What’s Your Muscle Type...I or II?

Why are some of us better at activities that require short bursts of movement, while others are more superior at longer distances? Well aside from the enjoyment or motivational factors of a certain activity, the answer might lie within our genetics. There are two types of muscle fibers found within the human body: type I and type II (which can be further divided into type IIA and type IIB). Each have different functional characteristics which are listed below:

Fiber Type	Size	Contraction Speed	Time to Peak Power	Fatigue	Amount of Energy Used per Unit of Activity	Efficiency
I (slow twitch)	smaller	slow	100 milliseconds	slower	low	most efficient
IIA (moderately fast twitch)	larger	fast	50 milliseconds	faster	moderate	more efficient
IIB (fast twitch)	largest	extremely fast	25 milliseconds	fastest	high	least efficient

The ratio of fiber types, within a particular muscle, varies from each person and is genetically predetermined. Muscles that are used during endurance activities like running, cycling, and swimming have a higher percentage of slow twitch fibers (type I). Movements that require explosiveness, quickness, and/or power such as lifting, sprinting, and jumping tend to utilize more fast twitch fibers (type II). In theory, those who have a higher number of fast twitch fibers have an advantage at activities that require those muscle types, over someone with fewer fast twitch fibers. The opposite holds true with slow twitch fibers and activities which require their use. There is no definitive evidence to suggest that slow twitch fibers can transform into fast twitch fibers or vice versa.

Our nervous system plays an important role in recruitment of muscle fibers. Most of the time, the limiting factor (when lifting weights or any other performance based movement) is the number of motor units the nervous system can recruit. This is seen in people who workout and increase their strength, but not the size of their muscles in comparison. By doing correct movement patterns over time (through power, speed, and explosiveness) we train our body and nervous system to become more efficient at type II muscle fiber recruitment and firing. As a result, we are able to lift more, jump higher, and run faster.

As we age, fast twitch muscle depletes more rapidly than slow twitch; thus, it is important to train and condition our body through resistance and endurance training to minimize the decrease in both fibers. Additionally, it is beneficial to work outside of your comfort zone (in a controlled manner) to maximize efficiency and performance.

Fiber type is just one piece of the puzzle when it comes to an individual’s fitness success; however, it is not the sole predictor. Other factors like proper conditioning, healthy nutrition, adequate rest, and mental preparedness all play a role in one’s performance. Find out how to get the most out of your muscle fibers by contacting Dan at (978) 807-8579 or visit seachangefitness.net to learn more.

Dan Kinsella is an Exercise Physiologist with a degree in Sports Medicine. He's a Certified Strength and Conditioning Specialist (CSCS) through the National Strength and Conditioning Association (NSCA), as well as a Certified Nutrition Specialist. To learn more about this article, or his personal training services, visit seachangefitness.net or call (978) 807-8579 to schedule your FREE consultation!