Types of muscle fibres

We train day in and day out for hours in the gym to develop big muscles but most of us do not know what are we training, what type of muscle is being trained? You may be wondering what is this all about? Muscles are muscles! You are right, well it is not important to go to the biological level to understand the muscles type but a basic understanding does help in the way you train. Keeping this in mind that most of us are not medical professionals i have described the muscle fibre types in a simple way.

Basically, there are two types of muscle fibres:
1) Slow twitch (Type I) muscle and
2) Fast twitch (Type II) muscle: These are further classified into Type IIa fast twitch and Type IIb fast twitch.

Type I Muscle Fibers
Type I muscle fibers have the slowest-contractile speed, the smallest cross-sectional area, the highest oxidative (aerobic) capacity, and the lowest glycolytic (anaerobic) 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
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, powerlifting, and bodybuilding.However, Type IIa fast twitch muscle fibers are intermediate and their properties lie between type I and type IIb muscle fibres.

How Type I & Type II Muscle Fibers Are Different
Type I fibers are different than type IIb fibers for many reasons. You can think of them as opposites. Type I is for long endurance activities while type IIb is for short fast bursts. Type I fibers are highly oxidative and are not likely to hypertrophy 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.

How Your Body Recruits Muscle Fibers
Even the small muscle groups in your body have over 100,000 muscle fibers. A motor neuron is what stimulates our muscles to contract. It carries impulses (messages) from our brain and spinal cord to our muscles. One motor neuron controls anywhere from 2-2,000 muscle fibers. A single motor neuron and the fibers it stimulates are called a motor unit. Each motor unit mainly contains muscles of its kind. Also, the motor unit fires with a frequency that is conducive to the fibers it stimulates. Simply put, a slow twitch motor neuron will cause the muscles in it to contract slowly while a fast twitch unit will fire quickly.The quicker it fires the more power it produces. If the activity is light, it will mainly stimulate type I muscle fibers. When it becomes too intense it will call upon type IIa muscle fibers. And finally, for the highest intensity movements, it will recruit the type IIb fibers. This is why type I fibers are called low threshold, and fast type IIb fibers are called high threshold. Low threshold because they are the first muscle fibers to be recruited and high threshold because they are only recruited under the most intense circumstances. Your body always activates its muscle fibers in this fashion.
An Example Of How Your Muscle Fibers Are Recruited
Say you were to help someone lift a heavy couch up a flight of ten stairs. You would use your hands as grips and let your legs do all the work. On the first step your legs will start to recruit type IIa fibers. By the 2nd or 3rd step your nervous system does not recruit more motor units. This being the case the first set of fibers rest and more type IIa fibers are recruited. Along with these, a number of type IIb fibers are called into play (to maintain fluent motion up the stairs).
As your journey continues more type IIa and type IIb fibers are recruited until by the last step they have all come into play. Your muscle fibers weren't twitching at maximum speed until the end of the stairs when they neared failure. The faster a muscle fiber twitches the greater the force is. At the beginning, the fibers weren't forced to twitch at maximum frequency to overcome the weight, but at the end they had to produce as much force as possible to overcome the weight. This is how recruitment is designed to maintain a certain amount of force.