This book, Physiology of Detraining and Performance Management, comprehensively examines the multidimensional de-adaptation process and cellular decline triggered by the cessation of training stimuli in athletes. Moving beyond the traditional perception of simple fitness loss, the book approaches detraining as an active biological energy-saving response. The first part explores the principle of reversibility, the hierarchical chronology of cardiovascular and neuromuscular decay, and moderating variables such as muscle memory, age, and gender. The second part presents evidence-based strategies developed to maintain performance during off-seasons or forced injury periods. Specific protocols tailored for endurance, strength, and team sports are detailed, primarily focusing on the Minimal Effective Dose (MED) approach. Furthermore, by incorporating wearable sensor technologies, mental training methods, and modern nutritional interventions targeting anabolic resistance, the book provides a comprehensive guide to turning the detraining crisis into a strategic recovery opportunity.