Naval Special Warfare (NSW) crewmen demonstrate diminished cervical strength and range of motion compared to NSW students

Abstract

BACKGROUND:

Cumulative effects of high-impact vertical forces, like those experienced on an open-ocean mission, could be an injury concern for the cervical spine.

OBJECTIVE:

Compare cervical range of motion (ROM) and strength measures between students and NSW Crewmen and secondarily to compare these measures between students and Crewmen separated into three groups based on years of service.

METHODS:

A volunteer sample of 186 students (age: 22.8±3.1 years) and 167 Operators (age: 26.9±5.1 years) completed cervical isometric strength (% BW) and ROM (°) measurements using a handheld dynamometer and CROM-3 device, respectively. Independent samples t-tests were done to evaluate differences between students and Crewmen. Analysis of Variance and appropriate post-hoc tests were calculated to compare students to Crewmen separated into three groups based on years of service. Data is reported as mean±SD and (student mean vs. Operator mean).

RESULTS:

Students demonstrated higher flexion strength (21.7±4.9 vs. 19.1±5.0) and greater ROM: flexion (54.3±10.6 vs. 51.2±9.2), and extension (77.0±14.4 vs. 71.3±11.4) than Crewmen. Students demonstrated greater flexion strength than Crewmen with ≤2 (19.4±5.1), 3–6 (19.4±5.3), and ≥7 (18.6±4.7) years of service.

CONCLUSIONS:

Students demonstrated greater cervical strength and ROM. A trend of decreasing ROM/strength in Crewmen with greater years of service was also observed.

Keywords

Neck military special operation forces

1 Introduction

Naval Special Warfare (NSW) requires that personnel maintain elite physical fitness, when compared to general Navy personnel, in order to successfully perform occupationally demanding missions in austere environments [1]. A unique personnel group specific to NSW are Special Warfare Combatant-craft Crewmen (SWCC), whose tasks range from mission ingress/egress for Sea, Air and Land (SEAL) teams and hostile waterway patrolling [2]. Unfortunately, there is limited epidemiological evidence of the rigors of SWCC occupational demands. However, small fast boats, similar to the Special Operations Craft –Riverine utilized by SWCC, have been shown to produce vibration and shock impulses to those passengers inside. Vibrations are a result of the running motor attached the boat, while shocks are typically vertical axis loads that occur as a result of becoming airborne or “cresting” a wave [2]. Physical discomfort, injury and occupational performance decrements have been linked to repetitive vertical shock loads and vibration exposure similar to that an Operator would endure on an open-ocean mission [3]. Further, in a previous survey of 154 Special Boat Operators, 65% reported at least 1 past injury and 94.8% of the injuries reported were related to tactical activities [2].

Anecdotal reports indicate a trend towards cervical spine related pain and/or pathology in long term Crewmen. Cumulative effects of high-impact vertical forces could be an injury concern for the cervical spine. However, only 6% of injuries reported from Special Boat Team personnel in the survey referenced above were related to the neck and/or upper back region [2]. While a useful description of injury data, the survey failed to take into account insidious pain the Crewman may have been enduring. Previous studies have demonstrated reductions in cervical strength and range of motion in those suffering from neck pain compared to those without pain [4 –6]. Furthermore, a study of the effects of different mission types on field tests in SEAL team members before, midway and after the mission revealed a temporary reduction in strength halfway through a 5-day speed boat mission [7]. Without adequate strength and range of motion of the cervical spine and surrounding musculature, stability and function of the region could be compromised during a boatmission.

Previous studies have noted high incidences of neck pain in military populations, such as helicopter pilots [8, 9]. Van den Oord et al. [8] also reported that total flight-hours were significantly higher in pilots with a history of neck pain than those without a history of neck pain. Occupational demands, such as carrying a helmet load or sudden perturbations, may have had a strong influence on the development of neck pain in that population. Previous studies have reported reduced lateral neck flexion strength and range of motion in fighter jet pilots with a history of neck pain versus those without a history [4, 5]. Further, prior studies on military pilots with occupational-specific spinal pain report that pain effects their concentration and job performance, which could negatively affect operational readiness [10]. If variations of cervical strength and range of motion are observed between Crewmen and Crewman Qualification Training (CQT) students, it could potentially be related to non-specific neck pain in SWCC, who have been exposed to more of the occupational stressors related to boating activities.

