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Muscle Fibers

It is generally accepted throughout the world that there are two different types of muscle fibers. Slow twitch (Type I) muscle and fast twitch (Type II) muscle fiber. From there, you can further categorize fast twitch muscle fiber into Type II-a and Type II-b.

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 II-b 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, power lifting, and bodybuilding. Type II-a muscle fibers are intermediate and their properties lay between types I and type II-b.

How Type I & Type II Muscle Fibers Are Different

Type I fibers are different than type II-b fibers for many reasons. You can think of them as opposites. Type I is for long endurance activity while type II-b is for short fast bursts. Type I fibers are highly oxidative (more oxygen supplied through blood) and are not likely to grow in size as much. Type II-b fibers are highly gycolytic (not as much oxygen supplied) and tend to grow in size more than type I fibers.

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 to 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 II-a muscle fibers. And finally, for the highest intensity movements, it will recruit the type II-b fibers. This is why type I fibers are called low threshold, and fast type II-b 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.

Recruitment In Low Rep Sets

Low repetition work (in the 1-5 rep range) provides an extremely unique adaptation. To overcome the weight, your body must recruit as many motor units as humanly possible. This will cause your nervous system to become more efficient at this process. Over time, you will learn to lift the heavier weight with all (or close to as possible) of your motor units in one rep. Power lifters are brutally strong for this reason. They can basically make all the their motor units fire at once.

How Does A Rep Range Affect Your Muscle Fibers & Strength Gains?

Energy Systems

No matter what sport you play, your body needs energy for top performance. Energy is supplied to your muscles from the food you eat. Your body breaks the food down into usable blocks of energy called Adenosine Triphosphate (ATP). ATP is the immediate source of energy for muscle contraction. Your body makes ATP available for muscle contraction through three main energy systems that are located within the muscle fibers. The energy system used depends on the intensity and duration of the activity.

• 1. The ATP-PC, or Creatine Phosphate System, does not require oxygen to produce energy.
  
  
• 2. Anaerobic Glycolysis uses glycogen stores in the muscle to produce energy without oxygen.
  
  
• 3. Aerobic Glycolysis uses muscle glycogen to produce energy and occurs in the presence of oxygen.
  
  
• 4. Oxidative Phosphorylation uses fats stored in the body to produce energy and also requires oxygen.

Effect of Event Duration on Primary Energy System(s) Used

ATP-PC System

The ATP-PC (Adenosine Triphosphate Phospho-Creatine) system is predominant in maximal or super-maximal activities lasting up to 20-30 seconds. As the duration of activity increases the ATP-PC system provides a smaller portion of total energy. The ATP-PC system is utilized during the transition from rest to exercise, and also during the transition from one exercise intensity to a higher intensity. A period of 30 seconds to 3 minutes is needed to replenish the energy in this system; however, during aerobic exercise ATP-PC reserves can be restored.

Anaerobic Glycolysis

As the ATP-PC system begins to fade after around ten seconds, a process known as Anaerobic Glycolysis begins to occur. Anaerobic Glycolysis is the primary energy source in activities lasting between 30 seconds and 3 minutes. Anaerobic Glycolysis continues to supply energy during exercise lasting up to ten minutes. This system breaks down muscle and liver glycogen stores without the use of oxygen. The byproduct of this system is lactic acid.

Aerobic Glycolysis

After about 3 minutes of exercise, Aerobic Glycolysis is the dominant energy system. Aerobic Glycolysis produces energy by breaking down muscle and liver glycogen stores with oxygen present. Because oxygen is present when this system is in use, there is no build up of lactic acid. This system does not produce energy as fast as the ATP-PC system or Anaerobic Glycolysis thus the intensity of exercise cannot be as high. This system has the capacity to produce energy for an hour or more.

Oxidative Phosphorylation

Oxidative Phosphorylation provides the body with energy during exercise of long duration and moderate to low intensity. This system breaks down the body's fat stores to supply energy to working muscles. As the intensity of exercise decreases, the body relies more on this energy system. This energy system can supply virtually unlimited supplies of energy. Endurance sports such as cross-country running, swimming, soccer and lacrosse all rely heavily on this system. However, speed and power can often be the determining factor in winning and losing. Therefore careful attention must be paid to developing both energy systems to achieve top performance.

Intensity Determines the System:

As the graph below shows, all three energy systems are active at any given time, but depending on the intensity and duration of the activity, different systems will be primarily stressed. High intensity, short duration activities stress the ATP-PC system. As the intensity slightly decreases and the time increases Glycolysis kicks in. Then as the intensity is further reduced and the time increased, the Aerobic System is primarily used.

Primary Metabolic Demands of Various Sports

This is an excerpt taken from a chapter in the ebook: Critical Speed Manual. The ebook was written by Mark Strasser M.S. CSCS, a professional strength and conditioning coach.

Critical Speed Manual
Decrease Your 40 Yard Dash with the Critical Speed Manual. If you are interested in gaining speed and training with a World-class strength and conditioning coach, then this is the most exciting training manual you'll ever read!

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