You might think that drinking a sports drink is just like drinking any other drink; you open the bottle when you are thirsty and drink up. You would be wrong. If you drink one of these scientifically-engineered drinks when you are already thirsty, you have missed many of its benefits. These drinks are not designed to be guzzled at the end of a long workout, but are meant to be sipped an ounce or two at time every 10 to 15 minutes during a workout. The sodium used in sports beverages are designed to maintain your blood volume during exercise, because it can decrease through loss of fluids, aka sweat. Plus you need to make sure there is enough blood to deliver energy to the working muscles in the periphery. Sports drinks also provide carbs, which help maintain blood sugar levels to keep the brain functioning and happy (the brain is fueled by carbs). Mental fatigue can cause muscle fatigue.
You should drink a sports drink, I mean sip a sports drink every 10 to 15 minutes, if you are sweating and your heart rate is elevated, as in exercise, for over a half an hour. All you need is a sip, which is about an ounce or two — between an 1/8 to a 1/4 of a cup or 30ml to 60ml. Now if you are just walking for 30 minutes on a cool, crisp day, water should do you just fine.