Therefore, the primary purpose of this study was to compare cervical range of motion and strength measures between CQT students and SWCC, with a hypothesis that CQT students would demonstrate greater cervical range of motion (ROM) and strength than SWCC. The secondary purpose of this study was to compare cervical range of motion and strength measures between CQT students and SWCC separated into three groups based on their years of service (these groups will be referred to as tertiles). The hypothesis for this aim was that SWCC with greater years of service would demonstrate lesser range of motion and strength than those with lesser years of service or CQT students. It is clinically important to understand why cervical pain issues are developing in this population despite the epidemiological evidence not reporting cervical pathology as a major issue. Quantifying any potential changes in cervical characteristics could help explain this problem and guide future interventions to reduce the risk of pathology and maintaining functional joint stability along with operational readiness.

2 Methods

2.1 Experimental design

A volunteer sample of CQT students and SWCC participated in a University of Pittsburgh Warrior Human Performance Center testing protocol that included evaluations of neck strength and range of motion. Cervical flexion/extension, rotation and lateral flexion were evaluated for both strength and range of motion measures. The primary analysis evaluated differences in cervical strength and range of motion between CQT and SWCC cohorts. To evaluate possible effects of years of service on these measures, a sub-analysis was conducted by dividing Crewmen into three groups based on their reported years of service and comparing those three groups to the CQT students.

2.2 Subjects

All subjects were healthy and reported no current musculoskeletal injuries that occurred within three months prior to testing. For the purposes of this study, an injury was defined as an incident affecting the musculoskeletal system that kept the individual from activity for at least 1 day. Potential subjects were not asked about current pain levels. However, if a potential subject indicated that they were suffering from pain that would limit their ability to complete testing, even if they were not missing activity, they were excluded. Levels of fitness were not controlled for in the study. However, physical training is often built into duty time in the military [11]. Additionally, adequate physical fitness is a job requirement in the U.S. Navy and each subject was physically active at least 5 days per week [12]. Verbal and written informed consent was given by the subjects and all subjects were provided the opportunity to ask questions and given the right to end testing procedures at any time. This study was approved by the University of Pittsburgh’s Institutional Review Board. A total of 186 CQT students and 167 SWCC completed the cervical strength portion and cervical flexion/extension range of motion. The initial 85 CQT students and 110 Crewmen completed lateral flexion and rotation range of motion, as well. Unfortunately, these tests were then eliminated from the protocol after a request to limit total testing time for the subjects. Thus, lateral flexion/rotation range of motion data is not available for every subject in this study.

2.3 Testing procedures

Cervical isometric strength was tested using a hand-held dynamometer (HHD) (Lafayette Instruments, Lafayette, IN). Subjects laid supine on a treatment table for lateral flexion and rotation testing with hands on the abdomen and pillows under the knees. A second researcher stabilized the opposite side of the subject for lateral flexion and rotation, as well. For flexion, subjects laid supine on a treatment table with their arms off the table in a “W” position. Before the flexion movement, subjects held their heads off the table around a 45° angle, and the HHD stirrup was positioned across the forehead with the bottom edge slightly above the eyebrow line. Prior to the lateral flexion movement, the HHD stirrup was placed just above the ear on the lateral side of the head with an assistant stabilizing the shoulder opposite of the testing direction. Prior to rotational testing, the HHD stirrup was placed horizontally over the frontal bone’s temporal line. The HHD stirrup was positioned perpendicular to the contour of the head prior to the flexion, lateral flexion and rotation movements. Prior to the extension movement, subjects laid prone on the treatment table with faces in a prone pillow, arms hanging over the sides of the table and a pillow under the lower legs. All cervical strength measures began with two 50% maximum voluntary effort (MVE) trials and two 100% MVE warm-up trials. Data was collected on three 100% MVE trials for analysis with a 60 second rest between trials. Subjects were instructed to “ramp” intensity over the five-second testing period (“push hard, then harder, then hardest”) so as not to cause a potential injury during testing. Reliability and precision of these measures has been described previously [6].

