We employed a prospective cohort design. Baseline data were collected in June 2015. The follow-up survey was carried out one year later, in June 2016.
A convenience sample was used. To recruit the baseline participants, we invited the employees of 36 daycare facilities located in Nagoya City, Japan, and its suburban area. There are 527 authorized daycare facilities in Nagoya City as of April 2022 . Although we did not sum up the exact number of workers in the 36 daycare facilities, we were reported that approximately 20 full-time workers and 20 part-time workers were averagely working in each daycare facility. Therefore, the total number of eligible participants was estimated 1440. To reduce costs, we did not distribute the questionnaire to all workers but only to those who were willing to participate in this study. Of the 36 daycare facilities, 34 had at least one respondent, while 2 had no respondent. The total number of respondents was 446, which resulted in an estimated response rate of approximately 30%. After excluding two participants without a valid response regarding LBP, we analyzed 444 subjects. Of them, 361 people (81.3%) participated in the follow-up survey.
The variables in this study were as follows. All the data were collected through a self-administered questionnaire.
Definition of LBP
The question “Where are you currently feeling LBP?” was used to determine whether the participants suffered from LBP both in the baseline and follow-up surveys. We provided the following five answer options to specify the location of pain: (1) feeling pain in the lower back only, (2) feeling pain in the lower and upper back, (3) feeling pain that extends to the hips and/or buttocks, (4) feeling pain and numbness that extends to the legs and feet, and (5) feeling pain in the shoulders, neck, and/or arms as well as the lower back. The participants were asked to answer “yes” or “no” to each option. Those who answered “yes” to any option were defined as subjects suffering from LBP. Those who answered “no” to all five options were defined as not suffering from LBP. The subjects who had LBP both in 2015 and 2016 were defined as suffering from persistent LBP. Those who did not have LBP in 2015 but suffered from LBP in 2016 were defined as experiencing the onset of LBP.
Pain intensity was assessed by the numerical rating scale (NRS) score [22, 23]. The NRS is a measurement instrument used to determine the degree of pain that ranges over a continuum of values and cannot be directly measured. In this study, the degree of LBP ranged from 0 to 10, as assessed by the NRS, where 0 indicated that no pain was felt, whereas 10 indicated the worst pain imaginable by the subject.
Disability in daily life due to LBP
The Roland-Morris Disability Questionnaire (RDQ) is a scale that was used to examine whether LBP affected daily activities such as standing, walking, sitting, dressing, and working. It comprised 24 items. The responses were “yes” and “no” for each item. The cumulative number of “yes” responses indicated the score, which ranged from 0 to 24. The higher the score, the worse the degree of disability in daily life. In this study, we used the Japanese version of the RDQ .
Impact of LBP on work
We examined the impact of LBP on work. The subject chose one of the following six options: (1) I cannot work without taking occasional days off; (2) I cannot work without taking an occasional break; (3) It hurts a lot, but I do not need to take a break; (4) I feel slight pain occasionally; (5) I would like a break or day off, but I cannot; and, (6) I do not have severe pain.
Psychosocial work characteristics
We employed the JDCS and ERI models to assess the psychosocial work characteristics. Both models are theory-based conceptual models for the assessment of adverse psychosocial job stressors.
Using the Japanese version of the Swedish Demand-Control-Support Questionnaire , we quantified job strain and social support of the subjects. Job strain was defined as the ratio of psychological demand to decision latitude. Referring to previous studies [26, 27], we defined the subjects in the top quartile of job strain scores as being exposed to high job strain and those with a social support score lower than the median as receiving low social support.
Using the Japanese version of the ERI questionnaire , we assessed whether the subjects were experiencing ERI and overcommitment. Those who had an effort/reward ratio score of 1.0 or higher were defined as experiencing ERI . Those in the upper tertile of overcommitment scores were defined as being overcommitted to work.
We collected information about the subjects’ sex, age, body mass index (BMI), smoking, and working conditions. BMI was calculated using the self-reported heights and weights. The working conditions included employment status (regular employment or not), occupation (teachers, cooks/nutritionists, and others), and working schedule (irregular or not).
First, we assessed the baseline characteristics of the subjects. Then, we examined the prospective relationships between the psychosocial work characteristics at baseline and LBP one year later using the follow-up survey data. We classified the subjects by the presence/absence of LBP at baseline to differentiate the effects of the psychosocial work characteristics on the persistence of LBP from those on the onset of LBP. For the comparisons, after calculating the prevalence of the persistence/onset of LBP, we used multiple logistic regression analyses to calculate the relevant odds ratios, adjusting for age, sex, BMI, smoking, employment status, occupation, and working schedule. As we could not follow-up all the subjects, we compared the baseline characteristics between those who did and did not participate in the 2016 survey. IBM SPSS 28 was used for the statistical calculations. P-values less than 0.05 were regarded as statistically significant.