National Academy of Sports Medicine (NASM) Group Fitness Instructor Practice Exam

Within muscle fibers, what are the two long protein filaments called that slide past each other to produce muscular contractions?

1. Collagen and elastin

2. Actin and myosin

3. Tropomyosin and troponin

4. Keratin and fibrin

The correct answer is: Actin and myosin

The correct answer is based on the fundamental mechanism of muscle contraction, which is largely dependent on the interaction between actin and myosin filaments. These two types of proteins are essential components of muscle fibers and are responsible for the sliding filament theory of muscle contraction. Actin is a thin filament, while myosin is a thick filament. During muscle contraction, myosin heads attach to binding sites on the actin filaments, forming cross-bridges. As these cross-bridges pivot and pull the actin filaments toward the center of the sarcomere, the muscle shortens and generates tension. This sliding action between actin and myosin underlies all voluntary muscle contractions, whether in skeletal, cardiac, or smooth muscle. The other options listed do not play this specific role in muscle contraction. Collagen and elastin are types of connective tissue proteins that provide structural support and elasticity to muscles and tendons rather than being involved in the contraction process. Tropomyosin and troponin are regulatory proteins associated with actin and play a role in the regulation of contraction, but they do not directly contribute to the sliding mechanism themselves. Keratin and fibrin are also not involved in muscle contraction; keratin is more related to the structure