What is Fast and Slow Twitch Muscle Fiber?

You will hear GMs and Scouts talk about athletes and muscle fiber. Here is a great article on the topic:

Fast and Slow Twitch Muscle Fibers
Does muscle type determine sports ability?
By Elizabeth Quinn, About.com Guide
Are you a better sprinter or distance runner? Many people believe that having more fast and slow twitch muscle fibers may determine what sports athletes excel at and how they respond to training.
Skeletal muscle is made up of bundles of individual muscle fibers called myocytes. Each myocyte contains many myofibrils, which are strands of proteins (actin and myosin) that can grab on to each other and pull. This shortens the muscle and causes muscle contraction.
It is generally accepted that muscle fiber types can be broken down into two main types: slow twitch (Type I) muscle fibers and fast twitch (Type II) muscle fibers. Fast twitch fibers can be further categorized into Type IIa and Type IIb fibers.
These distinctions seem to influence how muscles respond to training and physical activity, and each fiber type is unique in its ability to contract in a certain way. Human muscles contain a genetically determined mixture of both slow and fast fiber types. On average, we have about 50 percent slow twitch and 50 percent fast twitch fibers in most of the muscles used for movement.
Slow Twitch (Type I)
The slow muscles are more efficient at using oxygen to generate more fuel (known as ATP) for continuous, extended muscle contractions over a long time. They fire more slowly than fast twitch fibers and can go for a long time before they fatigue. Therefore, slow twitch fibers are great at helping athletes run marathons and bicycle for hours.
What Causes Muscle Fatigue?
Fast Twitch (Type II)
Because fast twitch fibers use anaerobic metabolism to create fuel, they are much better at generating short bursts of strength or speed than slow muscles. However, they fatigue more quickly. Fast twitch fibers generally produce the same amount of force per contraction as slow muscles, but they get their name because they are able to fire more rapidly. Having more fast twitch fibers can be an asset to a sprinter since she needs to quickly generate a lot of force.
Type IIa Fibers
These fast twitch muscle fibers are also known as intermediate fast-twitch fibers. They can use both aerobic and anaerobic metabolism almost equally to create energy. In this way, they are a combination of Type I and Type II muscle fibers.
Type IIb Fibers
These fast twitch fibers use anaerobic metabolism to create energy and are the "classic" fast twitch muscle fibers that excel at producing quick, powerful bursts of speed. This muscle fiber has the highest rate of contraction (rapid firing) of all the muscle fiber types, but it also has a much faster rate of fatigue and can't last as long before it needs rest.
Fiber Type and Performance
Our muscle fiber type may influence what sports we are naturally good at or whether we are fast or strong. Olympic athletes tend to fall into sports that match their genetic makeup. Olympic sprinters have been shown to possess about 80 percent fast twitch fibers, while those who excel in marathons tend to have 80 percent slow twitch fibers.
Are Athletes Born or Built?
Can Training Change Fiber Type?
This is not entirely understood, and research is still looking at that question. There is some evidence showing that human skeletal muscle may switch fiber types from "fast" to "slow" due to training.
These studies and journal articles offer more insight on muscle fiber research:
High-Intensity Training and Changes in Muscle Fiber
Nature vs. Nurture: Can Exercise Really Alter Fiber Type Composition?
Effects of Endurance Training on Muscle Fiber
What can I do to improve my performance?
Keep in mind that genetic differences may be dramatic at the elite levels of athletic competition. But following the principles of conditioning can dramatically improve personal performance of a typical athlete.
With consistent endurance training, muscle fibers can develop more and improve their ability to cope with and adapt to the stress of exercise.
Is fiber type the number one factor that makes an elite athlete elite?
Fiber type is part of a great athlete's success, but it alone is a poor predictor of performance. There are many other factors that go into determining athleticism, including mental preparedness, proper nutrition and hydration, getting enough rest, and having appropriate equipment and conditioning.

Are Athletes Built or Born?

