2.3.1. Off-line recruitment.

Recruitment flyers and brochures will be posted in cardiology outpatient clinics at Vanderbilt Heart (Nashville, TN, U.S.A) for self-referrals. In addition, the patient care providers will also reach out to their patients and provide their patients with our study brochure to allow the patient to contact the study team. We will also prescreen potential participants using the Epic medical records system for potential patients’ contact information. Initially, the study team will send an opt-out letter to the potential patients. If the patients do not contact the study team within two weeks by email or telephone call to opt-out, the trained research staff will call them, explain the study, and screen for interest and eligibility.

2.3.2. On-line recruitment.

We will send a message to potential participants who are registered in the Vanderbilt community-wide email distribution list and ResearchMatch (national web-based recruitment tool). Due to the in-person nature of the protocol, we will only contact those who are living in Nashville or surrounding areas. In the invitation email, there will be a link directing potential participants to the prescreening survey via REDCap. This pre-screening data will be saved into REDCap. Once they meet all the inclusion criteria, the trained research staff will call them to confirm their eligibility.

2.4.1. Baseline visit 1.

At Baseline Visit 1, eligible individuals will complete the written informed consent with assistance from the trained research staff. Those who are consented and complete the baseline survey questionnaires will commence a 7-day usual physical activity and glucose monitoring period. During this period, all participants will wear an activPAL3 device (PAL Technologies) on the thigh and a continuous glucose monitor (CGM, Abbott FreeStyle Libre Pro) on the upper arm and follow their usual diet and activity. Participants will receive $25 for the completion of the Baseline Visit 1.

2.4.2. Baseline visit 2.

After 7 days of activity and glucose monitoring, the participants will return to the VUSN Research Clinic. Baseline anthropometric assessments, blood pressure, and blood collection via finger prick will be completed. The activPAL and CGM will be removed at this visit. Participants will receive $25 for Baseline Visit 2.

2.6.1. Theoretical framework.

The Sit Less program employs a Habit Formation framework [31] to target the habit-like nature of sedentary behavior, which often occurs without conscious thought or intention [32, 33]. The program focuses on making individuals aware of their sedentary habits and encourages the adoption of new behaviors, such as taking breaks to stand or walk, through consistent practice in specific contexts. According to the Habit Formation framework [31], habit formation involves four phases: motivation, action initiation, behavior repetition in a context, and cue-association building. Through this process, over time individuals may rely on contextual cues rather than conscious self-regulation to initiate a behavior, making it potentially easier to perform and more sustainable with consistent environments [31]. The Habit Formation framework is widely used for sedentary behavior reduction in older adults [32].

The framework guides our intervention by: 1) bringing sedentary behavior into conscious awareness through prompts (e.g., vibrations from a Fitbit), and 2) making a new habit of sedentary breaks (5 minutes of standing/walking every 30 minutes of sitting) through context-dependent repetition (e.g., during TV commercials). To enhance habit formation, behavioral techniques supporting motivation and action planning will be used (Table 1) [34].

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Table 1. Relationship between intervention components, theoretical constructs, and behavioral techniques.

https://doi.org/10.1371/journal.pone.0302582.t001

2.6.2. Instructional goal setting session.

The PI will provide an instructional goal setting session during Baseline Visit 2. The PI will review the prior 7 days of activPAL3 data and identify targetable prolonged sedentary bouts with the participant. The PI and the participant will set two goals: 1) a sedentary behavior reduction goal aimed at reducing daily sedentary time gradually until they reach the goal of ≥ 120 minutes of sedentary time reduction; and 2) a sedentary break goal of 5 minutes of standing/walking every 1 hour and then increasing its duration to 10 minutes and frequency of the sedentary break to every 30 minutes [35]. These goals have been demonstrated to be feasible in stroke patients and CVD patients [8, 21]. Participants will have the autonomy to set their weekly goals, whether to maintain the same target as the previous week, or reduce sedentary time. We do not recommend reducing sedentary time by more than 30 minutes per week, but adjustments can be made based on the participants’ current sedentary hours and willingness.

