FitnessAI is an AI-powered fitness tool that creates personalized workout routines based on over 5.9 million workouts and 40,000 weightlifters' data. It adjusts sets, reps, and weights, while tracking user progress over time. The app recommends rest times and customizes future sessions to optimize results. Users can message real trainers for advice and accountability, making it an interactive experience. FitnessAI is praised for its ease of use, personalized approach, and data-driven optimization, making it a valuable tool for efficient and effective fitness training.