The SAMG physical activity condition will mirror the SASG group condition, but will be open to older adults from both genders. The same strategies to those used within the SASG condition will also be utilized (development of a unique program name, T-shirts, opportunities to socialize after the program), and will be offered three days per week (Mon, Wed, Fri, or Tues, Thurs, Sat). Older adults will also be recruited to be instructors for the group classes (= 65 years), with both males and females invited to occupy these instructional roles. (As with the SASG condition, training will take place between September and December 2013, through the respective YMCA centres. The YMCA will oversee the training of these volunteers through their Fitness Leader Training Program).

The third condition utilized within the RCT is designed to reflect 'standard' group based exercises that one sees in typical physical activity centres. Specifically, these classes are not restricted to participants on the basis of age or gender, and as such older adults will be participating in groups comprised of people younger than themselves as well as those of both gender.

Affective attitudes towards physical activity will be assessed using the procedures outlined by Ajzen (2002). Specifically, a 7-point semantic differential scale will be used, with anchors including "Enjoyable-Unenjoyable", "Pleasant-Unpleasant", "Interesting-Boring". Previous research with older adults has found support for both the internal consistency and predictive utility of scores derived from this instrument.

For pre-screening all individuals will complete the Physical Activity Readiness Questionnaire for everyone (PAR-Q+). If participants respond 'NO' to all of the (seven) questions within the PAR-Q+ they are cleared for participation in physical activity. If participants have a positive response to the PAR-Q+ (e.g., due to a current medical condition), participants automatically proceed to a second series of questions subsumed within the ePARmed-X. As a result of completing the ePARmed-X, they will either be cleared for participation in physical activity OR will be directed to their physician to obtain clearance before they can participate in physical activity. If older adults have a medical contraindication (flagged through the PAR-Q+ and ePARmedX) and have NOT been cleared to participate in physical activity by their physician they are then ineligible to participate in the study.

During the program, in-class task self-efficacy will be measured through a validated and reliable measure (Poag-Ducharme & Brawley, 1993). Self-efficacy to overcome barriers to exercise (with an emphasis on self-regulation) will use Bray and colleagues (2001) validated and reliable measure; using 0-100% confidence. An example self-efficacy question is: how confident they may be in "Completing the warm-up and stretching component of each class."

To measure both gender and ethnicity stigma, we will use Pinel's (1999) measure for "stigma consciousness." Each item is on a 6-point Likert scale. An example item is "I never worry that I will be viewed as being stereotypically [fe]male."

Diener et al's(2009) well-being measure of the relationship of flourishing and positive and negative feelings will be distributed post-class at each of the times listed above. This measure is on a 7-point Likert scale from strongly disagree to strongly agree with items such as "I am engaged and interested in my daily activities" and "I am a good person and live a good life."

Using methods from Beauchamp et al (2012), participants will answer a series of questions regarding their perceived similarity via a 9-point Likert scale. Surface-level perceptions will be assessed through items related to age, physical condition, and physical appearance similarity, whereas deep-level perceptions will be assessed through items related to attitude, belief, and value similarity. Exemplar items from this assessment included, "In my exercise class, I believe members are similar to me in terms of age."

Two-item assessment of how participants commute to the YMCA location: 1. Typically, how long does it take for you to get to your physical activity class (i.e., commute time):________________ mins and 2) What mode of transport do you use to get to your physical activity class (car, bus, bike, train, walk, taxi):_____________________.

To measure the group-interaction variables of interest (i.e., communication, cooperation, and competition) additional items are embedded within the PAGEQ. Communication will be measured through 6 items that can be further divided into task communication (e.g., 'members of our group talk about how often they should do physical activity') and social communication (e.g., 'people of this group talk about things that are happening in our lives'). Cooperation will be measured through 3 items (e.g., 'we all cooperate to help this group's program run smoothly') as will competition (e.g., 'There is friendly competition within the members to stay as healthy as possible'). Internal consistencies for the group-interaction variables were all previously acceptable: task communication (α = .94), social communication (α = .65), cooperation (α = .91), and friendly competition (α = .81) (Harden, Mama, Lee, Estabrooks, Under Review).

Physical activity outside of program will be measured by an adapted 3-step procedure outlined by Wilcox et al. (1999), specific for older adults. Participants will report on various activities that they engaged in over the last two weeks as well as how many times and the duration associated with each. Participants will also answer whether they experienced (a) no, (b) small, (c) moderate, or (d) large increases in heart and breathing rates while participating.

Functional fitness testing will be conducted based on a previously developed and validated functional fitness test for community-residing older adults (Rikli, R. E., & Jones, C. J. 1999). The assessment will include measures of upper and lower body strength and flexibility, aerobic endurance, dynamic balance, and Body Mass Index. These tests will be done at the participant's local YMCA by trained research assistants.