As I walked into a recent workshop, I received a gruff greeting from a wickedly ripped man with biceps the size of my head. John Hansen, former Mr. Universe and Mr. Natural Olympia, turned out to be a terrific instructor and I learned a lot from him. By knowing a little bit about your muscle fibers, you can make some simple adjustments to your activities to help meet your health goals.

In general, there are basically two different types of muscle fibers in the body, which are classified based on how they produce energy, and are referred to as being either slow-twitch (type I) or fast-twitch (type II). Slow-twitch fibers are associated with endurance activities such as distance running, and they use oxygen to create the energy that allow you to exercise. Fast-twitch fibers are used in powerful bursts of movement like sprinting and power lifting and they use glycogen to fuel activity.

Things to know about slow-twitch muscle fibers:

1. Slow-twitch fibers use oxygen to create adenosine triphosphate (ATP ), which is the chemical that fuels aerobic muscle contractions.
2. Slow-twitch fibers are red, because they contain more blood-carrying myoglobin.
3. Slow-twitch fibers can sustain force for an extended period, because they create their own energy, but they are unable to generate a significant amount of force.

Things to know about fast-twitch muscle fibers:

1. Fast-twitch muscle fibers are further broken down into two categories: moderate fast-twitch (type IIa) and fast-twitch (type IIb).
2. Moderate fast-twitch muscle fibers fatigue more rapidly than slow-twitch fibers. During moderate aerobic activities, such as swimming, the slow-twitch fibers are the first to contract. When these slow-twitch fibers become fatigued, the fast-twitch fibers kick in.
3. Fast-twitch are the most powerful and least enduring.
4. Fast-twitch fibers are responsible for the size and definition of muscles.
5. Fast-twitch fibers are generally white, because they do not contain much blood.

Ways to improve your workout: You can rearrange your fitness activities to more efficiently burn fat and increase muscle definition. A sample routine might look like this:

1. Five minutes or so of dynamic stretching
2. Weight training session with 8-12 repetitions of the exercise before reaching muscle failure with three sets and a 30-90 second break in between sets. Another option is to conduct supersets instead of taking breaks between sets.
3. 25 minutes or more of cardio
4. Cool down with static stretching.

There are several benefits to this sequence of events, especially if you are aiming to burn fat. With this structure, the body gets rid of more stored fat, as it burns glycogen while weight training and uses fat as fuel for cardio.

The final step to your most efficient workout can double as a tasty reward. Be sure to get a good source of protein within half an hour, to help your muscles recover. It is also suggested that having the protein with a few carbs helps your body absorb the protein more efficiently. Enjoy!