Limited variety of classes Two-minute review The Carol is a high-tech spinning bike for high-intensity interval training (HIIT) that requires you to exercise for just nine minutes at a time. It's a pleasure to ride, reacting in real time to your efforts to give you an optimized workout – if your wallet can stand it. The Carol, which stands for Cardiovascular Optimization Logic, is an AI-powered exercise bike that uses biometric data to create highly customized workouts. Using REHIT (reduced exertion high intensity training) Carol’s built-in tech works by calculating the user’s resistance levels, which in turn helps provide the ideal programme for you.