Participants
Participants between the ages of 18 and 65 years were recruited from a university medical campus and surrounding community. The healthy control group included 20 participants who reported no history of neck pain in the last year, and had a Neck Disability Index (NDI) score of less than 5 points at the time of enrollment. This cutoff was selected based on the classification scheme of Vernon and Mior[17] in which fewer than 5 points indicates no neck related disability. The neck pain group included 19 participants with a primary complaint of neck pain for at least 3 months prior to enrollment, and an NDI score of greater than 5 points. In order to generalize the study findings to individuals with a significant restriction of functional activities due to neck pain, only those individuals who met the Neck Pain Task Force[18] definition of grade I or II interfering neck pain with unilateral or bilateral symptoms located between the superior nuchal line and the superior spine of the scapula were included. The neck pain and control groups were matched based on sex, age, and body mass index. Ninety percent of the individuals in each group were right hand dominant. Exclusion criteria included any reported history of central nervous system impairment, signs or symptoms consistent with cervical nerve root compression or other non-musculoskeletal sources of pain, and prior surgery involving the cervical or thoracic spine. Although the majority of participants reported prior episodes of professional or self-treatment of neck symptoms, participants currently under the care of a health care professional for the treatment of neck pain were excluded due to the confounding effects of concurrent treatment on symptom severity. Medications were documented for all participants at the time of testing, and did not change across test sessions. All participants provided written informed consent prior to enrollment, and all study procedures were approved by the local Institutional Review Board.
Examiners
Two licensed physical therapists served as the examiners for this study. Both therapists participated in three, one-hour training sessions to standardize the examination procedures. Rater One had 2 years of clinical experience in an outpatient orthopedic setting, and was enrolled in a certification program for manual therapy. Rater Two had 23 years of clinical experience, with post entry-level certification in hand therapy.
Procedures
A repeated measures design was used to separately assess the inter-rater reliability of cervical and scapulothoracic impairment measures for individuals with and without chronic neck pain. Participants underwent one physical examination by each therapist on two different days in random order. As a safety precaution, therapists were not blinded to group assignment so that any worsening of symptoms could be monitored during the examination. However, all participants were instructed not to provide any clinical information or cues not related to the examination with the therapist prior to each session. Therapists remained blinded to the other rater's findings between examinations. Tests were performed in the same order for each examination, and tests that required multiple measurements by the same rater were performed consecutively with a 30-60s break between measurements. The examination included measures of cervical range of motion, strength, and endurance, as well as scapulothoracic muscle length and strength. Each examination lasted approximately 30 minutes. Sessions were performed at approximately the same time of day for each participant, with an average (SD) of 9 (4) days (range 3–14 days) between sessions.
Cervical active range of motion
Active cervical flexion, extension, and side bending range of motion was assessed using a gravity inclinometer (Medical Research Limited, Leer, U.K) with the participant sitting upright according to procedures described by Cleland et al[8]. Active cervical rotation range of motion was assessed using the same inclinometer with the participant lying supine as previously described by Hoving et al[10]. Three trials were performed in each direction to assess the intra-rater reliability of within-session measurements, and the average of the three measurements from each rater was used to assess the inter-rater reliability between sessions.
Cervical muscle strength
Cervical flexion, extension, and side bending strength was assessed using a HHD (FPIX 100kg load cell, Wagner Instruments, Greenwich, CT). For isometric flexion strength, participants were positioned in supine and asked to hold their head in approximately 30 degrees of flexion with the chin tucked while the examiner applied a force into the direction of cervical extension with the HHD centered on the forehead[19] (Figure1a). For isometric extension strength, participants were positioned in prone with the shoulders supported at the edge of the examination table and the head held against gravity just beyond the edge of the table. Participants were asked to hold their head in a neutral position while the examiner provided a force into the direction of cervical flexion with the HHD centered on the back of the head (Figure1b). Based on pilot testing, which revealed poor trunk and head stabilization in sitting and side lying during the assessment of isometric side bending strength, this outcome was assessed in supine. Participants were instructed to maintain a neutral head position with the back of the head resting against the examination table, while the examiner provided a force into the direction of side bending with the HHD centered on the contralateral side of the head (Figure1c). Participants were stabilized in supine using a 4-inch velcro strap placed across the chest at the level of the sixth thoracic vertebrae (T6) and across the pelvis at the level of the anterior superior iliac spine to prevent movement of the upper body as force was applied to the head. For all isometric force measurements, manual resistance was applied at a rate of approximately 3 kg∙F/s and the maximum force recorded by the dynamometer while the participant was still able to maintain the test position was considered the maximum isometric force. Only one maximum strength test was performed in each direction to minimize the potential for reduced force output with increased cervical pain due to repeated testing. Single trial results were used to assess the inter-rater reliability of isometric cervical strength measurements between sessions.