Active cervical ROM (flexion, extension, right/left lateral flexion, right/left rotation) was measured using the CROM 3 (Performance Attainment Associates, Lindstrom, MN). Cervical rotation ROM testing utilized a SKIL 8201-CL self-leveling cross-line laser (Robert Bosch Tool Corporation, Prospect, IL). Subjects were seated in a chair with feet hip-width apart, feet flat on the floor, and knees bent to a 90° flexion angle. Pillows were placed under the arms to relax the shoulders. All cervical ROM tests began with subjects’ heads in the Frankfort plane [13]. Subjects were instructed to move their head in the direction of the test until an “uncomfortable stretch or pressure” was felt. Prior to the rotation movement, a laser line was projected onto a wall that subjects were instructed to follow with their eyes during the trial. Three practice trials and three maximal trials were conducted, with 60 seconds of rest in between. The absolute angle, relative to the subject’s neutral position, was recorded for the three recorded trials and analyzed.

2.4 Statistical analysis

Normality of distribution was assessed using a Shapiro-Wilk test. In comparison of CQT and SWCC cohorts, strength and range of motion variables were analyzed using an independent-samples t-test to assess for normally distributed variables and a Mann-Whitney U test for variables that were not normally distributed. In a secondary analysis comparing CQT to SWCC cohorts separated into years of service tertiles, a one-way Analysis of Variance was used for normally distributed data. The Tukey-Honest Significant Difference test was used for post-hoc analysis of significant between-groups differences. For data that fell outside a normal distribution, a Kruskal-Wallis test was used. Years of service cohorts were defined as less than one to 2 years (≤2), 3 to 6 years (3–6), and 7 years or greater (7+). Significance was set a priori at p < 0.05. In order to evaluate the magnitude of differences, Cohen’s d effect sizes were calculated for statistically significant results. Per Cohen’s recommendations, d = 0.2 was considered a small effect, d = 0.5 was considered a moderate effect, and d = 0.8 was considered a large effect [14]. All statistical measures were obtained using IBM SPSS Statistics for Windows, Version 23 (IBM Corp., Armonk, NY).

3 Results

Demographics for CQT and SWCC are shown in Table 1. There were significant differences between CQT and SWCC in age (p < 0.001), weight (p < 0.001) and years of service (p < 0.001), but not height (p = 0.516). Significant differences between CQT and SWCC cohorts are shown in Table 2. Significant differences were demonstrated in cervical flexion strength (p < 0.001; d = 0.53) and right to left lateral flexion ratio (p = 0.004; d = 1.0). Significant differences were also demonstrated in range of motion: cervical flexion (p = 0.009; d = 0.31), extension (p = 0.001; d = 0.44), left rotation (p = 0.048; d = –0.29), right lateral flexion (p < 0.001; d = 1.6), and left lateral flexion (p < 0.001; d = 1.5).

