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

Fact 2: Isometric exercise is a sustained contraction of large muscles, such as weight lifting.

Given the two facts above, answer the question "Weigh lifting requires muscles" with the following options: 
- to move people
 - Energy.
 - stamina
 - it needs them
 - shortening
 - hydration
 - sensory neurons
 - Energy


 

     shortening    