Age, Weight, and Pain

My niece recently went camping and after sleeping on the ground, developed a very sharp pain in her lower back above her right hip. Her two children, sleeping in basically the same conditions, did not experience any pain. My niece surmised that this was because of their lower body weight distributed on the ground. My niece's husband told her that her reasoning was incorrect, and that the distribution of body weight was proportional, the inference being that a child could easily develop a painful pressure point in the same location, even though their overall body weight was only 30 to 40 percent of an adult's overall body weight. Who was correct? Does greater body mass make one more likely to develop painful pressure points, or would such painful spots develop just as readily in someone of much less size and weight?
- Tim Stevens (age 58)
Montgomery, AL, USA

This question involves a basic physics issue first described by Galileo. If you were to scale an animal up, say twice as big in every direction, there would be problems. The weight would go up by a factor of 2x2x2=8. The cross-sectional area of legs would only go up a factor of 2x2=4. So to support the bigger animal you'd need a different shape, with relatively thicker legs. Compare a deer and an elephant.

Us grown-ups are more like the elephants,with bigger weight per unit area. That's a factor here, as your niece said.

Although it's not a physics issue, I have to add that us old folks also are just plain more likely to get aches and pains, and we repair the damage to tissue much more slowly. I just got back from a family gathering where this was a major topic of discussion, to which I contributed too much. The younger people had other things to talk about.

Mike W.

(published on 03/22/2013)