Cervical muscle endurance
The isometric endurance of cervical flexors was assessed as described by Cleland et al[8] with participants positioned in supine, the upper cervical spine flexed, and the head held approximately 1 inch above the examination table (Figure2a). Participants were asked to maintain this head position for as long as possible. The endurance test was terminated when participants were no longer able to keep their head from touching the table, or when upper cervical flexion could not be maintained. Loss of craniocervical flexion was assessed by observing the position of the chin and the skin fold produced posterior to the mandible when the head was placed in the test position. A change in the thickness of this skin fold or visible motion of the chin was interpreted as a loss of craniocervical flexion, resulting in termination of the endurance test.
The isometric endurance of cervical extensors was assessed as described by Edmondston et al[20] with participants positioned in prone, the head held in a neutral position just beyond the edge of the examination table, and both arms at the sides with a 4-inch stabilization strap across the thoracic spine at the level of T6. A 2-inch velcro band was secured around the head with a fluid inclinometer placed over the occiput. A 2 kg weight was suspended from the headband, and participants were asked to support the weight while maintaining a neutral head position for as long as possible (Figure2b). The endurance test was terminated when the position of the head changed by more than 5 degrees from the horizontal, or a maximum endurance time of 5 minutes was achieved. Endurance time was measured using a handheld stopwatch. One trial was performed for each direction to minimize the potential for increased cervical pain with repeated testing, and this value was used to assess the inter-rater reliability of cervical muscle endurance between sessions.
Scapulothoracic muscle strength
Isometric strength of the middle trapezius, rhomboids, and lower trapezius muscles was tested bilaterally with participants lying prone and the HHD placed one inch proximal to the lateral epicondyle of the elbow. Participants were stabilized in this position using 4-inch velcro straps placed across the pelvis at the level of the posterior superior iliac spine and across the thighs just proximal to the knee joint. Test positions for each muscle were performed according to descriptions provided by Kendall[12] (Figure3a-c). Manual resistance was applied at a rate of approximately 3 kg∙F/s, and the maximum force recorded by the dynamometer while the participant was still able to maintain the test position was considered the maximum isometric force. Pilot testing revealed that three maximal effort trials of the scapulothoracic musculature were well tolerated by participants without an increase in primary cervical symptoms. Therefore, three trials were performed for each muscle with at least 60 seconds rest between trials to assess the intra-rater reliability of within-session measurements. The average of the three trials for each rater was used to assess the inter-rater reliability of isometric scapulothoracic muscle strength between sessions.
Scapulothoracic muscle length
Length of the latissimus dorsi and pectoralis minor muscles was estimated bilaterally using a standard ruler in the test positions described by Kendall[12] and others[8]. These muscles were selected for length testing based on their suggested effects on alignment and movement of the scapulothoracic region[15] and for comparison to a previous study[8]. Latissimus dorsi length was estimated as the distance from the lateral epicondyle to the surface of the examination table with the upper arms positioned in maximal flexion as participants lay supine with their knees bent and the lumbar spine in full contact with the table (Figure4a). Resting length of the pectoralis minor was estimated as the distance from the posterolateral aspect of the acromion to the surface of the examination table with participants resting in supine and both arms at the sides (Figure4b). Only one measurement was performed for each muscle to avoid changes in muscle length that can occur with repeated movements of viscoelastic tissues. Single trial results were used to assess the inter-rater reliability of muscle length measurements between sessions.
Data analysis
Descriptive statistics were calculated as means and standard deviations (SD). After confirming a normal distribution of all impairment measures using the Komologrov-Smirnoff test, point estimates and 95% confidence intervals (CI95) were calculated using intraclass correlation coefficients (ICC) to determine the intra-rater reliability of cervical range of motion and scapulothoracic strength measures obtained by each rater within the same session based on a two way mixed model for absolute agreement (ICC(3,1)). Inter-rater reliability across the two test sessions was calculated using a two way random effects model for absolute agreement. The average of three trials for cervical range of motion and scapulothoracic strength measures was assessed using ICC(2,3), whereas single measurements of cervical muscle strength, cervical muscle endurance, and scapulothoracic muscle length were assessed using ICC(2,1). The level of reliability is qualitatively described in the text according to Landis and Koch[21] with ICC values of 0.0-0.20 indicating slight agreement, 0.21-0.40 indicating fair agreement, 0.41-0.60 indicating moderate agreement, 0.61-0.80 indicating substantial agreement, and 0.81-1.00 indicating almost perfect agreement. ICC values were considered to indicate no significant agreement if the 95% CI included zero.
To determine the smallest change in each impairment measure that can confidently be considered to exceed measurement error at a 95% confidence level, the Minimum Detectable Change (MDC) was calculated according to the following formula[22]: MDC = 1.96 · √2 · SD · √(1 − test–retest reliability coefficient). MDC values were calculated separately for the neck pain and healthy control groups. Independent t-tests were used to compare cervical and scapulothoracic impairment measures between the neck pain and healthy groups, with each outcome computed as the average value across both raters. A 3-way ANOVA (group х rater х order) with repeated measures on the latter two factors was performed to identify any effects of the rater (Rater 1 vs. Rater 2) or test order (session 1 vs. session 2) on impairment measures. All statistical analyses were performed using SPSS software v.16.0.1 (Chicago, IL, USA).