In addition, the participant will be given the Ten Top Tips (TTT). The TTT is a booklet developed from the Habit Formation Theory and includes tips to add sedentary breaks into their daily routine [36]. The original TTT booklet [36] was obtained from Dr. Gardner [36], one of the developers of the Habit Formation theory [31], and it was applied to sedentary behavior with permission to revise it for the current study. The original TTT booklet suggested limiting TV watching to no more than one hour at a time and promoting light physical activities such as walking—by leaving the remote control by the TV, standing up during commercials, opting for stairs, and parking at a distance from the entrances of malls or offices [36]. It also included recommendations for stretching and muscle strengthening exercises [36]. We revised the original TTT booklet to include 8 tips, excluding the stretching and muscle training recommendations and incorporating the Fitbit and smart water bottle into daily routines. Although the suggested tips involve only light levels of physical activity, such as standing and walking for 3 to 5 minutes every hour—activities with no more risk than those encountered daily—we will instruct participants to stop these exercises immediately if they feel unwell or experience discomfort, and to promptly seek medical advice.

2.6.3. Activity tracker.

A Fitbit Inspire 2 (Fitbit Inc., U.S.A) will be provided to each intervention participant. This device alerts participants with a vibration and message on the watch face when they have been sitting for 60 minutes (i.e., “Move alert”). Participants can select the days and the time ranges to receive the move alerts. Fitbit provides sedentary behavior statistics including the longest sitting period (“Longest Stationary Period”) and total daily sitting hours (“Stationary Hours”) in the app. Participants will be encouraged to check the Fitbit app at the end of the day to monitor their longest sitting period and total daily sitting hours. They will be guided to apply the TTT booklet tips to interrupt their longest sitting period and decrease total daily sitting hours, aiming to achieve their overall goals of a total of 120 minutes of sedentary behavior reduction and having 5 minutes of a sedentary break every 30 minutes by the end of the intervention.

To ensure consistent use of the Fitbit, participants will be instructed to wear the device during waking hours, including while showering or swimming. They will also be encouraged to wear the Fitbit at night, although this is optional. Participants are required to charge the Fitbit once a week, and we will send a text message every Sunday reminding them to charge and sync their Fitbit. We will track participants’ wear status over 12 weeks. If a participant fails to sync their Fitbit or wear it for two consecutive days, they will receive an automated reminder text message. If no improvement is observed within another two days, research staff will contact the participant by phone to troubleshoot issues.

2.6.4. Smart water bottle.

We will provide a smart water bottle (HidrateSpark, U.S.A) to each participant in the intervention group as a natural motivator to have sedentary breaks. The device alerts participants with a color change and message on the Fitbit to keep them hydrated. The device provides the daily water intake summary in the Fitbit app. The app calculates an individualized water intake goal based on the participant’s weight, age, and activity levels. By increasing water intake, participants will need to get up more frequently to use the bathroom and to refill their water bottles, which will break their prolonged sedentary behavior.

2.6.5. Tailored text messages.

Three text messages each week will be provided to support and enhance the habit formation process. Using data collected from the Fitbit device, participants will receive algorithm-derived, individually tailored text messages updating them on their weekly goal progress mid-week and a review of sedentary behavior at the end of the week. We will start each week on Monday and provide an end of week message on Monday morning. A midweek message will be sent on Thursday. Thursday’s message will provide midweek sedentary behavior feedback. Sunday’s message will encourage participants to achieve their goal and sync the Fitbit to the app. Monday morning’s (end-of-week) message will be structured by 1) algorithm-derived feedback (praise, reinforcement or encouragement messages), 2) a weekly summary of sedentary time, the longest sedentary period, and a suggestion for extra sitting breaks, 3) one tip from the TTT booklet, and 4) setting of next week’s goal on sitting hours, duration, and frequency of sitting breaks (Table 2).

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Table 2. Examples of praise, reinforcement, and encouragement messages.

The underline indicates personalized information.

https://doi.org/10.1371/journal.pone.0302582.t002

To develop and provide text messages, we will take the following steps.

