Getting the wind knocked out of you is a commonly used idiom

Idiom

Idiom is an expression, word, or phrase that has a figurative meaning that is comprehended in regard to a common use of that expression that is separate from the literal meaning or definition of the words of which it is made...

 that refers to a kind of diaphragm

Thoracic diaphragm

In the anatomy of mammals, the thoracic diaphragm, or simply the diaphragm , is a sheet of internal skeletal muscle that extends across the bottom of the rib cage. The diaphragm separates the thoracic cavity from the abdominal cavity and performs an important function in respiration...

 spasm

Spasm

In medicine a spasm is a sudden, involuntary contraction of a muscle, a group of muscles, or a hollow organ, or a similarly sudden contraction of an orifice. It is sometimes accompanied by a sudden burst of pain, but is usually harmless and ceases after a few minutes...

 that occurs when sudden force is applied to the abdomen

Abdomen

In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...

 which puts pressure on the solar plexus

Celiac plexus

The celiac plexus or coeliac plexus, also known as the solar plexus, is a complex network of nerves located in the abdomen, where the celiac trunk, superior mesenteric artery, and renal arteries branch from the abdominal aorta...

. This often happens in contact sport

Contact sport

Many sports involve a degree of player-to-player or player-to-object contact. The term "contact sport" is used in both team sports and combat sports, medical terminology and television game shows, such as the Gladiators and Wipeout, to certain degrees...

s or by falling on the back. It results in a temporary paralysis of the diaphragm that makes it difficult to breathe. This can lead to anxiety and there may be residual pain from the original blow, but the condition should clear spontaneously in a minute or two.