I am adding force to an object and changing the maximum speed variable when I press LShift, just as a normal pc keymapping will do for things such as sprinting. The only thing is, RShift is working, but LShift is not, when it comes to inputting the keyboard key. Both options are available, but RShift controls both sides of the keyboard while LShift does nothing.
I noticed also, doing the same thing through the events tab directly to the key press, RShift is the only one that shows up.
Hope this helps!