The proportion of fast and slow-twitch fibers found in an individual's quadriceps muscle is primarily determined by the
A)individual's diet.
B)number of hours the individual spends each week engaged in anaerobic activity, like weightlifting.
C)the number of ACTN3 genes that person has
D)number of hours the individual spends each week engaged in aerobic activity, like running or cycling.

Question

The proportion of fast and slow-twitch fibers found in an individual's quadriceps muscle is primarily determined by the​ 
A)individual's diet. 
B)number of hours the individual spends each week engaged in anaerobic activity, like weightlifting. 
C)the number of ACTN3 genes that person has 
D)number of hours the individual spends each week engaged in aerobic activity, like running or cycling.

Quizplus · Accepted Answer

The proportion of fast and slow-twitch fibers is primarily determined by the number of ACTN3 genes that an individual has. ACTN3 is a gene that codes for a protein found in fast-twitch muscle fibers, so having more of these genes generally results in a higher proportion of fast-twitch fibers. Other factors, such as exercise habits and diet, can also have some impact on muscle fiber composition, but the genetic component is the most influential.

The Proportion of Fast and Slow-Twitch Fibers Found in an Individual's