Subjects selection
Thirty subjects with primary subacromial SIS (mean age 47.9 years; 20 women, 10 men) and twenty healthy subjects (mean age 46.6 years; 13 women, 7 men) voluntarily participated in the study. All subjects with SIS were screened by an orthopaedic surgeon to rule out calcifications, shoulder instability and rotator cuff tear. They were considered eligible if there was at least one positive finding in each of these categories [15]: 1) painful arc of movement during flexion or abduction; 2) positive Neer or Kennedy-Hawkins impingement signs; 3) pain on resisted lateral rotation, abduction or Jobe test. Other exclusion criteria were previous shoulder surgery, shoulder pain during neck movement and shoulder capsulitis. The healthy subjects had no history of pain, movement limitation or previous surgery to the shoulder and neck. All the participants read and signed an informed consent form. This study was approved by the Ethics Committee of the Quebec Rehabilitation Institute.
Study design and experimental procedures
All 30 subjects with SIS and 20 healthy subjects participated in the first measurement session and contributed to the evaluation of intrasession reliability. Ten healthy subjects were retested within 2 to 7 days to assess intersession reliability (mean 3.5 days). The subjects with SIS were participating in an intervention study following the first measurement session; therefore it was impossible to retest them without any intervention between measurement sessions. Subjects have to remain stable in time in order to evaluate reliability. Evaluators were blinded to the data from the first session when retesting.
At each measurement session, the kinematic patterns of the upper extremity was evaluated during a tasks that consisted of reaching out and pointing (with contact) to a target. The subjects were seated with their knees and hips at 90°, their feet flat on the floor and their lumbar spine supported. The reaching movements started with the upper extremity in a neutral position at the side of the body and the tip of the second finger in contact with a pressure switch (Figure 1). An auditory cue signalled the beginning of the movement. The target was located in the frontal plane and positioned at a distance equivalent to the subject's arm length and at a height equivalent to the position of the second finger when the shoulder was at 90° of abduction (Figure 2). It was a round target and the diameter of its center (bull's-eye) was 4.5 cm (Figures 1 and 2). With their second finger, the subjects had to point the center of the target. A pressure switch was also placed under the center of the target to signal the end of reaching. Reaching was performed at a natural speed, as if the subjects were performing daily life activities. Since it was necessary to have an end signal to analyse the data, if subjects overshot or did not touch the center of the target, the trial was cancelled and repeated afterward. The movement was practiced three times before data acquisition. Ten trials were recorded. A rest period of 30 seconds was given after each trial. The symptomatic arm was evaluated for the subjects with SIS. For the healthy subjects, the side was chosen to have the same proportion of dominant/non-dominant sides as evaluated in the SIS group.
Measurements
The upper extremity kinematics were characterized by the patterns of relative joint angles for the hand, elbow, shoulder, clavicle, and trunk. The Optotrak System (Northern Digital inc., Waterloo, Ontario, Canada) is an optoelectric system that was used to collect the 3-dimensional kinematic data of the upper extremity and the trunk (Figure 1). Three Optotrak 3020 position sensors were used. Triads of infrared light-emitting diodes were positioned on the hand (dorsal face), forearm (proximal to the styloid process of the radius), upper-arm (near the insertion of the deltoid), clavicle (lateral part of the clavicle) and trunk (top of the sternum) (Figure 2). A sampling rate of 100 Hz was used and data were digitally low-pass filtered at 8 Hz. As seen on Figures 1 and 2, the electromyography (EMG) activity of seven shoulder muscles was also recorded during the reaching task. However, the data of the EMG activity were not analyzed for this study.
Fourteen bony landmarks were digitized before the acquisition of data to recreate the coordinate systems [16]: C7 and T8 spinous processes, suprasternal notch and xiphoid process for the trunk; most ventral point on the sternoclavicular joint and most dorsal point on the acromioclavicular joint for the clavicle; most caudal point on the lateral and medial epicondyles for the humerus; root of the spine, inferior angle, acromial angle and most ventral point of processus coracoideus for the scapula (in order to estimate the glenohumeral rotation center by regression analysis [17]); and, most caudal-lateral and caudal-medial points on the radial and ulnar styloids for the forearm. Local coordinate systems and joint rotations were defined according to the Interntional Society of Biomechanics (ISB) recommendations [16]. The ISB recommends using the Grood and Suntay's convention to calculate joint movements of the upper extremity, except for the shoulder where a Cardanic y-x-y rotation matrix sequence was used [18].
In order to compare measurement, two periods (auditory cue to beginning of the movement and beginning to end of the movement) of 100 points were defined for the kinematics, with each point representing one percent of the period. Movement amplitudes were plotted for two degrees of freedom (DF) of the wrist (hand relative to the forearm: flexion/extension; deviation), one DF of the elbow (forearm relative to the arm: flexion/extension), three DF of the shoulder (humerus relative to the trunk: plane of elevation; elevation; rotation), two DF of the sternoclavicular joint (clavicle relative to the trunk: retraction/protraction; elevation/depression) and three DF of the trunk (trunk relative to the global system: flexion/extension; rotation; lateral flexion) [16]. Thereafter, the upper extremity angular position at the end of reaching and total joint excursion that occurred during reaching (difference between minimum and maximum absolute angles) were calculated in degrees.
Data analysis
The reliability of the joint position at the end of reaching and of the total excursion was calculated for each joint evaluated. For the healthy and SIS subjects, the level of intrasession reliability was analyzed by comparing the first two trials, the first five trials and then, the last five trials. For the healthy subjects, the level of intersession reliability was analyzed by comparing the mean of the first two trials of the first session to the mean of first two trials of the second session. The same intersession comparison was done for the means of the respective first five trials and ten trials. The relative reliability was estimated by calculating the intraclass correlation coefficients (ICCs) and its 95% confidence interval (95% CI) [19, 20]. ICCs values were considered to reflect: a poor reliability when below 0.20; a fair reliability from 0.21 to 0.40; a moderate reliability from 0.41 to 0.60; a good reliability from 0.61 to 0.80 and, a very good reliability from 0.81 to 1.00 [21]. The absolute reliability was calculated with standard errors of measurement (SEM) and its 95%CI, and minimal detectable change (MDC) [22]. The MDC was calculated by multiplying the z-score corresponding to the level of significance, the square root of 2, and the SEM [23]. A z-score of 1.65 was chosen to reflect an acceptable 90% confidence level for clinical application to individual patients [23]. Significant differences in reliability between groups and between numbers of trials were determined when the 95% CI of the ICC or the SEM were not overlapping. All analyses were conducted with the SPSS software (Version 12; SPSS Inc, 233 S Wacker Dr, 11th Fl, Chicago, IL 60606). The alpha level was set at 0.05.