance, and match injuries on a weekly basis, using standardized forms. No clear effect was identified for the intervention program using intention-to-treat analysis for overall injury burden, overall injury incidence or severe injury incidence. However, concussion incidence (1.2 vs 3.4 injuries/1000 player match-hours) and concussion burden (38 vs 102 days/1000 player match-hours) was 60% lower in the intervention group compared with the control group. Lower-limb injury incidence was also 40% lower for the intervention group over control group (3.3 vs 5.2 injuries/1000 player match-hours) although shoulder injury incidence (1.7 vs 1.0 injuries/1000 player match-hours) and injury burden (68 vs 45 days/1000 player match-hours) were higher for the intervention group. Further, clubs in the intervention group that had a greater compliance (≥ 85% to < 85% of possible sessions) indicated a likely beneficial 50% reduction in targeted injury burden .
Eckner et al. (2014) assessed the influence of neck size, neck strength, rate of force development, and muscle activation on head kinematics following loading in multiple planes . The participants in this cohort comprised of a broad range of ages, competitive levels, and sporting codes. The results of this study showed greater isometric neck strength and anticipatory activation to be independently associated with decreased head peak linear velocity and peak angular velocity after impulsive loading across all planes of motion (all p < .001). Further, neck circumference and sternocleidomastoid cross-sectional area were also significant (p < .001) in all planes of motion and remained significant when adjusted for age and sex (p < .001). This study reports that superior neck strength and anticipatory muscle activation are individually associated with a decreased kinematic response to impulsive forces applied to a subject's head .
Caccese et al. (2018) aimed to identify factors that contribute to head acceleration during soccer heading. This study utilized anthropometric measurements, isometric strength and electromyography of muscles of the neck and upper torso and kinematics of the head during active soccer heading in seasoned soccer players . The authors reported that the results suggest that greater head and neck size predicted lower peak linear and rotational accelerations. The results further showed that neck strength, specifically of the sternocleidomastoid muscle predict peak linear (β = − 1.544, p = 0.012) and peak rotational (β = − 0.117, p = 0.018) accelerations of the head. Technique-related predictors did not predict the same during soccer heading .
Eckersley et al. (2019) reported on the effects of cervical muscle strength on head kinematics using validated neck model simulations . This study examines plausible impacts to the head for different athletic scenarios, namely impact from a ball to the bare head in major league baseball and impacts between opposing player American Football helmets. The authors report that no consistent effect to the injury metrics for sport-related concussion (SRC) can be seen by changing neck muscle force in models. The results did show that tensed muscle activation conditions resulted in higher peak resultant angular acceleration values compared to relaxed muscle activation conditions. The authors conclude that impact location and impact scenario were greater determinants of SRC injury metrics than the protective capacity of cervical muscle activation. The results of this study do not support the hypothesis that greater cervical muscle force influences head kinematics during impact scenarios and neck strengthening programs and exercises will do little to reduce the risk of concussion .
In the most recently published study included in this scoping review, Becker et al. (2019) explored the effects of a 6-week strength training program on head acceleration during three different variations of headers on male soccer players . An interesting inclusion in this study design is that the researchers attempted to fatigue the trunk muscles of the participants to decouple the head-neck-torso alignment, thus resulting in an increased acceleration of the head. The results did not show a significant difference between the strength measurements of the control and intervention groups (p = 0.055). Neck flexion strength improved for all the groups, including the control group, who did not perform extra neck exercises. Neck extension strength improved for one of the intervention groups (youth team) but decreased in the other intervention (adult team) group, and in the control (mixed) group. The authors state that these results do not support the hypothesized preventative benefit of neck strengthening .
Framework Optional Stage: Consultation Exercise
As an additional stage in scoping review methodology as recommended by Arksey and O’Malley , two International Concussion Societies were consulted (International Concussion Society https://www.concussion.org/contact/ and The Center for Disease Control and Prevention, U.S. Department of Health and Human Services) for further possible information. These organizations were contacted due to their location as most of the studies identified in this review were conducted in North America. The literature sourced from this optional exercise provided insight into the broader discussion of concussion although the provided literature did not satisfy the inclusion criteria for this scoping review.