Table 1
Subject demographics

Primary Analysis

CQT SWCC

N 186 167

Age (years) 22.8±3.1 26.9±5.1

Height (cm) 178.3±6.9 178.8±6.6

Weight (kg) 82.2±8.7 84.7±9.1

Years of Service 0.91±1.4 5.2±3.9

Secondary Analysis

CQT SWCC ≤2 SWCC 3–6 SWCC ≥7

N 186 49 55 59

Age (years) 22.8±3.1 22.9±2.8 25.4±2.8 32.6±3.9

Height (cm) 178.3±6.9 179.0±7.6 178.6±5.6 179.1±6.1

Weight (kg) 82.2±8.6 84.5±9.6 84.6±9.3 86.1±10.0

Years of Service 0.9±1.4 1.5±0.6 4.1±1.0 10.1±2.5

Primary Analysis
N	186	167
Age (years)	22.8±3.1	26.9±5.1
Height (cm)	178.3±6.9	178.8±6.6
Weight (kg)	82.2±8.7	84.7±9.1
Years of Service	0.91±1.4	5.2±3.9
Secondary Analysis
	CQT	SWCC ≤2	SWCC 3–6	SWCC ≥7
N	186	49	55	59
Age (years)	22.8±3.1	22.9±2.8	25.4±2.8	32.6±3.9
Height (cm)	178.3±6.9	179.0±7.6	178.6±5.6	179.1±6.1
Weight (kg)	82.2±8.6	84.5±9.6	84.6±9.3	86.1±10.0
Years of Service	0.9±1.4	1.5±0.6	4.1±1.0	10.1±2.5

Table 2

Strength and range of motion differences between CQT students and SWCC

	CQT	SWCC	p-value	d
Strength (% BW)
Flexion	21.7±4.9	19.1±5.0	<0.001^*	0.53
Extension	30.2±5.8	31.0±6.1	NS	—
Flexion:Extension Ratio	0.75±0.3	0.62±0.1	<0.001^*	0.58
Right Rotation	21.6±3.8	20.6±4.1	NS	—
Left Rotation	21.5±3.7	20.9±4.1	NS	—
R:L Rotation Ratio	1.0±0.1	1.0±0.1	NS	—
Right Lateral Flexion	25.6±5.2	24.1±5.4	NS	—
Left Lateral Flexion	26.1±5.3	25.7±5.3	NS	—
R:L Lateral Flexion Ratio	1.0±0.1	0.9±0.1	0.004^*	1.0
Range of Motion (°)
Flexion	54.3±10.6	51.2±9.2	0.009^*	0.31
Extension	77.0±14.4	71.3±11.4	0.001^*	0.44
Flexion:Extension Ratio	0.73±0.2	0.74±0.2	NS	—
Right Rotation	65.9±7.2	67.2±7.1	NS	—
Left Rotation	67.0±8.3	69.2±6.8	0.048^*	–0.29
R:L Rotation Ratio	0.99±0.1	0.97±0.1	NS	—
Right Lateral Flexion	55.4±8.3	43.4±7.0	<0.001^*	1.6
Left Lateral Flexion	57.5±8.0	46.3±7.0	<0.001^*	1.5
R:L Lateral Flexion Ratio	0.97±0.1	0.94±0.1	NS	—

Data are presented as mean±SD. (^*) denotes statistical significance (p < 0.05). (NS) denotes a non-statistically significant difference (p > 0.05). (d) reflects Cohen’s d effect size.

Significant differences between CQT and SWCC tertiles are shown in Table 3. Post-hoc analysis revealed significant differences in cervical flexion range of motion between CQT and ≥7 (p = 0.002; d = 0.57) and SWCCs ≤2 and ≥7 (p = 0.014; d = 0.67). Significant differences between CQT and ≥7 were seen in cervical extension (p = 0.021; d = 0.46) Right and left lateral flexion range of motion differences were demonstrated between CQT and ≤2, 3–6, and ≥7 (p < 0.001; d = 1.5–1.8). Significant differences in cervical flexion strength were shown between CQT and ≤2 (p = 0.033; d = 0.46), 3–6 (p = 0.034; d = 0.45), and ≥7 (p = 0.001; d = 0.65). Significant differences were observed between CQT and ≥7 in lateral flexion strength ratio (p = 0.006; d = 0.70).

Table 3

Strength and range of motion differences between CQT students and SWCC separated by years of service tertiles

