Muscle contraction occurs when muscle fibers get shorter. Movement is based on the contraction of muscles. Movement happens when muscle fibers get shorter.
This justifies what answer for what question? Q & A: 
What do muscle fibers need to do to enable movement?
Options:
- (A) Bend
- (B) strength
- (C) exercise
- (D) ligt
- (E) It expands
- (F) shorten
- (G) Vibrates
- (H) stay warm
(F)