Fast or Slow – How do your muscles twitch?

Whether you realize it or not, you probably know the answer to this question. Do you excel at endurance in your workouts or long distance running? Maybe you are the first one at the finish line of a sprint race or the highest jumper in your group fitness class. The answer lies in the amount of fast or slow twitch fibers that make up your muscle. The ratio of these two types of muscle fibers you have determines which of  these types of activities comes easier to you.

Let’s look a little into the science behind this. Slow twitch fibers, also known as Type I, are great for endurance and don’t tire easily. They are reddish in color due to the abundance of blood vessels. These fibers use the oxygen from these blood vessels to make energy (ATP), making these fibers run aerobically (with oxygen). The soleus muscle locate below the calf is an example of a muscle primarily containing slow twitch fibers. It aids in our balance when we stand.

Fast twitch fibers, also known as Type II, allow for speed but tend to tire quickly. They are lighter in color because they do not have as many blood vessels, and they run primarily off of anaerobic energy (without oxygen).  The gastrocnemius (calf muscle) and hamstrings are primarily fast twitch muscles. There are actually two types of fast twitch fibers – IIa and IIb. IIa fibers use a combination of aerobic and anaerobic energy while IIb use only anaerobic.

Everyone is born with a certain number of each type of fiber which is why some of us excel at sprinting while others can run for hours. In general, most of us have about 50% of each type of fiber. There is speculation but no real determination yet as to whether you can train your fibers to switch to the other type to better suit your personal fitness goals. There are reasons genetically speaking why the Olympic sprinters and marathon runners are so good at their sport. In fact, these exceptional athletes sometimes have up to 80% of either fast or slow twitch fibers. The rest of us should not give up on our goals however, b/c training and determination can make up for a lot of what nature did not give us.

There are a few tests you can do to determine which type of fiber you predominately have. The most accurate (and least likely that you will do) is a muscle biopsy. This is how we know about the 80% fact in the Olympians. Less invasive methods are jump tests and maximum weight rep tests. Honestly though, if you have been exercising for a while, you probably have a good idea of your fiber type without having to take a test.

Although there is no concrete evidence that we can change the types of fibers in our muscles, it is possible through proper training to maximize the use of your fibers. If you have more slow twitch abilities but want to increase your speed you can do his by adding sprints, plyometrics, isometric exercises, and weight training. Also, if you want to increase your endurance, train for endurance even though it does not come easily. Your body adapts to the stresses you place on it, so if you train long and hard with a particular goal in mind you can accomplish it.

Be aware of your strengths and weaknesses when it comes to your muscles,  and tweak your workouts to become a more well rounded athlete. Never stop challenging your body!

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