Study design and subjects
The study subjects were nurses and caregivers who engaged in physical labor on hospital wards (i.e., patient transfer and carrying heavy objects). The recruitment period was from May 14, 2019, to August 21, 2019. Questionnaires and consent forms were distributed to the entire nurse and caregiver (n = 123) in the Musashigaoka Hospital. Nurses and caregivers who did not agree were excluded from the subject to carry out research based on the will of the individuals (n = 25). The study subjects were assigned to the CLBP group and healthy control (HC) group based on the inclusion criteria and exclusion criteria.
The definition of LBP was pain present from a lower rib edge to the gluteal fold [20]. The inclusion criteria were as follows: Male workers aged 20–40 years; LBP duration of > 3 months; and a score ≥ 1 on a numerical rating scale (NRS) for pain intensity during work in the past 4 weeks. Subjects were excluded if they had (1) a previous diagnosis of spinal disease (lumbar disc herniation, lumbar spondylolisthesis, or lumbar osteoarthritis), (2) pain in peripheral joints in an upper or lower limb, (3) the presence of neurological symptoms of a lower limb, (4) serious spinal pathology (cancer, inflammatory arthropathy, or acute vertebral fracture), or a diagnosis of neurological disease. The definition of HC had no history of LBP never before and no other diagnosis illnesses.
This study obtained ethical approval from the institutional ethics committee of Kio University (R2–01) and was conducted in compliance with the Declaration of Helsinki.
Procedure
Before the experimental task, the severity of the subjects’ LBP was assessed with the use of an NRS asking about the subject’s maximum pain intensity in the past 4 weeks, the TSK [21], the Pain Catastrophizing Scale (PCS) [22], the Fremantle Back Awareness Questionnaire (FreBAQ) [23], the Roland-Morris Disability Questionnaire (RDQ) [24], and Von Korff’s grading for the severity of LBP [25] as general measures of pain-related factors. The details of the questionnaires are as follows.
Pain assessment
The subjects’ pain intensity was assessed by an 11-point NRS (0 = no pain and 10 = highest possible degree of pain) to describe the subject’s maximum pain in the past 4 weeks (Pain NRS). The reason for focusing on maximum pain instead of average pain is that in our previous study, the subjects’ maximum pain intensity was indirectly associated with impaired trunk movement [26].
Pain-related psychological assessment
The subjects’ kinesiophobia was assessed by the 11-item Japanese version of the TSK (TSK-11) which shows better internal reliability, identical construction, and known group validity compared to the 17-item version [21]. This assessment was scored on a four-point scale from 1 (strongly disagree) to 4 (strongly agree), and the possible score ranges from 11 to 44; a higher score indicates a higher degree of pain-related fear [21]. For the assessment of catastrophic thinking, we used the four-item version of the PCS (PCS-4), a shorter version of the 13-item PCS [22]. This assessment was scored on a 5-point scale from 0 (not at all) to 4 (all the time), and the possible scores range from 0 to 16; a higher score indicates a higher degree of catastrophic thinking [22]. The PCS-4 was confirmed to have good internal reliability and internal consistency [22]. We used the TSK-11 and PCS-4 in this study because they have properties that are similar to those of the original scales but offer the advantages of brevity.
Body perception assessment
To assess the subjects’ body image of their lower back region, we used the FreBAQ [23], which is a nine-item questionnaire; it is based on a five-point response scale from 0 (never) to 4 (always), and the possible scores range from 0 to 36. A higher score indicates more disturbed perception.
Pain-related disability assessment
The RDQ was used to assess disability directly related to LBP [24]. This assessment is a 24-item questionnaire with a dichotomous scoring format; yes (= item is applicable), or no (= item is not applicable). A high score indicates a higher degree of LBP-related disability. For the evaluation of the severity of the subjects’ LBP, Von Korff’s grading was used as follows: grade 0 = no LBP; grade 1 = LBP that does not interfere with work; grade 2 = LBP that interferes with work but does not cause absences; and grade 3 = LBP that interferes with work, leading to sick leave [25].
After each subject’s assessment by the above-described questionnaires, a movement analysis was performed during a lifting task. We used a lifting task because it is a work-related activity that is widely recognized as a risk factor for LBP and has been used as an experimental task in kinematic studies [27]. After performing the lifting task, our subjects were also asked to complete task-specific questionnaires about pain, discomfort, pain expectation, and pain-related fear with the use of an NRS. These task-specific questionnaires were administered after each lifting condition.
