Design and procedure
In this cross-sectional cohort study, a population of Swedish AFP answered the MSP baseline questionnaire, performed eight tests of movement control of the spine (25), active cervical range of motion (ROM) in all six directions and isometric strength and endurance in cervical flexion and extension in a standardized order (as presented below). One of the authors (HL) administered the questionnaires and assisted during the strength and endurance tests. An experienced physical therapist (PT) who was blinded to the participants’ pain status performed the testing. For the strength and endurance tests, the PT asked participants about ongoing pain to determine whether the tests could be performed with regards to location and intensity. The Regional Ethical Review Board in Stockholm approved the study, DNR:2013/144-31/2 and DNR:2015/493-32.
Participants
All male AFP listed on flight duty at one airbase in Sweden during the period June 2015 to May 2016 were invited to participate in the study. Each AFP accepted the invitation and, following screening, all were included in the study (n=73 [36 FP, 18 HP and 19 RC]).
Questionnaire
The MSP questionnaire (1, 21-23, 26, 27) was used to gather information regarding age, height, weight, and musculoskeletal pain or injuries during the previous 12 months (one-year prevalence) and at present (point prevalence) for ten body regions. Present pain was rated as maximal pain intensity from 0-10 using the numerical pain rating scale (NPRS). For this study, only pain prevalence in the cervical and thoracic regions were reported and combined as cervico-thoracic region pain. Questions regarding flight hours during the previous 12 months and total (i.e., career) flight hours were added (Table 1).
Table 1. Demographic data of included participants; fighter pilots (n=36), helicopter pilots ( n=18), rear crew (n=19).
All
|
FP
|
HP
|
RC
|
p-value
|
Age (years)
|
39
|
(8)
|
35a,b
|
(7)
|
43a
|
(8)
|
43b
|
(7)
|
0.000
|
|
Height (m)
|
1.81
|
(0.06)
|
1.82
|
(0.06)
|
1.80
|
(0.06)
|
1.78
|
(0.06)
|
0.078
|
|
Weight (kg)
|
82
|
(9)
|
81
|
(7)
|
85
|
(12)
|
79
|
(7)
|
0.144
|
|
TFT (h)
|
1827
|
(1329)
|
1580a
|
(1097)
|
2880a,c
|
(1649)
|
1299c
|
(797)
|
0.000
|
|
ANFT (h)
|
97
|
(54)
|
105
|
(45)
|
104
|
(67)
|
73d
|
(55)
|
0.118
|
|
Data are presented as mean, SD with ANOVA.
TFT: Total flight time, ANFT: annual flight time (i.e., previous 12 months), FP: Fighter pilots, HP: Helicopter pilots, RC: Rear crew.
P-values in bold represent significant difference between FP, HP and RC. Post-hoc analysis with Bonferroni adjusted p-values (p=0.05/3): a: p < 0.017 between FP and HP. b: p < 0.017 between FP and RC. c: p < 0.017 between HP and RC. d: missing data, n=3.
Physical performance testing
Movement control test battery
The tests included in the test battery (neck flexion and extension in sitting, neck rotation in sitting (left and right), neck flexion in supine, chest lift, pelvic tilt and forward lean) are based on work by Sahrmann (28, 29) and Comerford and Mottram (30). They are used to analyse habitual movement patterns including relative flexibility (31) and/or challenge the ability to control movements in one region while moving an adjacent one (32). All tests have been presented in detail previously and showed moderate to almost perfect inter-rater agreement (prevalence and bias adjusted kappa coefficients = 0.57-0.84) and fair to substantial test-retest agreement (prevalence and bias adjusted kappa coefficients = 0.33-0.69) (25). Each test was evaluated with a dichotomous rating indicating whether the participant could (1) or could not (0) perform the movement according to the grading criteria (Appendix).
