Question

when the sarcomere shortens as a muscle contracts, which of the following is true?
both thick and thin filaments shorten.
both the thick filaments and the thin filaments move over each other in opposite directions.
thin filaments are stationary and the thick filaments slide over them moving towards the center of the sarcomere.
the thin filaments shorten.
thick filaments are stationary and the thin filaments slide over them moving towards the center of the sarcomere.
the thick filaments shorten.
rewatch

Explanation:

According to the sliding - filament theory of muscle contraction, during muscle contraction, the thick filaments (myosin) are stationary. The thin filaments (actin) slide over the thick filaments towards the center of the sarcomere. The lengths of the thick and thin filaments themselves do not change (they do not shorten).

Answer:

Thick filaments are stationary and the thin filaments slide over them moving towards the center of the sarcomere.