Fact 1: Muscle contraction occurs when muscle fibers get shorter. Fact 2: Exercise causes muscles to contract. Given the two facts above, what causes muscle fibers to get shorter?
Choose your answer from:
1). friction.
2). acetic acid.
3). heat or cold.
4). sweating.
5). electrically.
6). resistance.
7). Damages them.
8). exercise.
Answer:
8).