CVS Practice Test

What happens to the distance between Z lines during complete contraction?

1. It increases

2. It decreases

3. It remains the same

4. It varies

The correct answer is: It decreases

During complete contraction of a muscle, the distance between the Z lines decreases. The Z lines mark the boundaries of a sarcomere, which is the functional unit of muscle contraction. When a muscle contracts, thick and thin filaments slide past each other, a process known as the sliding filament theory. As the actin (thin) filaments slide toward the center of the sarcomere, the Z lines are pulled closer together. This shortening of the sarcomere translates into the overall contraction of the muscle fiber. Thus, in a fully contracted state, the distance between the Z lines is reduced, confirming that the correct answer is indeed that it decreases.