Slow Twitch vs Fast Twitch Muscle Fibers: What's the Deal?

There are two different types of muscle fibers.  We all have slow twitch (Type I) muscle fibers and fast twitch (Type II) muscle fibers. We can further categorize fast twitch muscle fiber into Type IIa and Type IIb. There are even Type IIbx, which I won't get into in this article.

TYPE I MUSCLE FIBERS SLOW-TWITCH-Type I muscle fibers have the slowest-contractile speed, the smallest cross-sectional area, the highest oxidative (aerobic or endurance) capacity, and the lowest glycolytic (anaerobic or power) capacity. They contract slowly and are able to hold a steady paced twitch for long durations without fatigue. Type I muscle fibers are predominately used in endurance activities. Long distance runners, swimmers, and cyclists mostly use Type I fibers.

TYPE II MUSCLE FIBERS FAST-TWITCH-Type IIb muscle fibers have the fastest-contractile speed, the largest cross-sectional area, the lowest oxidative capacity, and the highest glycolytic capacity. They are ideally suited for short, fast bursts of power. These muscle fibers are used in such activities as sprinting or powerlifting. Type IIa muscle fibers are intermediate and their properties lie between type I and type IIb.

HOW TYPE I & TYPE II FIBERS ARE DIFFERENT
Type I fibers are different than type IIb fibers for many reasons. We can think of them as opposites. Type I fibers are recruited in long, endurance activities, while type IIb are recruited for short, fast bursts. Type I fibers are highly oxidative and are not likely to hypertrophy (increase in size) as much. Type IIb fibers are highly gycolytic and tend to hypertrophy more than type I fibers. Type I fibers are also known as red fibers due to their abundant supply of blood. Type IIb fibers have little blood causing them to be white in appearance.