Customize Your Workout Results

Most people are aware that skeletal muscle adapts according to the demands placed on it. What is unclear to many exercisers however is how to customize their workout to achieve a specific outcome such as muscle hypertrophy (enlargement), strength or endurance improvement.

First, one must understand that muscles are composed of three different types of fibers - Type I fibers, Type IIA fibers and Type IIB fibers. Each type has unique biological characteristics and therefore must be trained according to their differences by using various exercise intensities and volumes. Second, one must understand that these three muscle types operate on an activation threshold continuum – the strength of the signal from the motor neurons and brain required to activate the fibers and cause them to contract. Thirdly, each muscle type has a unique activation speed or “twitch” Type I muscle is “slow twitch” whereas Type IIB muscle is commonly referred to as “fast twitch” fibers. Type I muscle is highly dependent on oxygen for its metabolism and energy generation and therefore are the “endurance” muscles that are primarily used for extended exercise such as marathon running. These muscles also have a low threshold of activation and slow twitch, and therefore are suited with high volume and low intensity/endurance exercise. Type IIB fibers rely very heavily on “oxygen independent” metabolism and are commonly referred to as being the “power muscles” of the body, being used for high intensity, quick and forceful actions like squats and power cleans. These fast twitch muscle fibers have a high threshold of activation and require large signal input from the motor neurons, being recruited during low volume, high intensity training. Type IIA muscle fibers are a hybrid between the two previously mentioned ones and therefore have both oxygen dependant and independent metabolic characteristics and a medium activation threshold and twitch response. These muscle fibers will be primarily called to action when performing moderate intensity exercise such as a bodybuilding (hypertrophy) routine. Keep in mind that since muscles operate on an activation threshold continuum, higher intensity exercise will not only activate and train the larger Type II fibers, but the Type I fibers also. 

The following is a summary format for resistance training (all ages and levels)