The main cheek muscle is the masseter, an important chewing muscle. In a normal horse, the jowls are fleshy and full. Stallions are known to have particularly full jowls.

A hollowed out appearance of the cheek muscles can result from a variety of problems.

This observation is commonly made in horses with "shear mouth" or steep angle of occlusion that prevent normal chewing movement. In this case, there is often very little side to side movement of the jaw and the muscles waste away because of disuse. Any condition that causes pain while chewing can reduce normal movement of the jaws and can also cause masseter muscle wasting.

Older horses that seem to chew normally can also have muscle wasting here. This area, like the top-line tends to lose muscle mass as a horse ages. However, older horses with poor teeth or other treatable dental problems that cause difficulty chewing often appear to have hollowed-out jowls or cheeks. This can be symmetrical or can affect one side more than the other.

Injuries that damage the muscles or the nerves that supply the chewing muscle will also cause muscle wasting and a hollowed out appearance. This usually occurs on only one side.