Testing Self-Control as a Behavior Change Mechanism to Increase Physical Activity
Phase: EARLY_PHASE1
Conditions: Sedentary Lifestyle, Self-control
Sponsor: Brandeis University

 
Project Title: Testing Self-Control as a Behavior Change Mechanism to 
Increase Physical Activity 
 
 
 
 
NCT04522141 
 
 
 
 
Study Protocol and Statistical Analysis 
 
 
 
Upload Date: 03/11/2022 
Document Date: 09/09/2020 
 
 
 

The overall goal of the study is to test the efficacy of a smartphone-based self-control 
intervention to increase physical activity among sedentary adults who are at risk for poor 
health. The specific aims of the study are threefold: The first aim is to test the efficacy of a 
self-control intervention to increase physical activity by comparing two experimental 
conditions (i.e., self-control treatment group vs. control group). It is expected that those who 
receive the self-control intervention, will show greater increases in physical activity compared 
to people in the control group, who do not receive the self-control intervention. Moreover, it 
is expected that effects in the self-control treatment group will last longer compared to any 
changes in the control group. The second aim is to compare changes in self-control between 
the self-control group and the control group. It is expected that people in the self-control 
group will show greater changes in self-control compared to people in the control group. The 
third aim is to examine self-control as a behavior change mechanism for physical activity. It is 
expected that those who show greater increases in self-control will also show greater 
increases in physical activity. Self-control is expected to mediate the relationship between 
condition and physical activity. 
Methods 
This study has been approved by the University’s Institutional Review Board at Brandeis 
University (#20100R) and the protocol has been registered at ClinicalTrials.gov 
(#NCT04522141). Informed consent will be obtained from each participant. Ethical issues 
will be continuously considered. There is a data and safety monitoring plan and a safety 
officer is assigned to the study. Any adverse events will be recorded and reported. 
Participants  
A total of 70 sedentary middle-aged adults, who are at risk for poor health, will be 
enrolled. Participants will be recruited from across the US by posting flyers on public boards 
in libraries, churches, shopping centers/malls, or grocery stores. Moreover, we will post 
advertisements online on Social Media, on Craigslist, and on findparticipants.com. As 

reimbursement, participants will receive a Fitbit Charge 4 step counter, which they can keep 
after the end of the study. In addition, as an added incentive, all participants will receive $25 
for completing all aspects of the study.  
Participants will be included if they indicate they want to become more physically 
active, are between 35 and 65 years, are fluent in English, fit enough to walk for at least 20 
minutes at a time, and own a smartphone (Android or iPhone) with a data plan. Participants 
will be excluded if they have experienced a fall or heart problem/condition in the last 6 
months, are currently participating in an exercise program to increase physical activity or 
currently using a Smartwatch (e.g., Fitbit, Garmin, Apple Watch etc.) to track their physical 
activity, already exercise at least 3 times per week and for more than 30 minutes at a time, 
and/or if a doctor has advised them to not walk due to health conditions.  
Study Design and Procedure  
The current study uses a two-armed randomized controlled trial design with 
assessments at pretest (baseline), at the end of weeks 1-7, posttest (week 8), and follow-up 
(week 12). Participants will be randomly assigned into two experimental conditions to ensure 
that the two conditions are fully randomized with respect to participants’ baseline 
characteristics (allocation concealment). Thirty-five participants are randomized into each 
condition. Also, participants are blinded in terms of their experimental condition.  
Recruitment flyers and advertisements specify: “Do you want to be more active, but 
find yourself making excuses to avoid exercise?” Flyers and advertisements also include a 
web link for interested participants. This web link will provide detailed information about the 
study and interested participants will be asked to fill out a screening questionnaire to check 
for all inclusion and exclusion criteria. If they are eligible for the study, they will be asked to 
provide their phone number and home postal address so we can send them the study materials. 
After that, participants will be randomly assigned into one of the two experimental conditions: 
The self-control treatment group or the control group. A research assistant will call all eligible 

