Study design and participants
This study was a randomized study in which participants were recruited by handing out leaflets in several different regions in Kumamoto, Japan. Forty-four older women over 65 years of age were recruited between August 2017 and September 2017.
After completing measurements before the intervention, participants drew a sealed envelope to determine whether they were allocated to the financial incentive group (FI group) or the FI plus social network incentive group (SNI+FI group).
During the intervention, two participants in the FI group and three in the SNI+FI group dropped out. Eventually, the study groups comprised 21 participants in the FI group and 18 participants in the SNI+FI group. The study flowchart is presented in Fig. 1.
Each participant in the FI group received a payment ranging from US$4.40 to US$6.20 per month, depending on the number of steps taken per day during the intervention. The SNI+FI group participants walked in groups of three people to utilize the power of peer pressure, in addition to receiving the financial incentive. The three people chose each other among the participants in the SNI + FI group after grouping by randomization. The groups of buddies remained the same over the three-month intervention in this study. The participants were informed that they would walk in groups of three people about once a week. Furthermore, their rewards were designed to reflect the largest number of steps taken among their “buddies.” The payment rewards ranged from US$4.40 to US$6.20 per month and were available for both groups. After the three-month intervention, the participants were not asked to walk with their peers. However, they could decide for themselves whether to walk or not.
Both groups underwent a three-month intervention between September 2017 and December 2017. Before the intervention, each group was assessed for age, body height, body weight, body mass index, and percentage of body fat. Measurements of physical activity and social capital were performed before and after each intervention. Additionally, the effects of the incentives on activity maintenance were measured six months after the intervention.
All participants provided written informed consent to participate in the study, which was approved by the ethics committee of Osaka Sangyo University (2017-JINRIN-016).
Blinding
Those assessing the outcomes were blinded to the grouping allocation; however, owing to the nature of the intervention, participants were not blind to their allocation. The FI group participants did not know about the SNI+FI group reward structure.
Anthropometric measures
Body height was measured to the nearest 0.1 cm. Body weight was measured to the nearest 0.1 kg using a digital scale. Body mass index (BMI) was calculated using the formula BMI = body mass (kg)/ (body height [m])2. The percentage of body fat was calculated using the formula: adult body fat% = (1.20×BMI) + (0.23×age) – (10.8×sex)-5.4 [34].
Physical activity
Before starting the study, a pedometer (EX-500, YAMASA TOKEI KEIKI CO., LTD, Tokyo, Japan) was given to each participant to measure the number of steps per day. Each participant also received a diary to record their daily step count (pedometer). For the evaluation of the number of steps, the average daily step count for one month was calculated.
Social capital
Social capital was assessed in both the cognitive and structural domains. Cognitive social capital was assessed by inquiring about trust and norms. Trust was assessed by a single item: “Generally speaking, would you think that most people can be trusted?” The responses were selected using a Likert scale [33]. From a previous study by the Japan Science and Technology Agency Index of Competence (JST-IC), the norm was assessed using four items of social participation from the JST-IC [35,36]. The four items were as follows: (1) Participate in regional events; (2) Participate in a neighborhood association; (3) Assume a managerial position or role such as the leader in a residents’ association; and (4) Engage in charity. These items were assessed using Yes=1/No=2, and the points were summed. Structural social capital was assessed by inquiring about the network using two questions. The first question was regarding “Relationship with neighbors.” This question included a four-point scale (none; would greet; would talk while standing; would consult with life concerns). The second question concerned the “Number of neighbors with whom one has a relationship.” This was also a four-point scale with the following possible answers: zero, four or fewer people, five to nineteen people, and twenty people or more.
For social capital, individual indexes were calculated by standardizing each item.
Statistical analysis
Data were analyzed using SPSS Statistics 20.0 (IBM Corporation, Tokyo, Japan). All descriptive and statistical data are shown as the mean ± SD. An unpaired t-test was used to compare the differences in age, body height, body weight, BMI, and percentage of body fat between the FI group and SNI + FI group before the intervention. Two-way repeated-measures ANOVA was conducted to compare the effects of the intervention and the six-month postintervention physical activity and social capital between groups. Post hoc analyses were conducted using simple main effects. The partial eta squared (η 2 p) was used to assess the effects size from the ANOVA analyses. The significance level was set at p < 0.05.