Muscle Fiber Types
Endurance sports are all about movement. And to move our bodies through space and time we use muscles. Muscles operate when a signal is sent from the central nervous system (CNS) to individual cells (also called muscle fibers) in your skeletal muscles. As a sufficient number of muscle fibers are recruited for the task, they then contract to produce movement toward your goal.
Skeletal muscle fibers come in different flavors. When you see the difference between the leg meat and breast meat of a chicken, you know that this is literally true—dark meat tastes different than light meat. These different color patterns are a result of the chicken legs consisting of a higher percentage of slow-twitch muscle fibers while chicken breasts contain a higher percentage of fast-twitch muscle fibers.
Slow-twitch muscle fibers—also called Type I—contract more slowly, just as the name implies. They are built to bring in oxygen and maximize the production of energy through the aerobic pathway. To those ends, they contain a greater number of capillaries and proteins called myoglobin to carry oxygen into the cells, along with a greater number of mitochondria which act as cellular factories for aerobic energy production. The iron-rich pigments associated with myoglobin are responsible for the color of dark meat.
Fast-twitch muscle fibers—also called Type IIb (or IIx)—contract more quickly and with more force than any other fiber type. They contribute substantially to shorter bursts of speed and do so through anaerobic energy production pathways. When a chicken is startled and flaps its wings to get away from a perceived threat, it recruits those fast-twitch fibers in the chest to move quickly. In contrast, the slow-twitch muscle fibers in the chicken’s legs are recruited for the long duration task of walking around the farmyard all day long.
There’s also a third muscle fiber type. These are called intermediate fast-twitch, or Type IIa muscle fibers. These muscle fibers possess some of the aerobic characteristics of the slow-twitch fibers as well as some of the increased contractile capability of the fast-twitch fibers.
When you decide to move your body, the first fibers to be recruited are the slow-twitch fibers. If the force demands are great enough, then the intermediate fast-twitch fibers are recruited and finally the fast-twitch fibers are called up for duty. The more force and speed you need for a given activity, the higher up the recruitment list you go.
Crucially, another factor that leads to recruiting higher up the list is fatigue. When your slow-twitch fibers become fatigued, the intermediate and fast-twitch fibers are recruited to share some of the burden. This is why even endurance athletes need to train all the fiber types. Even fast-twitch fibers are brought in to help out in endurance events as fatigue sets in. The athlete who has trained those fibers through faster paced speed work will be in a better position than the athlete who only trained by doing long aerobic workouts at lower end speeds.
Although it’s true that genetics have a great deal to do with what type of fibers predominate in your skeletal muscles, endurance training can increase the aerobic capabilities of even fast-twitch fibers. A world-class sprinter may never become a world-class marathoner, but that sprinter can nevertheless improve his or her marathon time substantially through endurance training.
TRAINING GUIDE CONTENTS
– Train with a Purpose
– The ABCs of Systematic Training
– The R&R of Training
– Begin with the End in Mind
2. Exercise Science Concepts
– Overreaching and Overtraining
– Energy Systems
– Aerobic Capacity
– Lactate Threshold
– Aerobic Threshold
– Muscle Fiber Types
3. Monitor Your Training Intensity
– What is Training Intensity?
– Key Indicators of Intensity
– Using Training Zones
– Training by Feel, or Perceived Exertion
– Training with Pace
– Training with Heart Rate
– Running with Power
4. Create Your Training Plan
– Prioritizing Your Events
– Overview of the Training Phases
– Choosing Your Periodization Schedule
– Filling in the Details of the Overall Plan
5. Create Your Weekly Workouts
– Creating Weekly Schedules
– Establishing and Developing Your Base
– Building Upon Your Base
– Peaking for Your Target Event
– Race Week and Race Day Warmup
6. Functional Strength
7. Recovery and Nutrition
About the Author