participants to receive their informed consent by phone, to help them installing the MindHike 
smartphone application on their own smartphone, and to answer any questions. Once 
informed consent is given, we will ship participants a Fitbit Charge 4 step counter, instruction 
materials, a face mask, and a stylus pen to their home postal address. 
Both the self-control treatment group and the control group will use the MindHike 
smartphone application and will use a Fitbit. The control group will receive a more basic 
version of the MindHike app which only sends them daily reminders to sync their step data, 
charge their Fitbit and asks them for their daily mood. The self-control treatment group will 
receive the version of the MindHike app which delivers the self-control intervention. 
MindHike is a smartphone application which can prompt participants with messages using a 
chatbot system. The app is pre-packaged, which means that the app will deliver the same 
dialogues automatically to all participant in each condition. There is no bi-directional 
communication, but participants click on predesignated responses to advance the dialogue.  
On the first study day, all participants will fill in a pretest questionnaire. The 
MindHike application will send a web link that forwards participants to this online 
questionnaire. All participants will be asked to wear their Fitbit every day during the entire 
study from when they wake up to when they go to bed. During the first study week, 
participants are only asked to wear the Fitbit to document the number of steps taken to 
establish an objective measurement of steps before the intervention. All participants will be 
contacted by the MindHike app twice each day for eight weeks. The first message is a push 
notification which reminds all participants to wear their Fitbit so they can get used to it. The 
timing of this Fitbit is set to 7 a.m. by default but participants can change the time if they get 
up earlier or later in the morning. The second daily push notification asks participants to open 
the MindHike app and to read through a short daily pre-packaged chat in the app. Again, the 
time for this second push notification can be customized by each participant. During the first 
week, this pre-packaged chat will only remind participants of both groups to sync their step 

data with the Fitbit app and to charge their Fitbit. After this baseline week, the study will last 
for 7 more weeks, and participants will still receive a push notification reminder to wear their 
Fitbit at customized times in the morning. Across the study, participants will also fill in 
weekly assessments of self-control at the end of each week. After the seven weeks, 
participants will fill in a posttest assessment. The MindHike application will send them the 
web link to the online posttest. After the posttest, participants do not use the MindHike 
application anymore, but we will contact them via e-mail for a last online follow-up 
assessment four weeks later. 
Control Group 
After the first study week (baseline week), all participants will receive a push 
notification which reminds them to open the app at least once per day. If they open the app, 
participants will receive a pre-packaged chat. These daily pre-packaged chats will differ 
between the two treatment groups. Although both groups will use the MindHike application at 
the same frequency, the daily pre-packaged chat for the control group will only include a 
reminder to sync their step data, charge their Fitbit and ask them for their daily mood.  
Self-Control Treatment Group 
The daily pre-packaged chat for the self-control treatment group will include a short 
coaching session to deliver the components of the self-control intervention. For this study, the 
intervention was adapted to specifically target self-control in the domain of physical activity. 
As part of the intervention, participants of the self-control treatment group set 
themselves a specific physical activity goal such as “Each evening after dinner, I will go for a 
20-minute walk”. The self-control intervention will teach participants different strategies on 
how to achieve their personal goal. In general, participants learn how to change their 
behaviors in ways that potentiate their desirable impulses or weaken their undesirable ones. 
The intervention is based on the process model of self-control (Duckworth, Gendler, & Gross, 
2016). This theoretical model will be used to describe the timeline of the development of 

impulses to participants. The goal of the self-control intervention is to target each stage of this 
theoretical model (i.e., (1) how to proactively select situations, (2) how to proactively change 
certain situations, (3) how to shift attention in a given situation, (4) how to change appraisal 
of a given situation, and (5) how to change behaviors in a given situation). Interventional 
components include specific behavioral tasks (e.g., If-then plans; Gollwitzer, 1999), self-
reflection tasks, and short film clips for psychoeducational purposes. Every week of the 7-
week intervention, participants focus on one of the five stages of the self-control process 
model, while the first week is an introductory week on self-control and the last week serves to 
practice all learned strategies in everyday life. Each stage of the five stages of the self-control 
process model will be introduced with a short video clip.  
Statistical Analyses 
Sample size and power considerations.  We conducted a power analysis for repeated 
measures and between-group effects based on the between-group effect size found in a 
previous physical activity intervention study (Robinson et al., 2019). The power analysis with 
an α error level of 0.05, a statistical power (1-b) of 0.80, a correlation of 0.50 between the 
pretest, posttest, and follow-up assessment, and a Cohen’s f = .30 suggests a sample size of at 
least N = 62. As we learned from previous similar smartphone-based physical activity studies, 
about 10% of participants who sign up for the study discontinue their participation. As such, 
we will recruit a total of 70 middle-aged adults. 
Data analyses. SPSS and R will be used to analyze differential effects between the 
two conditions. Multilevel models and the lme4 package in R will be used to examine group 
differences in change and maintenance of effects over time in objectively measured daily 
steps, self-reported physical activity, self-control as well as differential change over time in 
secondary outcomes such as Big Five personality traits, exercise self-efficacy, satisfaction 
with life, sense of control, and cognitive performance. 

Another focus will be to examine self-control as a mechanism of behavior change. We 
will use multilevel models to examine whether individuals who change more in self-control 
also increase more in their physical activity. Moreover, we will use PROCESS in SPSS and 
multilevel mediation analyses in R to test whether self-control mediates the relationship 
between condition and physical activity.