I am not Cathe but I am a personal trainer and can help you out. Fast twitch fibers are developed during anaerobic workouts. This can be weight training or all out sprint type activities. It depends what your goals are. Everyone is genetically different when it comes to their muscle fiber composition. For example, Olympic athletes that do endurance type activities are genetically gifted with a high percentage of slow twitch muscle fibers. While you cannot convert one type of fiber to the other, you can train your body so that slow twitch benaves more like fast twitch or vice versa. I am curious to know what your goals are in regards to your training. Please respond with more info.
Best wishes, Cyndie