Sulu showing off his abs
The abdomen was the part of a humanoid body between the thorax and the pelvis. The abdominal cavity was located here as well as several organs.
Another term for cramps was "post-prandial upper abdominal distension". (Star Trek IV: The Voyage Home)
According to an account that was anything but canon, during a party on the bridge of the USS Enterprise, a muscular Hikaru Sulu took off his shirt and said "look at these abs!" (VST: "Walk, Don't Run")
In 2153, Phlox prescribed Trip Tucker Aldebaran mud leeches against insomnia, of which he was supposed to place one on his chest and one on his abdomen. (ENT: "Anomaly (ENT)")
In 2268, Leonard McCoy was tortured by Vians, which caused many injuries, including internal bleeding in his chest and abdomen. (TOS: "The Empath")
In 2367, K'Ehleyr suffered from multiple stab wounds in her chest and abdomen after being attacked by Duras. (TNG: "Reunion")
In 2372, Samantha Wildman experienced a sharp pain in her abdomen due to the unborn Naomi Wildman shifting position. (VOY: "Deadlock")
In 2373, according to The Doctor, the abdominal organs of a deceased Borg drone were unremarkable. (VOY: "Unity")
According to Neelix in 2377, food laced with Talaxian wormroot caused abdominal spasms, which increased in severity for thirty to forty hours. (VOY: "Critical Care")