Fantasmo -- I agree that we are mostly inventing types of fantasy. However, some of what is imagined (or drawn second-hand historically) could have happened, even with those huge obstructive human spines.
The images we see in anatomy books also show that much of the extreme S-curve in the spine is because the medical image is supposed to be of a person standing up on their feet. But the spine has some flexibility. Think of how you can see a gymnast curl and move their torso around. If the bottom part of the pelvis is pulled forward then some of that curve straightens out. (Like thrusting out, or sitting, or having their knees lifted up.) People with lower back problems will lay on their back on the floor, and bend their knees with their feet on the floor. They can feel the small of their back (the curve) straighten out and touch the floor, as their knees come up, or if they pull their knees up to their chest.
Some of the ways of real historical pole impalement might have allowed for some of the memes and tropes of a fantasy (or dream-style) 'spit-roast' impalement story.
Some of the (few) historical drawings of impalement, may be from imagination of the artists, but some drawings do show the executed to be lifted and placed on a stake with the legs and knees held up in front. (Which would reduce the spine's curve). Or they are described (or drawn) on their backs with their legs lifted (also reducing the curve.). Or some have the executed on their face, on their belly, with their legs spread... but some of those descriptions include there being a small log sideways underneath their stomach and hips (curving the spine some). And there is the one testimony from the time of WW1, where the executed are facing the ground on knees and elbows, with bare ass in the air... which would curve part of the spine away from some of the path of an impaling pole.
The (few) historical descriptions of impalement sometimes will mention how the executioner in charge of the impaling had to be skilled (and lucky?) not to rapidly kill the executed. The executioner had to have the experience to visualize the human anatomy, and the location of the impaling pole. The rectum and vagina can flex and move. The spine can flex and twist, the chest can move. There are so many blood vessels and vital organs in the pole's path. The reality of impalement probably had a lot of bloody accidents and quick deaths, or other very uncomfortable events for the viewers to also see, smell, or hear. Like many other real-world executions, there are probably lots of details of the execution that would not be pleasant to observe close-up or from a distance. No matter the flexibility, skill, or endurance, having a pole through the executed would be agonizing most of the time they were aware. That's reality for us.
I prefer some fantasies.
Some impalement fantasies have idealized (and fantastic) circumstances of skilled & experienced executioners and assistants, With proper tools, taking their time moving an appropriate-sized pole in and checking the pole's position often. They do their duty so the executed will fulfill the expectations of the observers, and fit the customs and expectations of exotic historical justice (or oppression). The executed is attractive in their suffering enjoyment, thrashing, and groaning with much endurance, and 'good hair'. Perhaps in the impalement fantasy, the suffering or martyrdom slips into a surprising sensual realm of arousal, ecstasy, and orgasm, over and over. Hopefully, there will be a good fantasy shared the executioner, executed, and all the fantasy observers.