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When my 19-year-old son came home from college, he shared with me his attempts to build muscle on his lean body build. Most of us would want the days back again when our metabolism was churning ever so high and we could literally eat whatever was put before us and remain thin. But for my son, he was hoping he would be more muscular through his workouts and diet/nutrition intake. He was surprised through all his efforts, he hadn’t built up and out.

I then began to share with him the differences between fast and slow twitch muscle fiber and how they respond differently to training and physical pursuits. While I have the norm 50-50 combo of the two fibers, my son has mostly slow. I told him recognizing the differences, realizing your own muscle makeup and knowing the ideal training for your type will lead to the best results when trying to build up or slim down. He had never heard of the different muscle fiber types and suggested I write about it to inform others.

I stumbled onto the different types of muscle fiber in my own pursuit to try to understand why my lower half would build up from what seemed to be just looking at a weight while, on the opposite end, I could lift free weights until the cows came home and my upper body would never ever get bulky. With my own trial and error, I found out what exercises are best for muscle definition in my legs and butt, and what works ideal for my core, arms and shoulders. I must admit it gets challenging at times with my lower body; I can’t participate in step classes, cycling activities, and lower weight training on a regular basis without bulking up in what seems to be in a heartbeat to me.

Muscle fiber composition is already built into your genetics. The fibers are divided down into two main types and you can think of them as exact opposites. Type I, slow twitch, is for long endurance activities while Type II, fast twitch, is for short fast bursts.

Type I Fibers – Slow Twitch
The slow twitch muscle fibers are more efficient at using oxygen to generate more fuel for continuous, extended muscle contractions over a long period of time. They fire up more slowly than fast twitch fibers and can go for a long time before they fatigue. This fiber type is great for paced exercises and endurance activities, such as marathons and triathlons. When weight training, perform sets of high-rep lifts. Aim for 12 to 20 lifts per set at a comfortable weight, which builds strength without forcing your body to exert itself too strenuously.

Type II Fibers – Fast Twitch
The fast twitch muscle fibers do not need oxygen to create fuel and are excellent at generating quick, powerful bursts of speed and strength. These fibers fire up more quickly, but also fatigue easily. They are not effective in longer-term training, but are very useful in brief, high-intensity training, such as gymnastics, bodybuilding or powerlifting. When weight training, perform sets of low-rep lifts. Use heavy weights to make yourself stronger and to build up your fast twitch fibers.

Fast twitch can be further subdivided into Type IIa and Type IIb. Type IIa are known as intermediate fast-twitch fibers. They are a combination of Type I and Type II muscle fibers, using both aerobic and anaerobic metabolism almost equally to create energy. Type IIb are known as the classic fast twitch muscle fibers, using just anaerobic metabolism to create energy.

The distribution of muscle fiber differs in each individual. As mentioned earlier, examine your type and decide on your goals so that you can create an exercising program specific to your needs. Remember a person with more fast-twitch will do best with low reps and high sets and heavier weights. A person who with more slow-twitch will gain from high reps and low sets with sub-maximal weights. Read up and discuss a plan with a trainer at your gym as to what’s best for your own individual success.

Be Fit. Be Strong. Be Well.