Difference in Muscle Types

The muscles in our bodies are made up of three different types of muscle fibers, Type I, Type II a, and Type II b.  Each of our bodies contains both Type I and Type II fibers, yet most have more of either Type I or Type II.  The first, Type I is also called slow twitch muscle fiber, and it generates ATP predominately through the aerobic system of energy transfer.  Physical activities that utilize the Type I muscle fibers are endurance type events and low intensity sports such as long distance cycling.  Type I muscle fibers are extremely resistance to fatigue, yet they have low ability in terms of force output.  Athletes with a predominance of Type I muscle typically posses a lean body type, with a low body fat percentage (think of an Olympic marathon runner’s physique).  In terms of how to train to increase and enhance Type I muscle fibers, aerobic exercise of durations over 45 minutes is the most effective means. 

                The second type of muscle fiber categorized into two subdivisions, Type II a, and Type II b.  This type is referred to also as fast-twitch fiber.  Type II a is characterized by a fast shortening speed and well-developed capacity for energy transfer from aerobic to anaerobic sources.  Physical activities that generally rely on the performance of Type II a fibers are shorter distance running sports such as sprinting, and other power-driven sports such as power lifting and baseball.  Type II a muscle fibers have an intermediate resistance to fatigue, and athletes with a predominance of these fibers typically have larger muscles which allow them to produce high force output for long periods.  High intensity training such as sprinting and weight training is the most effective means of training this type of muscle fiber.

                The third type is another subdivision of fast-twitch muscle fiber, referred to as Type II b.  Our text defines this type as having the most rapid shortening velocity and the greatest anaerobic potential.  Often called the “couch potato fibers,” this type is prevalent in sedentary individuals and is believed to help these individuals respond to emergency situations.  These fibers have no resistance to fatigue and are extremely strong.  That being the case, they are susceptible to injury when used and often their use in situations leaves them damage beyond repair.  According to Fitness: The Complete Guide by Dr. Frederick Hatfield says that when sedentary people begin a training program, they immediately lose their Type II b muscle fibers.