In a proper fraction, the numerator is smaller than the denominator. This holds true no matter how large both are. As long as the numerator is the smaller of the two, the fraction is a proper fraction. For example, 998/999 is a proper fraction. Even though the numbers are both large, and they're only separated by 1, the numerator is still smaller than the denominator. Likewise, 1/999 is a proper fraction.
Due to the fact that the sural nerve is so close to the surface of the skin, it makes it much more likely to become injured due to compressions on the skin and the fact that most injuries occur on the surface of the skin.
In brief: Nerves. Inflammation, irritation or contact with a nerve will cause an electrical type sensation distal to the contact site. I occasionally get this in my lateral knee when i run and always in my forearm when I do dips. It's related to muscle or facial compression of the nerve while exercising. If you have this sensation frequently or without obvious mechanical compression, discuss it with your doctor. In brief: Nerves. Inflammation, irritation or contact with a nerve will cause an electrical type sensation distal to the contact site.