What are the major types of skeletal muscle fibers?

There are three major types of skeletal muscle fibers – Type 1 or Slow Oxidative fibers, Type 2A or Fast Oxidative fibers, and Type 2B or Fast Glycolytic fibers. 

Main Characteristics Of Type 1 Or Slow Oxidative (SO) Fibers

• Are also known as slow-twitch fibers
• Contract relatively slowly
• Have a high concentration of the pigment myoglobin and a rich capillary supply
• Contain numerous mitochondria and aerobic respiratory enzymes
• Produce ATP using aerobic respiration (oxygen and glucose)
• Generate low-power contractions over extended periods of time
• Are slower to fatigue
• Are unable to produce high tension

The ability to function for long periods without getting fatigued makes slow oxidative muscle fibers useful for stabilizing bones and joints, maintaining posture, producing isometric contractions, and making other small movements that don’t require large amounts of energy

Main Characteristics Of Type 2 A Or Fast Oxidative (FO) Fibers

• Contract rapidly
• Contain high amounts of mitochondria 
• Are able to generate ATP quickly, allowing it to produce relatively high amounts of tension within the muscles
• Produce ATP aerobically  
• Use aerobic respiration primarily but may also switch to anaerobic respiration or glycolysis
• Fatigue faster than slow oxidative fibers but slower than fast glycolytic fibers

Their ability to produce more tension makes fast oxidative muscle fibers useful for movements such as walking, which require more energy than that required to maintain posture but less energy than that required for explosive movements such as sprinting. 

Main Characteristics Of 2 B Or Fast Glycolytic (FG) Fibers

• Have large diameters
• Primarily use anaerobic glycolysis as their source of ATP
• Contain high amounts of glycogen, which is used in glycolysis to generate ATP quickly in order to produce high levels of tension rapidly
• Possess very little myoglobin and mitochondria because they not use aerobic metabolism 
• Have rapid contractions
• Fatigue more quickly as compared to SO and FO fibers

Their ability to rapidly generate high levels of tension make fast glycolytic fibers used for producing rapid, forceful contractions that result in quick, powerful movements. However, because they fatigue quickly, they can only be used for short bursts of high energy and not for prolonged periods of time.