Why Is it so Hard to Run on Sand?

First of all, if you’re an exercise physiologist, it’s not sand. It’s particulate matter. And the reason that running on it is so damn hard is that every time you attempt to push off it, it gives way, requiring runners to exert approximately 1.6 times the energy that would be required to maintain the same speed on solid ground. For walkers, make that 2.1 ot 2.7 times for energy.  The phenomenon is explained nicely in this piece in the Washington Post, which has some good news for those who make the effort to run in sand. The Post reports that a 1998 Turkish study of 60 men ages 15 to 21 showed that sand running
produced a greater increase in calf circumference over road running and
a larger boost in maximum aerobic power (VO2max). The calf size
increases came from the muscle overload effect; the VO2jump resulted
from the sand group’s working harder to keep up with the road runners. Son of a beach.