This study aims to determine how different types of exercise regimens (resistance, aerobic, or combined exercise regimens) can be used to minimize muscle loss and mobilize body fat in severely obese patients after bariatric surgery (BS), despite the rapid surgery-induced weight loss. Additionally, the study will examine how changes in the intestinal microbiota following BS act as a mediation factor that alter tissue-specific responses in muscle and adipose tissue. The study will also evaluate the effect of different types of exercise regimens on cardiometabolic markers, endocrine response, and physical function following BS. Sixty sedentary (regular exercise \<1 hour per week) candidates to bariatric surgery at Herzliya Medical Center will be recruited to participate in this six-month-long randomized control study. Participants will be randomly assigned to either one of three exercise regimens intervention groups (aerobic, resistance, or a combined exercise regimen combining aerobic and resistance exercises) or a control group that will receive standard care. Training will be supervised and matched for metabolic equivalent (METs). The intervention will take place at the Sylvan Adams Sports Institute at Tel Aviv University and an online platform. All measurements will be taken at a presurgical baseline assessment and throughout the study, and will include changes of muscle mass and adipose tissue distribution, measured by a 3-Tesla magnetic resonance imaging (MRI); body composition- will be assessed using multichannel bioelectrical impedance (Seca); Bone mass, will be measured by a Dual-energy X-ray absorptiometry (DEXA); microbiota composition and changes will be evaluated by stool samples that will be subjected to 16S ribosomal ribonucleic acid (16s rRNA) profiling and metagenomics analysis; fasting blood samples will be used to examine endocrine control and cardiometabolic markers; anthropometric measurements to assess surgery results associations with physical activity outcomes and physical function will be assessed by several validated performance assessments, including handgrip, six-minute walk, sit-to-stand, maximum voluntary contraction, and maximal oxygen consumption (VO2max).