Our muscles have fast and slow twitch fibers. Fast twitch fibers allow for sudden bursts of activity, such as sprinting, because they split ATP faster. They give us speed. Slow twitch fibers give us endurance and split ATP slower, like the names imply. Carbohydrate is important for speed, because it better fuels those particular muscle fibers. Carbohydrate replenishes glycogen which increases fuel availability to the fast twitch fibers.

http://www.indoorclimbing.com/muscles.html

This is why it is often said that sprinting, or lifting weights, or other "intense short burts" of activity tax sugar for energy, whereas slower endurance activities like walking burn fat better.

I have a tendency to hypoglycemia. I notice I become hypoglycemic very easily if I do a lot of upper body lifting, for example, much much more easily than if I walked for hours and hours. Walking all day will never give me a hypo. On the other hand, walking with a sack of books or heavy groceries? Almost always will.

At work (I am a cashier part time), if I lift too many jugs, I start to "shake" because this kind of activity uses a huge amount of sugar relative to fat. I come home at the end of the day and I am almost always shaking and low blood sugar. I am also often bloated at the end of my workday, which, I think has something to do with the hypoglycemia-inducing conditions (not sure what, but it has to be related... perhaps the constant raising of adrenal hormones is responsible?).
The absolute worst is when I have been sleep deprived for a few days, and then have work the next morning. The combination of all the lifting plus the metabolic state of sleep deprivation means my body just cannot possibly make enough sugar or energy to do what is demanded of it. In fact, I have taken to always having a liter of caffiene with me just so I have a way to keep my sugar level up, just so I can *function* at work. (God bless you diet pepsi, you wonder drug of sugar energy... a couple of swigs and I am like a whirlwhind for a short while).
Of course this results in horrible consequences of worse lethargy later on in th evening (and sometimes reaaally weird blood sugar/energy problems).

Walking, on the other hand, like I said it is far less likely to make me severely hypo (to the point where I shake, as in, a sudden dip). In fact, walking improves my fat metabolism tremendously, and, at the end of a long day of walking I can tell I am much more ketogenic. I am also almost always likely to lose water. Lifting a few heavy things is enough to make me "shaky" but if I walk from my house to the end of my city, not nearly as likely and my body burns fat more.

Endurance is all about fat and pro. Sudden, short, relatively intense activity is all about carbs. Moderate cardio, walking, etc = fats. Weight lifting = glycogen and carbs.

BTW the types of activity you do also determine how well your body uses certain fuels, and how capable you are of the activity. If you run, walk, or stand often you strengthen and increase the amount of slow twitch fibers, which means you gain endurance in those activities and can use fat for energy better. If you do a lot of lifting, sprinting, or other activities you improve your capacity to do these things, plus you increase how many carbs you can eat and tolerate (because you are constantly sucking your sugar low and using up glycogen that way).

That's why weight lifting has such a tremendous benefit for people with insulin resistance. It's the best way to improve how fast we clear out glucose and raises your need for sugar.