When hockey scouts evaluate players you will hear then talk about twitchy muscles. Hockey is a game of quick not fast (although fast will not hurt you). Many great hockey players benefit from having twitchy muscle fiber (See my blog on the topic).

This is a great Article on this topic:

Are Athletes Born or Built? How Genetics Influence Athletic Ability
How important are genetics in an athlete's success?
By Elizabeth Quinn, About.com Guide
Athletic records are broken year after year, and the limits of human performance continue to be debated. Just as soon as we think something can’t be done, someone comes along and shows us that it indeed can be done. There was a time when no one thought a human could run a four-minute mile. Roger Bannister did that in 1954 and soon, many others followed. Ultramarathons, Ironman Triathlons and 24-hour races are now commonplace.
Is there any limit?
Surely there must be some limit. So what factors limit performance? Most physiologists agree that the current limits have to do with our genetics – specifically genes that regulate our cardiovascular endurance and muscle fiber type, but some factors are much more variable. Things like nutrition, motivation, environment and advances in equipment (running shoes, swim suits, skis, bicycles) all allow for dramatic improvements in athletic performance.
Genetics and Sports Performance
Genetics shape us in many ways including our potential to excel in sports. Training, diet, and other factors play a large role in developing our potential, but our genes may also limit performance. You may have the genetic potential for being a champion athlete, but if you live a lifestyle of overeating and no exercise you are unlikely to achieve that potential. On the other hand, someone with limited genetic potential can find ways to compensate and become a solid performer.
Genetics have a large influence over strength, muscle size and muscle fiber composition (fast or slow twitch), anaerobic threshold (AT), lung capacity, flexibility, and, to some extent, endurance.
One major limitation for endurance athletes is cardiac capacity, or the heart’s ability to deliver enough oxygen (via the bloodstream) to the working skeletal muscles. This, too, is largely determined by genetics.
The other limitation for endurance athletes is the ability of muscle tissue to effectively use oxygen and create ATP (adenosine triphosphate), the fuel that allows muscular contraction and movement. (see: Creating Energy for Exercise.) The efficiency of this process is measured by something called VO2 max (maximum volume of oxygen).
How Genetics Influence Response to Training
Your genes may also determine how your body responds to training, diet and other external factors.
Research on aerobic endurance shows that some people respond more to training than others. So even if you have a low genetic potential for endurance, you may respond well to training and develop your potential more completely than someone with genetic 'talent' who doesn't respond to training.
Training also increases cardiac efficiency, but the extent of this increase may depend upon genetics. Genetically gifted athletes will have a much greater response to training and will have a large increase in the number of mitochondria in cells. (The mitochondria are organelles in cells that produce the ATP, so the more mitochondria a person has, and the more efficient they are.)
Other Factors That Affect Sports Performance
Genetics appear to have less influence over characteristics such as balance, agility, reaction time and accuracy. Many of these skills can be greatly improved with the proper training.
Sports Nutrition
An athletes diet and nutrition plan has an enormous affect on his or her athletic performance. No where is this more evident that when an elite athlete "bonks" or "hits the wall" during an event. Bonking is generally a result of glycogen depletion, dehydration or a combination. Athletes can avoid this by training the body to burn fat when glycogen stores decrease and by continually supplying the working muscles with energy during an event. (See: Energy for Exercise.)
Mental Skills Training
Practicing mental skills training such as imagery, visualization, and learning techniques for dealing with performance anxiety are all skills than any athlete can learn to master with practice. These techniques, along with learning the tactics and strategies of the sport, using proper equipment and avoiding injuries are all critical factors in sports success that have very little to do with genetics.
Although many elite athletes are blessed with the right genetics for their sport and a great training routine, even recreational athletes can make the most of their abilities with optimal conditioning, good nutrition and a positive mental attitude.

