Participants
In this cross-sectional, observational study, we approached all staff members (n = 470) of our general hospital (198 beds) between May and July of 2020. Informed consent from all participants was documented in writing. A self-administered questionnaire about katakori and related daily life was concurrently distributed to all staff members and collected within 2 weeks for the primary survey. Staff included nurses, medical office workers, physiotherapists, nursing assistants, doctors, and others. Exclusion criteria included being on leave during the survey period. For a subset of paired subjects with severe katakori and without katakori, propensity score matching was used for a secondary survey within 2 months after the primary survey.
The study protocol was examined by and received approval (#20010801) from our hospital’s ethics committee, which is guided by local policy, national law, the World Medical Association Declaration of Helsinki. This study was also registered in Japan’s national clinical trial site (UMIN000040483).
Assessment
The participants were initially asked to complete all questions specifically intended to assess individual characteristics (i.e., age, sex, body mass index [BMI], smoking habit, daily time using computers for work, daily time using mobile devices not work-related, physical and mental burdens in relation to work, regular exercise, and sleep satisfaction). BMI was calculated using self-reported weight and height. Smoking habit was dichotomized by the current status as Yes or No. Daily hours using occupational computers and non-occupational mobile devices were grouped into three categories (<3, 3-6, >6 hours). Levels of physical and mental burden could be Mild, Moderate, or Severe in response to the question, “How do you rate the physical and mental burden of your work?” Regular exercise habits were defined as continued exercise of at least 30 minutes/day on at least 2 days/week for at least a year, according to the Japanese Ministry of Health, Labour and Welfare. Sleep was assessed with the question, “How do you rate your sleep satisfaction?” Allowable answers were Satisfied, Not Satisfied, or Neither.
To assess katakori, the primary survey question solicited subjective assessment of bilateral or unilateral symptoms by frequency as Constant, Occasional, or None during the past 3 months. Subjects with katakori symptoms used a numerical rating scale (NRS) to describe the worst symptoms they experienced during the past month. The 11-point scale ranged from 0 = no symptoms to 10 = worst imaginable symptoms. In the primary survey, factors related to the onset of katakori (Constant or Occasional) were investigated in comparison with a reference group without katakori.
As a secondary survey, propensity score-matched pairs were selected from a group with bilateral severe katakori (Constant;NRS ≥ 8) and a control group without symptoms, to adjust for individual characteristics. To evaluate the stiffness of the upper trapezius muscle directly, an ultrasonic diagnostic apparatus (HI VISION Avius®;Hitachi, Ltd., Tokyo, Japan), equipped for real-time tissue elastography with a linear array transducer, was used to measure muscle elasticity between 3:00 pm and 5:00 pm on a subject’s work day. An acoustic coupler was connected to the transducer via a plastic attachment as a reference for strain in the control individuals. The transducer was placed on the superior trapezius, corresponding to the middle point of the spina scapulae in the sitting position. Strain was induced by repetitive light compression with the transducer. Bilateral muscle strain was measured by a physician, masked from participants’ survey data, twice per side. The strain ratio (control reference/muscle) was automatically calculated and was used to compare muscle elasticity (the higher the strain ratio, the stiffer the muscle). The average of all obtained values for each subject was calculated. In our pilot study with 10 volunteers (5 with and 5 without katakori), the repeatability of ultrasonic measurements was determined by the intraclass correlation coefficient (ICC1,2). The ICC was 0.76 (95% confidence interval [CI]:0.49-0.90), indicating good reliability of this method21).
Limb/trunk muscle volume was measured with a bioelectrical impedance analysis (BIA) device (InBody 770® body composition analyzer; InBody Co., Ltd., Seoul, Korea) in the standing position. Muscle mass was measured as tissue volume excluding fat and bone mass. Limb/trunk muscle masses were determined directly from lean mass values provided by the device. Limb/trunk muscle mass indices were acquired by dividing each muscle mass by body height squared (kg/m2) and were compared between the severe katakori and non-katakori groups.
Statistical analysis
The primary analysis included all participants who responded to the questionnaire. Continuous variables, categorical variables, and ordered variables are presented as means ± standard deviation, stand-alone numbers, and medians with interquartile range, respectively. Demographic data were assessed using the χ2 test, unpaired t-test, and Mann-Whitney U test. Factors related to katakori were analyzed using single logistic regression. Results were initially reported as crude odds ratios (ORs) with 95% CIs. Furthermore, multiple logistic regression adjusted for the following independent variables:age, sex, BMI, smoking habit, computer use time, mobile device use time, physical burden, mental burden, regular exercise, and sleep.
In the secondary survey, we used 1:1 nearest-neighbor propensity score matching, based on the 10 aforementioned characterization variables, to create well-matched groups, inasmuch as the primary survey was an unmatched, case-control analysis of all participants. Differences in strain ratio and trunk muscle mass were compared between severe katakori and non-katakori groups using the non-paired t-test for data with a normal distribution and the Mann-Whitney U test for non-normally distributed data.
Statistical significance was set at P < 0.05. Data were analyzed using the freely available EZR (Easy R, ver. 1.40, Jichi Medical University, Kanda Yoshinobu).