	CQT	SWCC ≤2	SWCC 3–6	SWCC ≥7	Between Groups	P-value	d
					Differences
Strength (% BW)
Flexion	21.7±4.9	19.4±5.1	19.4±5.3	18.6±4.7	CQT vs. ≤2	0.033^*	0.46
					CQT vs. 3–6	0.034^*	0.45
					CQT vs. ≥7	0.001^*	0.65
Extension	30.2±5.8	32.1±6.4	31.1±6.0	30.4±5.7	NS	NS	—
Flexion:Extension Ratio	0.75±0.3	0.61±0.1	0.63±0.1	0.62±0.1	CQT vs. ≤2	<0.001^*	0.63
					CQT vs. 3–6	<0.001^*	0.54
					CQT vs. ≥7	<0.001^*	0.58
Right Rotation	21.6±3.8	21.5±4.7	20.2±4.2	20.2±3.4	NS	NS	—
Left Rotation	21.5±3.7	21.3±4.6	20.6±4.2	20.7±3.7	NS	NS	—
R:L Rotation Ratio	1.00±0.1	1.01±0.1	0.98±0.1	0.98±0.1	NS	NS	—
Right Lateral Flexion	25.6±5.2	25.6±5.9	23.6±5.3	23.2±5.0	NS	NS	—
Left Lateral Flexion	26.1±5.3	26.5±5.3	25.4±5.5	25.3±5.3	NS	NS	—
R:L Lateral Flexion Ratio	0.99±0.1	0.97±0.1	0.94±0.1	0.92±0.1	CQT vs. ≥7	0.006^*	0.70
Range of Motion (°)
Flexion	54.4±10.4	54.7±7.6	51.0±8.6	48.2±11.5	≤2 vs. ≥7	0.014^*	0.67
					CQT vs. ≥7	0.002^*	0.57
Extension	76.9±14.5	71.4±10.6	73.1±9.0	70.3±14.1	CQT vs. ≥7	0.021^*	0.46
Flexion:Extension Ratio	0.73±2.0	0.78±0.2	0.71±0.1	0.73±0.3	NS	NS	—
Right Rotation	65.9±7.2	68.7±7.3	66.8±7.5	65.8±6.5	NS	NS	—
Left Rotation	67.0±8.3	69.6±5.3	69.4±8.4	68.4±6.5	NS	NS	—
R:L Rotation Ratio	0.99±0.1	0.99±0.1	0.97±0.1	0.96±0.1	NS	NS	—
Right Lateral Flexion	55.4±8.3	43.7±7.8	44.0±6.7	42.0±6.8	CQT vs. ≤2	<0.001^*	1.5
					CQT vs. 3–6	<0.001^*	1.5
					CQT vs. ≥7	<0.001^*	1.8
Left Lateral Flexion	57.5±8.0	46.3±7.0	47.0±7.2	45.0±6.8	CQT vs. ≤2	<0.001^*	1.5
					CQT vs. 3–6	<0.001^*	1.4
					CQT vs. ≥7	<0.001^*	1.7
R:L Lateral Flexion Ratio	0.97±0.1	0.95±0.1	0.94±0.1	0.94±0.1	NS	NS	—

4 Discussion

Anecdotal discussions among study personnel and on-site Crewmen indicated that cervical pain was a common issue, especially as Crewmen aged. Thus, it appeared that cervical pain was a unique problem specific to this population. This knowledge, along with evidence from previous studies on military personnel suffering from occupational specific neck pain, led to the present study. Similar to previous studies on aviators, it could be postulated that cumulative effects of SWCC tactical demands may lead to neck pain, which could subsequently reduce cervical spine strength and range of motion. The purpose of this study was to compare CQT to SWCC in cervical strength and range of motion measures, and to compare CQT to SWCC years of service tertiles to examine potential differences based on increased operational years. The hypotheses for this study were that CQT would demonstrate higher cervical strength and range of motion at the cervical spine than the SWCC cohort. This was largely supported, as students outperformed SWCC in all measures except cervical extension strength and rotation range of motion. The secondary hypothesis was that SWCC with more years of service would demonstrate lesser strength and range of motion measures than CQT and SWCC with lesser years of service. This hypothesis was not statistically supported, but a trend of decreasing range of motion and strength was observed in SWCC with greater years of service.

