Adherence to an Exercise Program and Pre- and Post-Changes After Bariatric Surgery

Bariatric surgery is an effective treatment for severe obesity; however, the rapid postoperative weight loss period is frequently accompanied by unfavorable changes in body composition, including reductions in lean mass and muscle performance, which may compromise functional capacity during recovery. Therapeutic exercise is commonly recommended as part of metabolic rehabilitation; nevertheless, evidence regarding short, structured programs initiated early after surgery-and particularly regarding changes in muscle quality-remains limited. This study will evaluate within-subject changes in muscle quality, body composition, and functional response following a concurrent exercise program in adults after bariatric surgery. Additionally, the association between exercise adherence and the magnitude of change in primary outcomes will be explored using multivariable analytical models. This is a single-center, prospective, longitudinal, quasi-experimental study with a one-group pre-post design. Participants will be recruited from BUPA Clinic Santiago and will include men and women aged 18 to 60 years who underwent sleeve gastrectomy or Roux-en-Y gastric bypass, are PAD voucher beneficiaries, and have a medical indication for metabolic rehabilitation. After eligibility screening and informed consent, participants will enter the protocol at postoperative week 4, when baseline assessments will be performed; the intervention will begin immediately after baseline testing. The intervention consists of a 5-week concurrent exercise program combining aerobic and resistance training, delivered as two supervised in-person sessions per week (10 sessions total), approximately 60 minutes each, plus one autonomous home-based session per week guided by a written exercise plan provided by the responsible professional. Training progression will be individualized according to participant tolerance and safety parameters. Aerobic intensity will be monitored using the Borg Rating of Perceived Exertion scale, progressing from moderate levels at the beginning of the program to higher perceived exertion toward the final week. Resistance training loads will be prescribed using relative intensity based on one-repetition maximum (1RM) estimation and progressed weekly, focusing on major muscle groups and emphasizing controlled technique. Vital signs may be monitored as clinically appropriate before and after sessions and/or functional testing. The home-based session will be considered a complementary recommendation and will not be systematically recorded for adherence calculation. After completion of the 5-week intervention, participants will undergo a 1-week post-program period without supervised sessions (rest week). Post-intervention assessments will then be performed during the following week. Therefore, outcomes will be measured at two time points: baseline (pre-intervention) and 7 weeks after baseline (after 5 weeks of exercise and 1 week of rest). Muscle quality will be assessed by musculoskeletal ultrasound of the rectus femoris using Philips Lumify, and images will be analyzed with ImageJ following a standardized approach to quantify parameters such as muscle thickness and metrics related to fat infiltration/echo intensity and cross-sectional area, according to the protocol used. Body composition will be evaluated using bioelectrical impedance analysis (InBody S10). Functional response will be measured using the One-minute Sit-to-Stand Test (1min-STST), recording repetitions and perceived exertion using the modified Borg scale; additional functional measures may include handgrip strength depending on the final protocol specification. Quality of life will be assessed as a secondary outcome using the Moorehead-Ardelt II questionnaire. Adherence will be assessed exclusively as attendance to the 10 supervised in-person sessions during the 5-week intervention period, calculated as: adherence (%) = (attended supervised sessions / 10) × 100. For descriptive purposes, adherence will be categorized as high (≥80%), moderate (60-79%), or low (\<60%). Statistical analyses will be conducted using SPSS. Data distribution will be assessed (e.g., Shapiro-Wilk), and within-subject pre-post changes will be analyzed using paired parametric or nonparametric tests as appropriate (paired t-test or Wilcoxon signed-rank test), with effect sizes estimated to support clinical interpretation. To explore associations, correlation analyses (Pearson or Spearman) will be used, and multivariable linear regression models will be constructed to examine the association between exercise adherence (continuous variable) and changes in primary outcomes; complementary analyses using adherence categories (high, moderate, low) will also be performed. Because bariatric surgery produces expected weight loss during the intervention period and this may influence outcomes independently of exercise, weight change will be included as a covariate in regression models, defined as Δweight (kg) = post-intervention weight - baseline weight, where negative values indicate weight loss. The results are expected to provide clinically relevant evidence on within-subject changes and on the association between adherence to an early concurrent exercise program and muscle-related outcomes (muscle quality), body composition, and functional recovery after bariatric surgery, contributing to optimization of postoperative metabolic rehabilitation strategies.