Table 1. Main variables and questions used to answer this study`s research aims.
Dimensions assessed Main variables and questions used
Sociodemographic variables* Age, gender, body weight, smoking, level of education, total household income, cohabitation and occupation.
Membership †  Assessed using the question: Are you still a member? Response options: “Yes” or “No”.
Exercise attendance and drop-out †  Assessed using the questions: Exercise is a subset of physical activity that is planned, structured, and repetitive, and has the improvement or maintenance of physical fitness as a final or an intermediate objective. With respect to this, have you been exercising? Response options: “Yes” or “No”, How often have you exercised per week on average at the fitness club? Response option: "Number of sessions”.
Exercise patterns at the fitness club †  Assessed using the questions: What is your average exercise duration at the fitness club (do not include time used for shower and travel)? Response option: “Minutes”, Do you usually exercise individually (e.g. tread mill or resistance exercise) or at group exercise classes (e.g. aerobic or cycling)? Response options: «Exclusively individually», «Mainly individually», «Both individually and at group exercise classes», «Mainly group exercise classes», «Exclusively group exercise classes» or «My choice of exercise mode is varied and random», Do you usually do resistance (e.g. use of machines or free weights) or endurance exercise? Response options: «Exclusively resistance exercise», «Mainly resistance exercise, but supplemented with endurance exercise», «Equally distributed between the types of exercise methods», «Mainly endurance exercise, but supplemented with resistance exercise», «Exclusively endurance exercise» or «My choice of exercise method is varied and random», Have you attended group exercise classes? Response options: «Yes» or «No», How often have you attended the following group exercise classes: Yoga/Pilates, Aerobic/Zumba, Resistance exercise, Cycling, Circuit exercise? Response options: «rarely or never», «once a month», «two to three times a month», «once a week», «two to three times a week», «four to five times a week» or «six to seven times a week».
Use of a personal trainer †  Assessed using the questions: Have you used a personal trainer (PT) to achieve your exercise goals? Response options: «Yes» or «No», How many PT-sessions did you bought on your last purchase? Response option: «Numbers of sessions».
Use of the fitness club`s childcare †  Assessed using the question: Have you used the fitness club`s childcare? Response options: «Yes», «No» or «I do not have children».
Exercise patterns outside the fitness club †  Assessed using the questions: Have you been exercising outside the fitness club? Response options: “Yes” or “No” and How often have you exercised per week on average outside the fitness club? Response option: "Number of sessions”.
*Answered at onset of fitness club membership,† Answered after three, six and 12 months follow-up.