1. 1. Creation of weekly text message template: The team will create the weekly message templates for praise, encouragement, and reinforcement messages.
2. 2. Fitbit data transfer: We will transfer the Fitbit data from the participants’ Fitbit accounts into REDCap on Thursday morning and Monday morning to provide Thursday (mid-week) and Monday (end-of-week) messages. There are several Fitbit data management platforms that summarize physical activity data, but none summarize sedentary behavior data. Therefore, the team developed our own data collection interface using Fitbit’s Web Application Programming Interface (API). This platform provides secure data acquisition and management tools that facilitate remote data collection from Fitbit. All Fitbit’s minute-level data will be imported into REDCap.
3. 3. Fitbit data aggregation and calculation: Fitbit provides minute-level data of prelabeled “sedentary” time and step counts, as well as summary data for “light level”, “moderate”, and “vigorous” activity time, and does not provide meta-data shown in the Fitbit app (“longest stationary period” and “total stationary hours”), nor an algorithm to replicate the daily sedentary behavior meta-data. Therefore, we will reproduce the Fitbit app’s sedentary behavior meta-data. First, we will exclude days with less than 10 wearing hours [37] and remove the sleep period detected by Fitbit from calculation. The “total stationary hours” from Fitbit will be calculated by the sum of all sedentary minutes that had accompanying heart rate data during a selected day. The “longest stationary period” from Fitbit will be calculated by the longest uninterrupted period of the day at which zero steps were taken.
4. 4. Integration of Fitbit data into the text message template: The team will take advantage of logics built into the REDCap environment to integrate information from the collected Fitbit data into our weekly text message template using the following algorithm. If the participant achieves the week’s sedentary hours goal for > 5 days, they will receive a praise message. If the participant achieves the week’s sedentary hours goal for 3–5 days, they will receive a reinforcement message. If the participant achieves the week’s sedentary hours goal for < 3 days, they will receive an encouragement message. The examples of praise, reinforcement, and encourage messages are provided in Table 2. The text messaging algorithm system has been used successfully before [38].
5. 5. Send text message by using Twilio and REDCap: The customized text message will be sent to the respective participant using the Twilio plug-in for REDCap.
6. 6. Ask next week’s goals via text message: This study will use REDCap’s function to administer a survey via SMS conversation to set a participant’s next week’s sedentary hours goal and numbers/frequency of sedentary breaks. Participants may respond to the SMS survey with any kind of alpha-numeric text. Participants’ answers on next week’s goals will be automatically saved into REDCap.

2.9.1. Feasibility and acceptability.

Feasibility will be determined by analyzing dropout rates and intervention compliance. The dropout rate will be calculated as the percentage of participants who withdrew consent. Participants who drop out will explain their reasons. For compliance, we will confirm if over 75% of participants wore their Fitbit for at least 5 valid days each week (with a valid day being over 10 hours of wear) for 75% of the study duration (9 weeks), and if the Sit Less group responded to 80% of goal-setting texts. These criteria are derived from metrics used to assess the feasibility of consumer-based wearable physical activity trackers in other digital behavior change studies [40].

Acceptability will be assessed through two satisfaction questionnaires and exit interviews. Due to the absence of a standardized satisfaction questionnaire for multi-technology interventions, we merged two questionnaires to assess overall satisfaction with the Sit Less program and its individual components (Fitbit, Fitbit app, Fitbit’s Move Alert, smart water bottle, and text messages). Satisfaction with the overall program and each technology (Fitbit, Fitbit app, Fitbit’s Move Alert, smart water bottle) will be measured using modified items from Lyons et al.’s questionnaire [41]. Satisfaction with the overall program will be evaluated with six items like “I would recommend the program” and “The program is likely to reduce sedentary behavior.” Additionally, the perceived ease and continued intention to use each technology component will be evaluated with 11 items such as “I found the Fitbit/Fitbit app/Smart water bottle convenient to use.” and “I would continue to wear the Fitbit”. A separate questionnaire by Burner et al. [42] will assess the text messaging component, evaluating the relevance of the message contents to the participants, as well as the frequency, motivation, and timing of the messages. All responses will be made on a Likert-type scale from 1 (strongly disagree) to 5 (strongly agree). Our threshold for acceptability outcomes is established so that at least 75% of participants must either agree or strongly agree with each statement on the satisfaction questionnaires [42].

In addition to the satisfaction questionnaires, an exit interview (n = 20) will be conducted by the PI or trained research staff and audio-recorded. For the exit interview, we will develop a semi-structured interview guide and thematic analysis. The interview guide will be structured to capture additional feedback on the acceptability and satisfaction of the intervention, barriers and facilitators to sedentary behavior reduction, and perceived health impacts of sedentary behavior reduction.

2.9.2. Changes of sedentary behavior.

Sedentary behavior will be assessed by three indicators including total daily sedentary time, prolonged sedentary time (time spent sitting >30 minutes and >60 minutes), and number of sit-to-stand transitions. These three sedentary behaviors will be measured by 7 days of activPAL 3 (PAL Technologies, U.K) device monitoring prior to the beginning of the intervention/control protocol and again after the conclusion of the protocol for both study arms. The activPAL 3 monitor is a tri-axial accelerometer that also includes an inclinometer, worn on thigh (Fig 4). Due to the activPAL 3 position on the thigh, the activPAL 3 monitor is the most sensitive device to distinguish sitting (horizontal position) and standing (vertical position) [43, 44]. The activPAL 3 has demonstrated great test-retest reliability and criterion validity with direct observation to classify sitting, standing, and stepping [45].

