Q. Can you explain the difference between fast-twitch and slow-twitch muscle? How do they impact my strength-training routine

A. Slow-twitch (i.e. type 1) and fast-twitch (i.e. type 2) muscle fibers differ in a number of ways. This is reflected in the various names for these fibers. For example, slow-twitch fibers are named such because of the speed at which they contract when stimulated. Fast-twitch fibers, as you might expect, contract much quicker. Slow-twitch are also called red fibers due to their deep red color, and fast-twitch fibers are called white because they appear very pale. The color is determined by the amount of myoglobin in the tissue. Myoglobin is akin to hemoglobin that is found in the blood. Myoglobin carries oxygen within muscle tissue that is needed for oxidative metabolism energy production. The extra myoglobin in slow-twitch fibers plays a role in their ability to resist fatigue. Slow-twitch fibers are also better than fast-twitch fibers at using fat for energy. By contrast, fast-twitch fibers burn primarily glucose. There are a number of other metabolic and structural differences, but this much should suffice for our purposes.

As for your second question,
no, you do not need to alter your training routine in hopes of somehow directing the emphasis toward one type or the other. Strength training will require that you use resistance approximating 80–90% of your one-repetition maximum (1RM). If you are training just to build muscle you will still need to use weights in the 70–85% 1RM range. In a study comparing various levels of resistance on fiber response by fiber type, we see that all fiber types (i.e. fast and slow twitch) will respond to the same relative extent within a wide range of weight loads. So rather than thinking about fiber types when planning your training, think about what you specifically want to improve; if it is strength, building muscle, or just athletic ability you will need to train in a manner that best elicits that specific adaptation. The details on how to do that will have to wait for another day.