Yes - Bananas have potassium, the next most important electrolyte next to good old sodium. They're the best natural form of potassium you can get. Canteloupe is a close second.
I believe that calcium and magnesium are also electrolytes, thus their inclusion in Sport Legs.
Ken: They come in both powder and pill form. I use both. I put what I think I'll need in my bottles in powder form, then supplement with pills if necessary. If the legs start feeling tight (prelude to cramps), I start popping them.