1. Participants
The study participants were 66 students from Hiroshima University who had completed the measurements (42 men, 24 women; mean age of 21.7±1.60).
2. Study design
On the first day of the study, Day 1, after informed consent to participation in the study was obtained, accelerometers were distributed. Participants were instructed how to fill in the activity record. The participants wore an accelerometer for 7 days; on day 8, the accelerometer was returned and questionnaires on psychological indices were distributed, completed, and then collected. The activity record of 9 PM the preceding day to 9 PM on the day was submitted by e-mail from 9 PM to 0 AM. Of the consenting participants, 58 were included after exclusion of 8 patients whose physical activity data were insufficient, with 20% or more data missing because they had not worn the device for 33.6 hr or more.
3. Measurements
3-1. Amount of physical activity
Using a wrist-worn tri-axial accelerometer device (UW-301BT Life Log; Hitachi Ltd., Tokyo), we measured metabolic equivalents (METs), an indicator of physical activity intensity, per minute from the resultant signal. Participants were instructed to wear the accelerometer continuously on the wrist of the nondominant arm, except during bathing. Because participants started wearing the device at different times of Day 1, data of 165 hr in all from 0 AM on Day 2 to 9 PM on Day 8 were used for analyses. The total METs values per hour were calculated as the amount of physical activity for the hour. When 0 of METs per minute was included in the hour, the amount of physical activity for the hour was referred as a missing value without calculation.
3-2. Pleasure from the activity record
During the experimental period with the accelerometer, participants recorded their activities per hour into an activity record of electronic file and rated the amount of pleasure from each activity on a scale of 10. To minimize failures in recording them, participants were instructed to send the table for the day by e-mail at a scheduled time (from PM 9 to AM 0) every day.
3-3. Psychological indexes
Participants answered the following questionnaires on Day 8 when they finished the measurement of physical activity and had filled out the activity record.
i. Japanese version of the Beck Depression Inventory – Second Edition (BDI-II)9
The BDI-II is the most widely used self-report inventory to measure depression severity. Scores of 0–63 and higher indicate a higher severity of depression10.
ii. Japanese version of Behavioral Activation for Depression Scale (BADS)11
The BADS was invented to measure depression-related behaviors and dysfunction in the context of behavioral activation therapy12. It comprises four subscales of Activation (AC), Avoidance/Rumination (AR), Work/School Impairment (WS), and Social Impairment (SI). Its reliability and validity have been documented.
4. Calculation of the index for Physical Activity – Pleasure
To assess the association between the amount of hourly physical activities (PA) from the accelerometer and the hourly pleasure (PL) level, which is recorded subjectively with the activity record, the correlation coefficient between them for each hour was calculated for each person as an index for Physical Activity – Pleasure (PA-PL). Pleasure levels for the activity of sleeping in the activity record were all corrected to 0. Figure 1 shows the time-series plot of PA and PL of representative samples along with the value of their PA-PL index.
5. Statistical analysis
To examine how the calculated PA-PL index affects individual behavior characteristics, we performed multiple linear regression analysis using the PA-PL index as the explanatory variable and the scores of BADS subscales as dependent variables. The scores of BDI-II, age, and gender were included as covariates for the analysis. All analyses were performed using “statsmodels” pakage (ver.0.11.0), a library for statistical analysis in Python.
6. Ethical consideration
We conducted the study after providing all participants a written explanation describing the study purpose, the voluntary nature of participants’ participation, the confidentiality of the participants’ personal information, and the publication of the study results. All participants provided their written informed consent. This study protocol was approved by the Ethics Committee for Epidemiological Research of Hiroshima University and adhered to relevant ethical guidelines.