Multiple measures of biomedical, psychological, and behavioural outcomes and measures will be used to assess the intervention effectiveness; these are discussed below.
Key outcome biomedical outcome measures will be taken to monitor changes in cardiovascular disease risk and body weight and composition as a result of the intervention [7,9,45]. All the biomedical outcome measures will be conducted at baseline, 6 and 12 week periods unless specified otherwise.
Within a three day period prior to the clinic appointment, blood lipoproteins including low-density lipoprotein (LDL), high-density lipoprotein (HDL), total cholesterol (TC), triglycerides (TG), and blood glucose and insulin levels will be taken at an approved pathology laboratory using a venous blood sample from each participants.
Body weight (kg) will be recorded with participants wearing light clothing and no shoes, using a digital body composition monitor scale (Omron, model HBF 362). Height (cm) will be measured at baseline, to the nearest 0.1 cm using a stadiometer with participants wearing light clothing and no shoes. Body mass index (BMI) will be calculated using both the weight and height measurements using the formula weight in kilograms divided by the square of the height in meters (kg/m2). Waist-hip ratio (cm) will be calculated by measuring waist circumference in standing position at the narrowest area between the iliac crest and lateral lower rib, and hip measurement from the largest circumference of the lower abdomen. This will be measured to the nearest 0.1 cm using a circumference measuring tape (Seca 203). To ensure greater accuracy the measure will be taken twice and the average of the two readings will be used.
Systolic blood pressure (SBP) and diastolic blood pressure (DBP) will be measured using an automated, blood pressure monitor (A & D Medical, model UA-851) with participants in a supine position with the tested arm at the level of their heart for at least a minimum of 10 minutes before and during measurements . The measure will be taken three times and the average reading will be used. Body composition including bone mass, body fat percentage, and muscle mass will be measured using the whole body dual-energy X-ray absorptiometry (DEXA, Lunar Prodigy; Lunar, Madison, WI, USA). Participants will be required to lie on a bed during the DEXA scanning with no jewellery or metal clothing. DEXA scans will be performed at baseline and 12 weeks. The dual-energy X-ray absorptiometry machine will be calibrated and a phantom scan undertaken daily.
A number of behavioural outcome measures will be taken at baseline, 6 and 12 weeks, unless otherwise specified. The 24-item Bailey Dietary Screening Questionnaire will be used to identify nutritional risk. Three levels of nutritional risk include: at risk, possible risk, and not at risk. The measure has been found to have adequate sensitivity and specificity when compared with nutritional risk based on dietary reference intakes . Sensitively measures indicate that 83% of individuals were correctly classified as positive by the screening tool. Specificity measures indicated 75% of individuals correctly classified as negative by the screening tool. An example item from the questionnaire is: “how often do you usually eat fruit as a snack?” Participants are asked to respond on a Likert scale ranging from 0 (never) to 4 (3 or more times a week).
An 18 item version of the Three Factor Eating Questionnaire, (TFEQ-R18), will be used to measure eating behaviour examining three cognitive and behavioural domains of eating; these include cognitive restraint, uncontrolled eating, and emotional eating. Good reliability has been reported . An example of an item within the emotional eating scale includes “I start to eat when I feel anxious”. The item response occurs on a 4 point Likert scale from 1 (definitely true) to 4 (definitely false).
Physical activity and diet intake diary
Prior to each clinic visit, participants will complete a 3 consecutive day record of their physical activity and dietary intake. Participants will complete the ‘food, drink and exercise diary’ record including 1 weekend and 2 week days . For the physical activity section, participants will complete a 3-day activity log adapted from Bouchard and colleagues . Participants will record an activity code from 1 to 9 that corresponds to the physical activity carried out during each five minute period; the time and activity of physical activity is thus recorded. In addition, participants will record their belief about the accuracy of their physical activity log. The physical activity log information will be assessed by examining energy expenditure. Each activity code is related to various levels of Metabolic Equivalent. The measure has been found to have high reliability with a coefficient of 0.96. For the food diary intake section, participants will record a 3-day food log containing information about their drink or food intake, time consumed, location, amount, drink or food description and method of preparation, and belief about the accuracy of their food log. The diary will be used to evaluate the participant’s compliance with the recommended diet and physical activity guidelines. The program Food Works will be used to analyse the food content and nutrients .
