Here's why:

* Muscle cells are specialized to contract, which is the process of shortening and thickening. This contraction is what allows for movement.

* Relaxation is the opposite of contraction, where the muscle cells lengthen and return to their resting state.

There are three types of muscle tissue:

* Skeletal muscle: Attached to bones, responsible for voluntary movement.

* Smooth muscle: Found in the walls of internal organs like the stomach and bladder, responsible for involuntary movement.

* Cardiac muscle: Found only in the heart, responsible for pumping blood.

All three types of muscle tissue work together to allow for movement and function of the body.