Hereditary spastic paraparesis (HSP) has a few different names, but the common theme is that the person afflicted with this rare nerve disease has weak and stiff leg muscles. Only about 20,000 people in the United States have the disease and as it’s name indicates it is genetic, but recessive. The simple form is relegated to only affecting the legs, but there are complicated versions that can affect the upper body as well. Although you are born with the disease, it is progressive so may not show until young adulthood.
Whenever I look something about the disease I use this site. I find it to be the best organized and most comprehensive: Spastic Paraplegia Foundation