Different Muscle Fiber Types and Their Functions.
Did you know the human body has over 600 muscles? Your skeletal muscle is categorized into three different fiber types. Each type has a different contraction speed and not surprisingly, each has a special function within your body.
Slow twitch muscle fibers are called type I and they are recruited first for any type of activity you do, whether it be strenuous or mild. Slow twitch fibers are your body’s aerobic specialists. They are fueled by blood fats called triglycerides and the aerobic energy system (the oxygen you breathe). They help you go for long periods of time at a light or moderate pace. Distance runners have a lot of slow twitch fibers and they train them to be resistant to fatigue for as long as possible.
Fast twitch, or type II muscle fibers are recruited for strenuous activities requiring power, speed or strength such as heavy resistance training (weight lifting, power training, etc.) and interval training. They are fueled by your anaerobic energy system (glycogen, ATP, creatine phosphate in muscle cells) and help you go fast and hard. The downside is these fibers fatigue quickly.
Fast twitch fibers come in two sub types: type IIa and type IIb. Fast twitch type IIa fibers are basically a hybrid of the slow twitch type I fiber and the super fast type IIb fiber, utilizing both the aerobic and anaerobic energy systems equally. They have attributes of both power and endurance and are used for extended periods of relatively heavy activity, like carrying heavy grocery bags up a flight of stairs.
The type IIb fibers are the ones responsible, along with adrenaline, for helping you work at the highest intensities, for only brief periods of time. These muscle fibers fatigue very quickly, usually in less than 30 seconds. They are reserved for only the most difficult of tasks and unfortunately most trainees never recruit them during exercise. If you are not training these fibers, you are missing out on some important components of fitness, namely, explosive power, speed and strength. You are also missing out on hypertrophy, i.e. an increase in muscle mass.
How Muscle Fibers Are Recruited
When your body moves, it recruits muscle fibers to perform the work. Individual fibers are not called into action however. They band together in bundles called motor units, which are controlled by individual motor neurons.
Slow twitch muscle fibers are grouped into small motor units which have a low “firing threshold”. So when you perform any activity, whether lifting your arm or doing a sprint, you recruit slow twitch muscle fibers first. Activities requiring increasing amounts of strength and speed are met by increasingly larger motor units containing the fast twitch type II fibers needed to do the job.
By the way, motor units always contain the same type of muscle fibers. In other words you can’t have slow twitch and fast twitch fibers in the same motor unit. If that were possible, you would have some fibers lying dormant while others were being recruited. The dormant fibers would not perform any work and would be “carried along” for the ride in the motor unit by the activated fibers. Fortunately for us, Mother nature is smarter than that!