Experimental task
An experimental task that involved lifting an object was used. The subjects were asked to lift a box (520 × 365 × 305 mm) placed on the ground (Fig. 1). The subject’s start position was standing with the feet shoulder-width apart, and the centerline of the box width was placed to match the center of the subject’s feet. The box was placed so that there was no space between the subject’s toes and the box. The reason for these controlled factors is that an earlier study reported that the positional relationship between the subject’s feet and the box affected the low back load during lifting [28].
In the present study, the subjects were asked to initiate lifting the box as quickly as possible upon hearing the start cue, and to lift the box to waist-height. The weight of the box was 10, 30%, or 50% of the subject’s body weight. We used a block design in which the subjects first performed at the 10% of body weight condition and then the 30% of body weight condition, and finally the 50% of body weight condition. We used this design because another study of object-lifting reported that the weight of the object affected the lifting performance that follows [29]. After completing several practices with an unweighted box, our subjects performed the lifting task five times for each weight condition. They had a 1-min rest between trials.
Instrumentation
We recorded trunk kinematic data during the lifting task by using a three-dimensional (3D) motion capture system with a four-charge-coupled device (CCD) camera (KinemaTracer, KisseiComtec, Matsumoto, Japan). The system recorded the displacement of color markers at a sampling frequency of 60 Hz. A total of 12 markers (30 mm in dia.) were attached to parts of the subject’s body and the box as shown in Fig. 1. Using palpation, a physical therapist with 10 years of experience identified the appropriate anatomical landmarks by using the technique suggested in Gray anatomy for students [30] and attached 11 markers to the subject: the thoracic spine (Th12 spinous process), lumbar spine (L3 spinous process), pelvis (S1 spinous process), bilaterally on the iliac crest, great trochanter, lateral femoral epicondyle, and lateral malleolus. The 12th marker was placed on the box.
Coordination analysis
The recorded kinematic data obtained by the 3D motion capture system were low-pass filtered with a second-order recursive Butterworth filter with a cutoff frequency of 6 Hz. To define an upper lumbar angle, a vector was created based on the color markers placed on the Th12 spinous process and the L3 spinous process. A lower lumbar angle was defined as a vector-based on the color markers placed on the L3 spinous process and S1 spinous process. The Upper and lower lumbar angles were calculated using the angle between each of these vectors and the vertical axis.
We divided the time series of trunk movement into flexion and extension phases, and we calculated the trunk coordination pattern of each phase (Fig. 2) [31]. The flexion phase began with the start of trunk flexion motion and ended when the box was raised. The extension phase started when the box was raised and ended when the trunk had resumed an upright position. In accord with previous research, we conducted a relative phase angle analysis by the following procedures [12, 32]. The moment the box left the ground was identified by the vertical axis of the marker attached to the box. The upper and lower lumbar angular displacement and angular velocity data were time-normalized to 100%. Before the plotting of the phase diagram, the angular displacement and angular velocity data were normalized to − 1 to + 1 intervals using the following equation:
$$Normalized\ angle:\left(\left[ angle-\mathit{\min}\ angle\right]/\left[\mathit{\max}\ angle-\mathit{\min}\ angle\right]\right)\times 2-1$$
$$Normalized\ angular\ velocity: angular\ velocity/\mathit{\max}\ angular\ velocity$$
The normalized angular displacements were plotted versus the normalized angular velocity to generate the phase plane for each segment (Fig. 3). For the quantification of the phase plane trajectories, the phase angle was derived using the following equation:
$$\varPhi =\mathit{\tan}^{-1}\left( Normalized\ angular\ velocity/ Normalized\ angle\right)$$
For an index of upper-lower lumbar coupling, we calculated continuous relative phase (CRP) curves. The CRP was defined as the phase angle difference between the upper lumbar and the lower lumbar (i.e., φlower lumbar − φupper lumbar). To compare the CRP curves between the groups and each condition, we calculated the mean absolute relative phase (MARP) values for the upper and lower lumbar. A MARP angle closer to 0° indicates a more in-phase motion pattern between two segments, and a MARP angle closer to 180° suggests a more out-of-phase motion pattern. It was suggested that a more in-phase coordination pattern might indicate increasing protective behavior in the performance of a task [13]. Herein, the MARP was derived using the following equation:
$$MARP={\sum}_{i=1}^P \mid \kern0.50em \varphi CRP \mid i/P$$
To quantify the variability of coordination patterns, we calculated the deviation phase (DP). DP values closer to 0° indicate lower coordination variability or more coordination stability. The DP was derived using the following equation:
$$DP={\sum}_{i=1}^P\ SDi\kern0.5em /P$$
Each variable was averaged for each condition.