Active cervical ROM
The Cervical Range of Motion (CROM) 3 device (Performance Attainment Associates, Roseville, MN) was used to measure active cervical ROM (degrees, in the following order: flexion, extension, axial rotation left and right and lateral flexion left and right) (Fig 1 A). The CROM 3 has two gravity-controlled balls as well as a magnetic compass to measure movements in all three movement planes and has shown good reliability (33). For testing, participants sat on a bench in a neutral upright position with hips and knees in 90˚, hands resting on their thighs. To ensure full active ROM, they performed three repetitions of each movement. The highest value was used for analyses. To control movement in adjacent body regions during measurements of flexion and extension, the PT stood on the side of the bench with their hands on the participant’s sternum and thorax. To stabilize during measurements of rotation, the PT kneeled on the bench behind the participant with their hands on the participant’s shoulders. To stabilize during measurements of lateral flexion, the PT stood in front of the participants and held one hand on the opposite shoulder of the side being flexed.
Isometric strength and endurance in cervical flexion and extension
A fixed dynamometer (Advanced Force Gauge, Mecmesin Ltd, Slinfold, West Sussex, UK) measured the maximal voluntarily contraction (MVC) in cervical flexion and extension in an upright sitting position, similar to Lo Martire et al. (34) (Fig 1 B-C). Participants warmed up using a rowing machine for 7-8 minutes in a self-selected pace, followed by five gradually increasing submaximal isometric contractions against the test leader´s hand in flexion and extension. Participants wore a firm headband which was attached to a dynamometer positioned in line with the centre of the headband. To measure flexion, the back of the participants rested against a rigid square block, their knees were flexed so that only the tips of their toes were in contact with the floor, and a strap over the sternum fixated the arms and thorax. To ensure proper movement and prevent injury, participants kept their chin down (i.e., slight cranio-cervical flexion) to avoid protraction. Up to three submaximal isometric contractions were performed to ensure proper alignment and good position of the fixation and headband. Three trials were performed with a gradual onset of force to maximum for about three seconds with one-minute rests between trials. The gradual increase was intended to avoid injuries and falsely high values (18).The average of the two highest measurements was used as the MVC and were multiplied with the lever arm (measured with a ruler as the vertical distance between the tragus of the ear and the spinous process of C7) to calculate the torque (Nm). After three minutes of rest, they were accustomed with the force needed to obtain 50% of their MVC. The assisting test leader kept the time and noted their perceived fatigue during the endurance test at 15 second intervals using the Borg CR10 scale. The test leader monitored the force and gave verbal feedback to the participant if the force deviated 10 N from the intended torque. The test was interrupted if: (i) the participant was unable to maintain the correct torque after two encouragements by the test leader, (ii) a fatigue rating ³ 7 on the Borg CR10 scale was reached, or (iii) any pain was experienced. To measure the MVC and endurance of the cervical extensors, participants sat on the bench with their sternum against the rigid square and the same procedure as for flexion was performed.
Figure 1 A-C. A: The CROM 3; B and C: Isometric flexor and extensor strength and endurance tests.
Data handling and statistical analyses
Demographic data are presented as relative frequencies or means with standard deviations/min-max. Normal distribution of data was checked visually with bar charts and by skewness and kurtosis. In cases of non-normal distribution, both parametric and non-parametric tests were performed, and parametric results were chosen if similar outcome was achieved. Differences of age, height, weight, flight hours, active cervical ROM and isometric strength and endurance between FP, HP and RC were evaluated with ANOVA (with Bonferroni post-hoc adjustment). Movement control tests were analyzed with Pearson Chi-square tests (with Bonferroni post-hoc adjustment when appropriate). Binary logistic regression analyses presented as odds ratios (OR) with 95% confidence intervals (CI) were performed to investigate movement control, ROM and isometric strength and endurance association with cervico-thoracic pain. First, univariate associations were checked, and factors associated with a p-value < 0.20 were deemed suitable to use in the multivariate regression model. Thereafter, factors associated with cervico-thoracic pain with a p-value > 0.05 were sequentially removed from the model to identify the model of best fit. Confounding, defined as a > 10% change in OR between the adjusted and crude model, was checked for a priori for possible confounders; age, group (i.e., FP, HP and RC) and pain in adjoining body regions (i.e., lumbar [25%]) and shoulder regions [11%]). No such change was evident and thus the crude model was presented. IBM SPSS Statistics for Windows, version 27 (IBM Corp., Armonk, N.Y., USA) was used to analyse the data. A p-value < 0.05 was considered statistically significant.