The most effective phase to stretch muscles is during the cool-down period of a workout. After exercise, muscles are warm and more pliable, making it an ideal time to improve flexibility and promote recovery. Stretching during the cool-down can help relieve muscle tightness, prevent soreness, and enhance overall performance in future workouts. Incorporating targeted stretches focuses on muscle groups worked during exercise can yield long-term benefits, aiding in injury prevention and promoting mobility.