Assessment of task-specific pain-related factors
To assess task-specific pain-related factors, we used an NRS (0 = no feeling and 10 = highest possible degree of feeling) that concerns the subject’s maximum pain, discomfort, pain expectation, and pain-related fear that occurred during the lifting task. All of the above assessments were conducted in each (10, 30, 50% body weight) lifting condition (i.e., three times). The subjects were asked to respond verbally to the NRS at the end (not the beginning) of each condition, based on a previous study [33]. We used these retrospective assessments because asking the subjects questions about their pain expectation and pain-related fear before they performed the lifting task might lead them to imagine a context other than the study’s task setting. The subjects were asked the following questions at the end of each (10, 30, 50%) condition: (1) “How much pain in your back did you feel when lifting the box?” (pain), (2) “How much discomfort in your back did you feel when lifting the box?” (discomfort), (3) “How much pain did you anticipate when lifting the box?” (pain expectation) and (4) “How much fear did you feel when lifting the box?” (pain-related fear).
It was reported that such task-specific measures of pain-related factors were more useful for the prediction of a limited lumbar range of motion in CLBP patients compared to general measures of pain-related pain (e.g., the TSK and PCS) [34].
Statistical analyses
The statistical analyses of the results were performed as follows. After using the Shapiro-Wilk test to confirm the normality of the basic information of the subjects (i.e., age, height, and weight), pain-related factors (i.e., the scores on the Pain NRS, TSK-11, PCS-4, RDQ, FreBAQ, and the task-specific measure of pain-related factors), and kinematic factors (the MARP and the DP), we compared these variables between the CLBP group and HC group using the Mann-Whitney U-test and Fisher’s exact test. To compare the difference in task-specific measures of pain-related factors and kinematic factors in each condition and group, we used a two-way repeated-measures analysis of variance (ANOVA). We performed a hierarchical multiple regression analysis to analyze the relationship between kinematic factors and pain-related factors. All statistical analyses were performed using HAD ver. 14.8 [35]. The level of significance for all analyses was set as p < 0.05.
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1.
Mann-Whitney U-test and Fisher’s exact test
We compared the subjects’ age, height, weight, LBP duration, and general measures of pain-related factors (Pain NRS, TSK-11, PCS-4, RDQ, and FreBAQ) between groups using the Mann-Whitney U-test. To compare the occupational category and LBP of severity, we used Fisher’s exact test.
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Two-way repeated measures ANOVA
To compare the differences in task-specific measures of pain-related factors and kinematic factors in each condition and group, we used a two-way ANOVA. The binary factors were weight condition (10, 30, and 50% of body weight) and group (CLBP group and HC group). For post hoc comparisons, the Bonferroni method was used for multiple comparisons.
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Correlation analysis
We used Spearman’s rank correlation coefficients to analyze the relationship between kinematic factors and task-specific measures of pain-related factors. In the correlation analyses, we focused on the variables in which there were significant interactions and main effects.
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4.
Hierarchical multiple regression analysis
A hierarchical multiple regression analysis was performed to test the hypothesis that pain-related fear contributes to the in-phase trunk motor pattern. We used a hierarchical multiple regression analysis because we needed to consider the effects of confounding factors, as impaired motor behavior in LBP patients involves complex interactions of various pain-related factors [14]. We thus performed the hierarchical multiple regression analysis with kinematic variables for which there was a significant interaction and main effect, plus demographic variables (age and LBP duration), general measures of pain-related factors, and the task-specific measurement of pain-related fear.
In model 1, age and LBP duration were the independent variables. In model 2, the general measures of pain-related factors (Pain NRS, TSK-11, PCS-4, FreBAQ) were added to model 1 as independent variables. In both models, we selected factors associated with impaired trunk movement in CLBP that had been reported as independent variables in an earlier study [17, 36, 37]. In model 3, the task-specific measurement of pain-related fear was added to model 2 as an independent variable.