RecruitingBehavioural interventionExercise Training After Upper Gastrointestinal Cancer SurgeryThis randomized controlled trial aims to determine the optimal exercise intensity for improving postoperative recovery in patients with upper gastrointestinal cancer. Participants who have undergone curative surgery will be randomly assigned to high-intensity interval training (HIIT), low-intensity continuous training (LICT), or usual care. The exercise interventions will be performed under supervision three times per week for eight weeks. The primary outcome is peak oxygen uptake (VO₂peak), assessed using cardiopulmonary exercise testing. Secondary outcomes include physical function, body composition, patient-reported outcomes, and biological and mechanistic markers such as inflammatory biomarkers, muscle-related factors, and gut microbiota. This study will also explore potential mechanisms underlying exercise-induced adaptations and their association with clinical outcomes.