Self-reported intervention compliance questions were also incorporated at week 6 and 12. The 5 items will be used as a measure of the participant’s self-reported compliance to the behaviour changes of dietary intake and physical activity. For example, “on average, during the past 6 weeks, how often did you engage in at least 30 minutes of planned exercise?” with item responses ranging from 1 (7 days/week or Always) to 5 (Never).
Participants will be instructed to complete the outlined psychological measures before and after viewing the video presentation intervention unless specified otherwise. Variables from the HAPA will be measured to assess the psychological mediators that may explain the effects of the intervention on the behavioural and biomedical outcomes. The HAPA measures include items adapted from Barg and colleagues’  measure of assessing risk perception for developing cancer for physically inactive middle aged adults. Risk perception will be assessed using four items on a 6 point Likert scale ranging from 1 (strongly disagree, much lower, not likely) to 6 (don’t know/refused) to measure perceived risk of developing obesity related risk factors. For example, “I think it is likely that I will develop health problems related to obesity at some point in my life.” Outcome expectancy will be assessed using six items with responses ranging from 1 (strongly disagree, not at all effective) to 5 (strongly agree, extremely effective) to question participants about the effect of diet and physical activity on health risks related to weight gain. For example, “I think that consuming fewer calories per day is a very important way to help me to lose weight”. Intention will be assessed with four items that ask participants how much they plan and intend to follow the diet guidelines and/or to participate in the recommended physical activity amount over the next 12 weeks. Responses range from 1 (strongly disagree) to 5 (strongly agree). For example, “I intend to participate in daily physical activity with a minimum of 30 minutes of planned exercise on each individual occasion over the next 6 weeks. Action self-efficacy will be measured using 10 items regarding participants’ confidence and ability to engage in the recommended diet and physical activity guidelines over the next 12 weeks. Prior to answering the questions, participants would have heard the recommendations during the video presentation. Items are on a 5 point Likert scale ranging from 1 (not confident, not likely, strongly disagree) to 5 (completely confident, extremely confident, strongly agree). For example, “if it were entirely up to you, how confident are you that you would be able to follow a diet that requires you to consume fewer calories per day on each individual occasion over the next 6 weeks?” Maintenance self-efficacy will be measured using 18 items on a 5 point Likert scale ranging from 1 (not confident) to 5 (completely confident) to measure barriers to engage in the recommended physical activity or diet guidelines. For example, participants were asked “How confident are you that you will do daily physical activity with a minimum of 30 minutes of planned exercise during your leisure time on each individual occasion over the next 6 weeks even if…” followed by a list of barriers, such as, but not limited to bad weather and feeling tired. The participants planning will be assessed using two items ranging from 1 (strongly disagree) to 5 (strongly agree) that ask participants if they had made a detailed plan about when, where and how they would engage in physical activity or follow the diet guidelines. For example, “I have made a detailed plan about when, where, and how I will do daily physical activity with a minimum of 30 minutes of planned exercise on each individual occasion over the next 6 weeks”.
Further variables that relate to the HAPA were adopted from Hagger and colleagues . Motivation will be assessed at baseline by asking participants to respond to six items that question motivation to participate in physical activity and change their diet. For example, “how motivated are you to change your diet on each individual occasion over the next 12 weeks?” Item responses are based on a 6 point Likert scale ranging from 1 (not at all motivated, no effort at all) to 6 (extremely motivated, all my effort). In addition four items exploring engagement in mental imagery exercises will be used. Participants will be asked to indicate the level to which they were able to imagine the task. For example, “to what extent did you visualise in your mind (imagine) exactly how you might reach your diet goals (example, consuming fewer calories per day) on each individual occasion over the next 6 weeks?” The items are on a 6 point Likert scale ranging from 1 (indicating did not imagine), to 6 (indicating clearly imagined).
