In this corner, wearing the black and red, fighting out of the sports science lab…genetics! In the other corner, wearing Texas Longhorn orange, fighting out of the school of hard knocks…hard work! This six-round series will detail the differences in which genetics and hard work team up to build a machine out of man or woman.

Genetics is the study of heredity and variation in all forms of life. It follows codes and disciplines assigned by deoxyribonucleic acid (DNA), which is responsible for the blueprint of every living organism known as the genome. DNA is composed of nucleic acids adenine, cytosine, guanine, and thymine. In a normal DNA strand, cytosine binds to guanine and adenine binds to thymine.

The distribution of muscle fiber types is made up by DNA. There are two main muscle fiber types—Type I (slow twitch) and Type II (fast twitch). Type I muscle fibers are mostly aerobic and contain numerous mitochondria. They are largely involved in endurance-related activities like cycling and distance running. Type I fibers are slower to contract but have a greater resistance to fatigue, which prevents them from burning out.

Type II fibers branch off into two subsections, Type IIa and IIx. Type II muscle fibers are just the opposite of Type I fibers. They contract quickly but tend to fade faster. They mainly rely on the ATP-CP system for energy, which tends to deplete rapidly as well. Type II muscle fibers are more favorable for activities such as sprinting and weight lifting.


The distribution of the muscle fibers for most of the general population is 50:50. Professional athletes tend to have a bias one way or another depending on their competitive sport. Distance runners can be in upwards of 85 percent slow twitch fibers while sprinters can be 85 percent fast twitch fibers. Your muscle composition can help determine what sports you may excel or rather what you have potential to be good at.

For sedentary people, a large amount of Type IIx fibers tend to show up in muscle biopsies. According to an article written by Paul Moses, “In the early 1990s, Geoffrey Goldspink of the Royal Free Hospital in London suggested that the fast IIx gene constitutes a kind of ‘default’ setting. This hypothesis has held up in various studies over the years that have found that sedentary people have higher amounts of myosin IIx in their muscles than do fit, active people. Moreover, complementary studies have found a positive correlation between myosin IIa and muscle activity” (Moses, 1997).

From a training perspective, it makes sense that the Type IIx fibers are the default fibers. The more anaerobic activities an untrained person performs, the more likely those intermediate fibers are to become more efficient fast twitch fibers. However, if the untrained subject gravitated more toward aerobic activities, the fibers would be able to build up oxygen capacity through the increased size and number of mitochondria.

Your genetics play a big role in mapping out whether or not you’ll achieve a particular level in a specific sport. Being 6’5” gives you a great advantage in basketball, but you’ll have to pull a bar for 30 seconds to complete a deadlift. Being a ripped 185-lb sprinter is fantastic in the 100-meter dash, but you may not have the endurance to tackle a mile competitively.


Moses, P (1997) “Muscle Structure and Function” from Davin’s Bodybuilding and Fitness Pages. Here.

DNA molecule photo courtesy

Muscle fiber picture photo courtesy