Diagnoses were established using The Structured Clinical Interview for DSM-IV. Clinical symptoms were assessed using the Scales for Assessment of Positive and Negative Symptoms (SAPS/SANS) [33, 34] and the Beck depression and anxiety inventories (BDI; Beck Depression Inventory, BAI; Beck Anxiety Inventory) [35, 36].

Aerobic fitness was determined by a cardiopulmonary exercise test to establish VO2 peak, an index of the maximum capacity of an individual’s body to transport and use oxygen during incremental AE activity. All tests were completed on weekdays at ~ 10 am and were performed on an electronically braked cycle ergometer [37] with a Viasys Encore metabolic cart [38]. The equipment was calibrated prior to every test. Continuous 12-lead telemetry was monitored via CardioSoft electrocardiogram software [39]. Participants completed a 5-min resting baseline, a 3 min of no-resistance warm-up, and a ramping exercise protocol of 10–15 watts to peak exercise with a target of exercise for 8–12 min. Exercise was terminated when the subject reached maximum capacity (VO2 plateau; 85% of maximal heart rate (HRmax; 220-age); respiratory quotient ≥ 1.1; or self-reported exhaustion indexed via Borg Scale). A 3-min active recovery period completed the test. We used VO2 peak scores (ml/kg/min) in all analyses.

We employed three complementary measures of SF including self-reports (Provision of Social Relations Scale, PSRS) [40], informant reports (Specific Levels of Functioning Scale, SLOF) [41], and clinician-administered interviews (Scale for the Assessment of Negative Symptoms, SANS) [33]. The PSRS is a 15-item self-report scale measuring social relationships with family and friends. Items are scored on a 5-point scale (from 1 = “Very much like me” to 5 = “Not at all like me”) with higher scores indicating lower support and relationship quality. The PSRS was found to have alpha coefficients of 0.85, 0.80, and 0.78 among individuals with schizophrenia, bipolar disorder, and healthy controls, respectively [42], as well as solid test–retest reliability of 0.75–0.87 [43]. The SLOF is a 43-item scale assessing multiple domains of daily functioning. Items are rated on a 5-point Likert scale with total scores ranging from 43 to 215 and higher scores indicating better overall functioning. The SLOF was found to have alpha reliability of 0.62 [41]. An informant version of the SLOF was administered to caregivers of all study participants, typically first-degree relatives who were in regular contact with study participants. For data analyses, we used the total score of the SLOF’s Interpersonal Relations subscale. The SANS is a 25-item clinician-administered scale developed to measure negative symptoms in schizophrenia, which have been linked to SF [44]. Items are rated on a 6-point scale (range 0–5), with higher scores reflecting increased severity. For the purposes of assessing SF, we used an Asociality subscale item from the SANS, which indexes the ability to feel intimacy and closeness with others (item #20). The interrater reliability for this individual item of the SANS was found to be 0.90 [33].

Demographic and clinical information was collected including age, sex, primary language, reading ability, and baseline prescription of antipsychotic medication (as indexed by chlorpromazine equivalence). Additionally, we characterized the participants’ social network, as indexed by item #5 of the Heinrichs-Carpenter Quality of Life Scale (QLS) [45], a semi-structured interview. The item indexes the scope and degree of the participant’s involvement in her/his social network—the extent to which other people concern themselves with the participant, care about her/his fortunes, or know about her/his activities.

We employed a proof-of-concept parallel assignment, single-blind RCT design with participants randomized to receive standard psychiatric treatment (“treatment as usual”; TAU) or attend a 12-week AE program in addition to TAU. Participants were randomized in the order they entered the study. After satisfying the inclusion/exclusion criteria, participants completed the diagnostic, clinical, aerobic fitness, and SF measures. Raters conducting clinical interviews and informant surveys, as well as the technicians conducting the aerobic fitness assessments, were blind to the participants’ treatment status. Following the 12-week period, participants completed a follow-up assessment of clinical, aerobic fitness, and SF measures. Participants received $270 for completing the research assessments. Additionally, those randomized to the AE arm received $5 reimbursement for each AE session they attended (paid weekly) to defray the costs of round-trip public transportation.

All participants received standard psychiatric care over the course of the study, which included regular meetings with a psychiatrist, psychologists, social workers, and/or psychiatric nurses as determined by their pre-study individual treatment plan. Treatment schedules were determined for each participant by their clinical team and no attempts were made to inform or influence treatment.

Participants randomized to the AE intervention underwent a 12-week, 3 sessions/week, 1-hour AE training program informed by the American College of Sports Medicine and federal guidelines, which recommend 150 min of moderate-intensity AE per week. Moderate-intensity AE involves activities that expend 3.0–5.9 times the energy expended at rest and are broadly defined as activities in which the participant is able to talk while engaging in the activity. All AE sessions were held in a small gym located in the medical center. The AE equipment included two treadmill machines, a stationary bike, an elliptical machine, and two active-play video game systems (Xbox 360 Kinect, Microsoft) with whole-body exercise software [29, 46]. The AE sessions were led by a certified exercise trainer (B.S. degree in Therapeutic Recreation). The sessions opened with a 10-min trainer-led warm-up period, followed by 45-min AE using the equipment, and ended with a 5-min cool-down period. The trainer was present during all AE sessions for guidance and support, along with a research assistant who assisted with equipment set-up and collection of AE-related behavioral data.

Training fidelity was indexed by 1) the number of AE sessions attended over the 12-week program (maximum 36) and 2) in-session AE intensity. The latter was set for participants individually based on their maximal heart rate (HRmax), as determined during their baseline VO2 peak assessment. Target minimal AE intensity was set to 60% of HRmax in Week 1, 65% in Week 2, 70% in Week 3, and 75% in Weeks 4–12. In-session training intensity was monitored using Polar RS400 HR monitors (a wireless-enabled digital watch and chest strap) that participants wore during each session. The monitors were programmed to emit a soft beep when the participant’s HR was lower than the individually targeted AE intensity level, which cued the trainer to encourage the participant to achieve their individual target goal.

Data Analyses: Data analyses were conducted using IBM SPSS ver. 25. All tests were two-tailed and the significance level was α = .05. Determination of predictors of change in SF from baseline to 12-week follow-up was examined using three hierarchical step-wise regression analyses, using data from the PSRS, SLOF, and SANS as dependent variables. Change in aerobic fitness was entered in block 1 and demographic and clinical variables were entered in block 2. Clinical and demographic variables included age, sex, baseline antipsychotic medication use, baseline anxiety and depression scores, and baseline social network. Shapiro–Wilk tests indicated that the primary dependent variables were normally distributed. The AE and TAU intervention groups were compared using multivariate analyses of variance with a repeated-measures design, with time and group designated as within-subject and between-subject factors, respectively. We focused our analyses on study completers, but also present results using an intention-to-treat approach with baseline observations carried forward.

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