Goal setting will be measured by asking participants six open ended questions. For example, ‘What are your goals for physical activity?’ The answers to these questions will be “coded” appropriately based on whether they are appropriate and realistic. Intervention checks will also be included to ensure that participants engaged with and completed the implementation intention and mental imagery exercises appropriately and according to protocol. That is, participants will be asked to record their goals; If-then plans; and a list of the steps they imagined during the intervention process. The answers to these questions will be “coded” appropriately based on whether they are appropriate and realistic. These variables will be measured as they are expected to mediate the key components of the intervention. That is, the effect of the mental imagery component of the intervention variable on the primary and secondary outcomes is expected to mediate the HAPA variables of self-efficacy, motivation, outcome evaluation, and intention. Similarly, the effect of the if-then plan component of the intervention on the primary and secondary outcomes is expected to be mediated by the HAPA planning variable.
Imagery ability will be measured at baseline as it may moderate the effectiveness of the mental imagery intervention and should therefore be included as a potential covariate. Imagery ability will be measured using the Betts Questionnaire upon Mental Imagery  is a 35 item questionnaire used to measure the clarity and vividness of their mental imagery ability. The questionnaire instructs participants to imagine, for example “Visualize a mental image of a friend you see on a regular basis” and rate the image vividness on a Likert scale. Sheehan suggests that the measure is valid and has predictive value. The scale has also been found to be reliable .
The variables of quality of life, depression, anxiety, and stress will be measured as a control variables to ensure the intervention does not have adverse effects on quality of life or emotion. These variables will be measured using the two listed questionnaires. The Impact of Weight on Quality of Life Questionnaire (IWQOL - Lite) is a 31 item version, self-reported measure that assesses the effect of obesity on quality of life in five domains: physical function, self-esteem, sexual life, public distress, and work . For example, a physical function item includes “because of my weight I have difficulty picking up objects” with responses made on 5 point Likert scales ranging from 1 (never true) to 5 (always true). Depression Anxiety Stress Scales-21 (DASS-21) will be used to measure symptoms of depression, anxiety, and stress. At baseline, 6 and 12 weeks, participants will rate their symptoms over the past week by answering 21 items using a 4-point Likert Scale anchored by 0 (did not apply to me at all) and 3 (applied to me very much, or most of the time) . An example item from the questionnaire includes “I couldn’t seem to experience any positive feeling at all”.
A series of 3 (condition: control, intervention, and intervention with text message) × 2 (time: 6 and 12 weeks) mixed model multivariate analyses of covariance (MANCOVAs) with time as a repeated measures variable will be used examine the effects of the intention over time on study outcome variables. Baseline measures will be covariates in the model. A separate MANCOVA will be conducted for each set of dependent outcome variables: biomedical, behavioural and psychological. Pending significant multivariate effects, follow-up univariate analysis of covariance (ANCOVAs) will be used to identify differences for individual variables within each outcome set. Post-hoc follow-up tests will also be conducted to locate individual differences at the time intervals. Mediation analyses will be conducted by mediated multiple linear regression analysis with dummy-coded intervention conditions as the independent predictor variables, the constructs from the HAPA included as mediating variables, and the primary and secondary biomedical outcome variables as the dependent variables. Mediation will be confirmed through significant bootstrapped indirect effects using Preacher and Hayes’  algorithms for multiple mediation and the PROCESS macro. Missing data will be handled by multiple imputation using regression analysis with the Markov-chain Monte Carlo (MCMC) estimation method. In addition, we will adopt a full intention-to-treat analysis with last measured data points carried forward in order to provide a conservative estimate of the effects.
Sample size has been determined by a power analysis using the G*Power 3.1 Program, suitable to evaluate differences between the three condition levels greater than 10%, with statistical power set at > 0.80 and alpha set at p < .05 . A large effect size is assumed as previous studies that have used implementation intentions and reminder text messages , as well as studies using mental imagery and implementation intentions to increase exercise behaviour , have found large effect sizes. Given the estimated effect size, the total sample size required post-test is 66 and considering a potential 35% drop out rate and eliminations of cases due to missing data or spoilt questionnaires based on previous trials , we plan to recruit 102 participants at baseline or until the required sample size has been achieved.