Participants
Sample size for the reliability study was estimated in order to obtain a target reliability coefficient (ICC) of 0.80 (95 % confidence interval: 0.6–1.0). Based on five experimental trials over two individual testing sessions, a minimum of 28 participants was needed considering the abovementioned requirements [13]. The study conducted by Lee et al. [14] is the only one reporting a statistically significant difference between both groups with regard to MPT in axial rotation, and using 24 patients with LBP and 24 control subjects. Based on the study conducted by Lee et al. [14] the present study have been conservative with a sample size of 29 patients with LBP and 30 control subjects. All participants were aged between 18 and 65 years old, and were recruited through newspaper advertisements and from physiotherapy clinics in Montreal, Quebec, Canada. All participants spoke French and English. Patients with LBP had lumbar or lumbosacral pain, with or without radicular pain, for at least four weeks (non-acute phase), and a score above 12 % on the Oswestry Disability Index (ODI) [15] to allow a minimal important change of 10 % [16] to occur. Patients with non-acute LBP were targeted because exercise is not a primary intervention strategy for acute LBP [1]. General exclusion criteria were: surgery of the pelvis or spinal column; a specific lumbar pathology (fracture, infection or tumor) or scoliosis; systemic or degenerative disease; body mass index over 30 kg/m2; having begun an exercise program in the last three months; pregnancy and claustrophobia. Additional exclusion criteria for patients with LBP were: having one positive neurological sign in two of three test categories: (1) Achilles and patellar tendon reflexes; (2) Reduced strength in myotomes; (3) Reduced sensation in dermatomes; and litigation relative to the back injury. Exclusion criteria for the control subjects were back pain in the preceding year or a history of back pain lasting more than 1 week.
Before testing, each participant was informed of all experimental procedures and provided their informed written consent. All procedures were approved by the ethics committees of the Centre for Interdisciplinary Research in Rehabilitation of Greater Montreal (CRIR) (ethical registration number: CRIR-738-0512).
Lumbar stabilization program
An 8-week lumbar stabilization exercise program (two 30-min sessions/week) was provided to the patients with LBP in local physiotherapy clinics, without any co-intervention allowed. The patients were encouraged to do the exercises at home. Briefly, the exercise program focused on motor control of the deep trunk muscles [17], followed by gradual inclusion of overloading exercises designed to improve endurance and strength of the paraspinal and abdominal muscles [18] (further details in the Additional file 1).
Questionnaires
The ODI was used to assess LBP-related disability [15] while the 11-point (0 to 10) numeric pain rating scale (NPRS) was used to assess the current, best and worst levels of pain intensity during the last week, so as to average the three ratings [19].
Lumbar proprioception assessment
Lumbar motion sense, or motion perception threshold (MPT), was evaluated using a custom built apparatus similar to ones used in previous studies [14, 20] (Fig. 1). The MPT test measured the smallest amount of axial trunk rotation a participant could perceive. This measure was preferred over the measure of lumbar joint position sense because previous findings suggest that this test better discriminates between patients and control subjects [14], and produces more reliable data [20]. During the test, the lumbar spine is passively rotated in the transverse plane (trunk rotation) by rotating the lower body (seat). The seat is positioned on a 16-inches diameter high quality ball bearing (Silverthin Bearing Group, Preston, WA, USA; model SG160CPO) designed to minimize vibration. Movement is driven by a stepper motor (Applied Motion Products, Watsonville, CA, USA; model HT34-504) at a slow, steady rate to minimize tactile cueing. The resolution of the angular measurement was 0.09°.
Participants were positioned in the apparatus so that the L5/S1 joint aligned with the stepper motor shaft. The participant’s upper body was secured to the backrest with a 4-point seatbelt to minimize vestibular feedback. Starting from the neutral (zero) position, the seat was rotated, either clockwise or counter-clockwise, at a constant rate of 0.2°/s. As soon as motion was perceived, participants stopped the rotation by pressing a switch, and immediately stated the direction of movement. To remove trials where the subject would have guessed the motion perceptions, the trial was rejected if the direction perceived did not correspond to the true direction [14]. This was done in an attempt to reduce potential noise in the data. Participants were returned to the neutral position following each trial. All trials were performed with eyes closed and noise-canceling headphones.
Prior to testing, participants were given five familiarization trials (or more; until the task was properly understood), randomized for direction. This was followed by ten experimental trials also randomized for direction (five trials per direction).
All testing was done at the start of the study (session 1) and after 8 weeks (session 2).
Home-exercise adherence
Home-exercise adherence was assessed with one question at the end of the 8-week clinical program: How many times have you done your exercises as prescribed in the last week? The frequency per week was divided by the recommendation of the physiotherapist to obtain a ratio, in accordance with the most common definition of adherence, which is defined as “the extent to which the patient follows medical instructions” [21]. The ratio may vary between 0 and 1, one being given when the frequency was equal or higher than prescription. The measurement of adherence was not carried out during the course of the 8-week clinical program to avoid influencing positively the adherence behavior (e.g. desirability bias).
Data analysis
MPT scores were expressed as absolute values, in degrees. No significant differences between left and right rotations were detected, so the data were combined. Outliers (outside the 2-interquartile range) were removed from the 10 MPT scores, to reduce variability. The mean of MPT value, for each participant, was used for analysis.
Reliability statistical analyses
The reliability of the MPT data from the 30 control subjects was assessed within the generalizability theory framework [22], using a 2-way (2 DAY × 10 TRIALS) ANOVA for repeated measures. The computed sources of variance were used to calculate the dependability coefficients (ϕ) and standard error of measurement (SEM) [23]. D-study (decision study) results are reported, based on averaging data from 1, 5 and 10 trials within the same testing session (averaging across days is impractical), and may be interpreted in the same manner as an intraclass correlation coefficient [24].
Between-group and between-session statistical analyses
The healthy subjects whom participated to the reliability study were also used as a “control” group here. This allows testing whether the patients with LBP had proprioceptive deficits at baseline and also allows to estimate if systematic changes that could be detected during the treatment in the patients are attributable to the treatment of to the learning of the measurement protocol. MPT was assessed using a 3-way ANOVA (GROUP: LBP and control subjects; SEX: male and female; DAY: session 1 and 2). Significant interactions or main effects were further analyzed using a post hoc Tukey-Kramer test.
In patients with LBP, clinical outcome measures (NPRS, ODI) were assessed using a 2-way ANOVA (SEX; DAY). Partial Pearson’s correlations were also carried out between the change (session 2–session 1) of the MPT measure and the corresponding change of the clinical outcome measures, accounting for the baseline (session 1) MPT measure.
The analyses above were done with NCSS statistical software (version 8.0 for Windows), with the significance level set at P < 0.05.