Hockey is a Mental Game

We spend a lot of time as parents, coaches, and players focusing on skating and skills development as well as strength conditioning. Every player needs to develop a winning mental game. I advise every player I work with to read the book "Hockey Tough" by Saul L. Miller. It's a great introduction to the mental side of the game.
As noted by Mark Messier in the introduction "Hockey Tough" is about strengthening the mental game for hockey or any sport. Saul Miller points out that "hockey places extreme demands on the mental capacities of the athlete". Saul Miller notes that the five (5) keys to being hockey tough are:

1. Having the desire and drive to be the best player you can be...
2. Being hockey smart. Its having a positive productive on-ice focus.
3. Being in control of your emotions.
4. Being determined and committed to doing what's necessary to improve.
5. Having the strength and resilience to weather the bumps, bruises, frustrations and fatique of a long season.

There are also a number of great techniques that Saul Miller discussed as well to help players achieve a mental edge. I highly recommend this book!

The NCAA Rules

NCAA eligibility will be jeopardized if a student-athlete or parent reaches a verbal or written agreement with an agent. It is permissible however for a student-athlete to have an advisor but not an agent, without jeopardizing his/her eligibility at an NCAA school. Under NCAA regulations, student-athletes and parents are permitted to receive advice form a lawyer or other individuals concerning a proposed professional sports contract, provided that the advisor does not represent the student-athlete directly in negotiations for the contract. In this regard, it is permissible for an advisor to discuss with a student-athlete or parent the merits of a proposed contract and provide suggestions about the type of offer a student-athlete should consider. In order to maintain NCAA eligibility at an NCAA school, however, a student-athlete may not use this advisor as a link between the student-athlete and the professional sports team. Rather, a student-athlete must view the advisor as an extension of the student-athlete’s own interests and not as a source to contact a professional team.

If a student-athlete uses the advisor as a direct contact with a professional team, the advisor shall be considered an agent, and the player may have jeopardized his or her eligibility at an NCAA school. For example, an advisor may not be present during discussions of a contract offer with a professional team or have direct contact (i.e., in person, by telephone, email or mail) with the professional sports team on a player’s behalf. Finally, it is important to note that in order to maintain eligibility at the NCAA school, if a player receives assistance from an advisor, the player or parent will be required to pay that advisor at his or her normal rate for such services. A student-athlete will be deemed ineligible if he/she accepts any transportation or other benefits from any person who wishes to represent such student-athlete in the marketing of his or her athletic ability. This rule does not prohibit a student-athlete from having a meal with someone who wishes to assist in negotiations, provided each party pays for the actual cost of any meals and arranges for transportation separately.

NCAA rules were changed in 1992 to allow a student-athlete, parent or legal guardian to negotiate directly with a professional league team. If a student-athlete reaches a written or verbal agreement for any portion of the terms of a professional contract, the student-athlete will have jeopardized his or her eligibility at an NCAA school. It has been noted that no matter how vigorously you may be persuaded that it is permissible to reach a verbal agreement for the terms of a contract, NCAA regulations do not permit a student-athlete to reach a verbal agreement for the terms of a contract. The receipt of any items of value will jeopardize a student-athlete’s eligibility at an NCAA school. In order for a student-athlete to retain NCAA eligibility, the student-athlete would not be permitted to receive any kind of entertainment expenses from the professional team. This includes the representative of the professional team purchasing a meal for the athlete or parents or paying expenses for the athlete to visit the city of the professional league team.

NCAA rules do permit, prior to collegiate enrolment, a student-athlete to receive an expense paid tryout with a professional league team, provided such a visit does not exceed 48 hours and any payment or compensation in connection with the visit is not in excess of actual and necessary expenses. Thus, a student-athlete may receive an expense paid visit from a professional team or professional combine, provided that the student-athlete participate in tryout activities that allow that club to evaluate the student-athlete. A student-athlete may receive such a visit either prior to or subsequent to a draft. Additionally, a student-athlete may try out at his/her own expense with any team for any length of time, provided the student-athlete ceases the tryout when he/she becomes a full-time student at any NCAA school.