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Fig 4. Participant wearing activPAL 3 on thigh for sedentary behavior and physical activity measurement.

https://doi.org/10.1371/journal.pone.0302582.g004

2.10.1. Physical activity.

Time spent standing and walking (stepping), number of sitting-to-standing transitions, and number of step counts will be measured using the activPAL 3 device. The activPAL 3 has demonstrated greater criterion validity compared to video observation for measuring physical activity [46].

2.10.2. Cardiometabolic markers.

Height, weight, and waist and hip circumferences will be measured using a validated stadiometer, digital scale, and flexible tape measure, respectively. All measurements will be recorded to the nearest 0.1 cm and 0.1 kg. Blood pressure will be measured using an automated and validated blood pressure monitor. The device will take three readings at 2-minute intervals and the mean of three BP readings will be recorded. For blood samples, participants will be required to fast overnight (at least 8 hours). Cardiometabolic markers including fasting lipids, insulin levels, HbA1c, and hs-CRP levels will be collected via finger prick by a trained research staff member, using dried blood spot cards for measurement and analysis at ZRT Laboratory (Beaverton, OR, U.S.A). Lastly, 24-hour glucose levels will be measured by the use of a continuous glucose monitor (Freestyle Libre 2, Abbott, U.S.A) over a 7-day period at baseline and post-intervention. The 24-hour glucose control will be evaluated by mean 24-hour glucose levels, glucose management indicator, glycemic variability (%), % time below range (glucose < 70 mg/dL), % time in range (glucose 70–180 mg/dL), and % time above range (glucose >180 mg/dL) [47].

2.10.3. Patient-centered outcomes.

Confidence in reducing sedentary behavior will be measured using the 12 items from the Self-Efficacy Questionnaire for Physical Activity and Sedentary Behavior (previously reported Cronbach’s α = 0.79) [48]. The items assess the level of confidence for specific sitting behaviors and sedentary breaks. Habit strength for sedentary behavior will be assessed by using the validated Self-Report Habit Index measure (previously reported Cronbach’s α = 0.91) [49]. This 7-item index was adapted to assess the degree to which sedentary breaks (standing/walking) become habitual [49]. Fear of movement will be measured by using the Fear Avoidance Belief Questionnaire. The questionnaire consists of 5 items regarding patients’ beliefs about the relationship between perceived discomfort and movement, which can lead to fear of movement and avoidance of physical activity/exercise. The words “pain” and “back” were changed to “heart discomfort” and “heart” respectively, as used by Ahlund et al. [50]. The questionnaire was also validated for patients with CVD. Depressive symptoms will be measured using the Patient Health Questionnaire 9 (PHQ-9) [51].

2.14.1 Data monitoring.

As a single-site, low-risk behavioral study, we will not establish a Data Safety and Monitoring Board, nor conduct interim analyses. Instead, the PI (CP), supervised by the coauthor (SM) and aided by the study team, will bi-monthly monitor enrollment, data quality, adverse events, and participant dropout rates. Data from the continuous glucose monitoring and activPal devices will be assessed for validity immediately after return. Should data quality be below standards, the team will review the monitoring protocols and measures will be re-taken accordingly.

2.14.2. Harms and auditing.

The study team, along with the PI, will hold weekly meetings to assess the progress of the study, and review any potential problems or adverse events. Expected adverse events include muscle soreness, discomfort from frequent sedentary breaks, skin irritation from the activPAL and Fitbit, and local redness, inflammation, or bleeding at the CGM sensor site. Serious and unexpected adverse events related to the study protocol will be reported to the IRB within seven days. Annually, the study will undergo continuing review and be granted approval by the IRB.

2.15.1. Ethics.

The research study complies with the Declaration of Helsinki and has been reviewed and approved by the Vanderbilt University Medical Center IRB (#220416). Written informed consent will be obtained from all subjects involved in the study with the assistance of the trained research staff. Any necessary protocol amendment will undergo IRB approval and be reflected in the clinical trial registry.

2.15.2. Confidentiality.

Participant confidentiality is ensured by coding personal information with a four-digit number and storing participant identifiers separately from research data. An anonymous email account will be used for the Fitbit and HidrateSpark apps to maintain anonymity. All electronic records will be password-protected and accessible only to authorized team members, with paper documents secured in the VUSN Research Clinic. After seven years post-study, all identifying data will be destroyed.

2.15.3. Dissemination plan.

When we complete the analyses of data, the data will be accessible through a data repository. We will disseminate the study findings by publishing the results in scientific journals and all participants will be provided with a summary of the findings.