4.1 Strength

There were statistically significant differences between CQT and SWCC in age and weight, as SWCC were ∼4 years older and ∼2.5 kilograms heavier. While statistically significant, these values would not be expected to be clinically significant due to the minimal difference in weight and the fact that healthy, active persons typically do not reach maximum physical capacity until ∼30 years of age [15]. CQT also demonstrated greater cervical flexion than SWCC counterparts. Weakness in cervical flexion strength tests have been described previously in civilian populations currently suffering from neck pain compared to healthy controls [16]. Both CQT and SWCC demonstrated flexion/extension isometric strength ratios that are consistent with previous literature (0.60–0.78) [17], but CQT students had a 20.9% higher strength ratio due to greater flexion strength. Direct value comparison of the current study to previous literature is difficult given the uniqueness of this study’s testing procedures and those previously used in the literature, which vary widely in implementation [17 –19]. Due to repeated identification of reduced neck strength in the literature related to cervical pathologies [4, 17], along with the moderate to large magnitude of effect (d = 0.53–1.0) for statistically significant differences between groups, further research should investigate the reasons behind decreased cervical strength in this population. For example, Crewmen could have adapted contraction strategies, such as increased rate of force development, to stabilize the cervical region as a result of perturbation. These adaptations would not be observed in isometric testing and may warrant further study.

4.2 Range of motion

CQT students demonstrated significantly greater cervical flexion, extension and bilateral lateral flexion range of motion than SWCC, while the Crewmen reported greater motion in left cervical rotation. Normative values for cervical range of motion have been previously established using the CROM device [20]. In comparison with values obtained on healthy males aged 20–29 years old, both CQT and SWCC demonstrated lesser than normal values for cervical flexion and right rotation, while SWCC was substantially below normative values for cervical extension. CQT had greater range of motion than normal values for extension and lateral flexion while SWCC hovered around the average for lateral flexion and left cervical rotation. Interestingly, Crewmen demonstrated significantly more left rotation than students but not right rotation. The difference between average left rotation and right rotation was less than a degree, however, which may not be clinically meaningful. Future study on rotation range of motion in this population may be necessary to better understand this finding.

One possible explanation for range of motion differences observed in this study is the potential for Crewmen to be suffering from subclinical cervical pain due to occupational hazards. Previous studies on military pilots have found reduced cervical range of motion in those with a history of neck pain compared to pilots with no prior history of neck pain [5, 6]. Lee et al. [21] reported reduced cervical range of motion values in those with subclinical neck pain compared to healthy controls, concluding that early musculoskeletal changes could be identified prior to developing neck pain at a level that requires clinical intervention. Further research should investigate if individually perceived pain levels could be effecting cervical range of motion in this population. An additional consideration for future study would be, if pain is not effecting range of motion, to investigate if reduced range of motion is actually an advantageous adaptation to enhance stability of the head and neck to perturbations caused by the boats.

4.3 Occupational demands

In order to investigate if the occupational demands of SWCC play a central role in reduced neck range of motion and strength, CQT students were compared to grouped by years of service tertiles (≤2, 3–6, ≥7 years). SWCC 3–6 and ≥7 years had lesser cervical flexion range of motion than CQT, but the only significant difference was observed between CQT and 3–6. Interestingly, SWCC with ≤2 years of service were able to achieve significantly more range of motion on average than the ≥7 cohort. CQT also demonstrated greater range of motion in both right and left lateral flexion than each SWCC tertile. CQT demonstrated greater cervical flexion strength and more balanced flexion to extension strength ratio than each SWCC tertile, while registering more optimally balanced lateral flexion strength ratios.

The hypothesis that range of motion and strength would decrease in SWCC populations with more years of service was not statistically supported. There was an observed trend, albeit at a nonsignificant level, towards lesser strength and range of motion in the ≥7 cohort. Contrary to expectations, however, there were no significant decreases in performance from the ≤2 cohort to the ≥7 cohort of Crewmen, with the exception of cervical flexion range of motion. Further, only cervical flexion range of motion demonstrated significant between-groups differences between SWCC cohorts; all other significant differences were between one or more SWCC cohort and CQT students. Therefore, the difference observed between cohorts in the primary analysis of this manuscript cannot be fully explained by the cumulative effects of years of service alone. Age is a factor that, while not analyzed in this study, could have played a role in decreased strength and range of motion in the group with the most years of service. Military service is a complex variable as not all years of service are comparable. Variations in perceived intensity, individual stress levels, job requirements, and missions completed could be responsible for the lack of a uniform trend in performance reduction.

