Fact 1: Muscle contraction occurs when muscle fibers get shorter.

Fact 2: Movement is based on the contraction of muscles.

Given the two facts above, answer the question "What do muscle fibers need to do to enable movement?" with the following options: 
- Bend
 - strength
 - exercise
 - ligt
 - It expands
 - shorten
 - Vibrates
 - stay warm


 

      shorten   