Question

Which myosin isoform is found in fast-twitch muscle?

a) type I
b) type IIa
c) type IIb
d) typestudent submitted image, transcription available below
e) none of the above

Answer 1

The myosin isoform found in human fast-twitch muscle fibers is type IIx or type IId, both representing the same subtype that is capable of the quickest contractions.

Step-by-step explanation:

The myosin isoform found in fast-twitch muscle is type II. Within the fast-twitch category, there are several subtypes, including type IIa (oxidative), type IIx (also known as type IId), and type IIb. The question specifically asks which type is found in fast-twitch muscle fibers, which are responsible for quick, anaerobic movements like weight lifting. Type IIa are oxidative and still relatively slow compared to other fast-twitch fibers. Type IIx/D is less dense in mitochondria and myoglobin but is the fastest in humans capable of the quickest contraction. Type IIb is even less dense in mitochondria and myoglobin and is typically found in small animals such as rodents. Since the question seems to focus on human muscle, the correct answer would be (b) type IIx or type IId.