Targeting these observed differences in SWCC with a program that aims to increase cervical strength could be beneficial [22, 23]. Increasing strength to the neck musculature has been shown to reduce risk of injury and alleviate neck pain in civilian populations [23]. Further, in patients undergoing cervical rehabilitation, increasing isometric strength has been shown to reduce pain levels [22]. There is limited evidence for reducing pain in clinical populations with range of motion rehabilitation [24]. However, assessing range of motion can be a useful tool for identifying both subclinical neck pain and cervical pathology, such as whiplash associated disorders [21, 25]. Additionally, increases in neck strength and range of motion in cervical rehabilitation patients can be used as objective indicators of directing patient management [26]. Incorporating both strength and range of motion measures is important to identification of cervical pathology and should be considered in this population. It is important to note that previous studies on cervical rehabilitation were done in civilian personnel, and further study on the military to understand its effectiveness in this population is necessary, given the unique occupational demands.

5 Conclusion

This study noted significant differences in cervical strength and range of motion measures between Crewman Qualification Training students and Special Warfare Combatant-craft Crewmen. Neck pain needs to be quantified in this population, at a level above anecdotal evidence, and investigation of a potential relationship to cervical musculoskeletal characteristics should be addressed. If anecdotal observations are confirmed, operational readiness and individual long-term health are serious concerns for this population if neck pain is not identified and quickly treated [10]. Investigations on military pilots revealed that at least half of the population suffering from low back pain felt that concentration and work-effectiveness was negatively affected [27, 28]. There is little reason to believe that cervical pain would not elicit a similarly negative effect on SWCC. Continued monitoring is critical to identifying those at potential risk of cervical neck pain considering that early musculoskeletal changes have been noted in those prior to developing subclinical neck pain and cervical pathologies like whiplash disorders [21, 25]. Further research should examine the incidence of neck pain in Special Warfare Combatant-craft Crewmen and the possible relationship to musculoskeletal characteristics that were reported in this study. If neck pain is observed frequently in this population, investigating the effect of pain on job performance, as described above in military aviators, is a crucial next step. If neck pain is effecting performance, or even perceived individual performance, attempting to reduce pain by a specific rehabilitation program should be investigated, as well. Rehabilitation that focuses on strengthening the musculature of the neck and shoulder has been shown to be effective in reducing neck pain in civilian populations [17 , 29].

There are limitations to this study. Anecdotal observations are not considered a high level of evidence, and further research is necessary to quantify these observations in a more objective manner. Previous injuries could have affected the data and were not controlled for in this study, however, as mentioned above, subjects were excluded from participation if they had a current musculoskeletal injury or were injured in the previous three months. Testing Crewmen longitudinally would be more appropriate given the propensity towards neck pain in older Crewmen but that type of study design was not feasible due to time-limitations of subjects and varying assignment locations. Additionally, there are a limited number of outcome measures included in this study. Crewmen’s time is highly valuable and the outcomes included were for clinical feasibility purposes. It is difficult to fully understand the nature of the problem without including more comprehensive outcome measures such as joint position sense, visual search and target acquisition, and subjective ratings of pain and/or disruption of occupational effectiveness. Therefore, while pain may play a role in the measures presented in this study, other outcome measures may also help explain the differences in strength and range of motion. Finally, it is difficult to describe clinical meaning of the differences shown in the present study due to the highly unique population and testing procedures. By providing effect sizes for statistically significant differences, it was the goal of the authors to mitigate that difficulty slightly by providing magnitude of the differences to reinforce the findings and need for future study.

Conflict of interest

None to report.

Footnotes

Acknowledgments

This study was funded by the Office of Naval Research, grant number #N00014-11-1-0929. The opinions or assertions contained herein are the private views of the authors and are not to be constructed as official or reflecting the views of the NSCA, Department of the Navy, Department of Defense, nor the U.S. government.

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