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 type of ACTN3 genes that person has.
D) number of hours the individual spends each week engaged in aerobic activity, like running or cycling.

Question

Quizplus · Accepted Answer

The proportion of fast- and slow-twitch fibers in an individual's muscles is determined by the type of ACTN3 genes that person has. Specifically, individuals with one or two copies of the "RR" version of the ACTN3 gene are more likely to have a higher proportion of fast-twitch fibers, while individuals with two copies of the "RX" or "XX" version of the gene are more likely to have a higher proportion of slow-twitch fibers. Diet and hours spent engaged in aerobic or anaerobic activity may play a role in overall muscle development, but they are not primary determinants of fiber type proportion.

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