Which type of muscles have a predictable tendency towards tightness?

The correct answer is postural muscles.

Postural muscles are muscles with the function to sustain posture in the gravity field. In response to stimuli such as over-use, under-use or trauma, postural muscles tend to shorten. This type of muscles is sometimes called  tonic muscles because they have high resting tonus.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.