# Comparative leg sizes

When I was a child, my father showed me the classic essay "On Being the Right Size", J. B. S. Haldane. It talks (among other things) about how large animals need stockier legs to support their weight. I weigh about the same per leg as a 350 lb deer, but my legs are closer in diameter to those of a 1500 lb horse. Why?

• Nice question. :-) I guess he was wrong or was talking about averages and not individual species... Commented Nov 11, 2014 at 3:03
• @inf3rno: he was clearly talking about averages, based on the body mass increasing with the cube of linear dimensions and the weigh bearing capability of legs increasing with the square. I suspect the answer is humans stress their legs in ways that deer and horses do not, so it is advantageous to have stronger legs, but that is why I asked. Commented Nov 11, 2014 at 3:18
• There is no way your legs are close in size to a horse. Commented Nov 11, 2014 at 4:36
• @canadianer: they are not as long, but in diameter they seem close. I haven't been that close to horses, so I may be off. When I asked a farrier this question he seemed to think it was close. Commented Nov 11, 2014 at 4:38
• You need to view the chapter, not just the look inside. Qualitatively, the horse femur appears ~2.5 times the diameter of the human femur. Same for the tibia. Commented Nov 11, 2014 at 5:01

## 1 Answer

There is an interesting relationship between the length of an animals femur, the thickness and the mass of the thickness. A point that should be taken into account is that bones can different densities and so might be able to support varying amounts. The muscle in you leg also does nothing for the support of you weight it is (basically) all in the bone. Really looking purely at the leg won't tell you much you need to strip back the tissue and get you measurements straight from that. Given the different gate strategies of various animals this can lead to differing levels of muscle surrounding the leg and could be confounding your answer.

Walter Lewin a professor of physics at MIT did a lecture series where in the first lecture he went off on one about the relative sizes and thickness of animal femurs and it is really interesting (http://www.youtube.com/watch?v=PmJV8CHIqFc - between 11 minutes and 22 minutes). The short answer was that the ratio between the thickness and the length stays similar relative to the length.