ENERGY SYSTEM: Aerobic
USED FOR: Steady burning of fuel for any moderately paced, sustained cardio training (running, cycling, swimming, etc.)
HOW IT WORKS: For longer efforts, this system provides a sustainable supply of adenosine triphoshate (ATP), the molecule that causes contractions in your muscles. For the first 15 to 30 minutes, fuel comes mainly from glycogen stored in your muscles. Muscle fat contributes more fuel for up to 90 minutes, and after that, adipose fat (the jiggly kind) is burned as well. You have about 1,400 calories' worth of stored glycogen in your muscles and another 400 in your liver—enough for a 1.5-hour run.

ENERGY SYSTEM: Anaerobic glycolysis
USED FOR: Sustained speed for high-paced efforts, extended resistance training, intervals
HOW IT WORKS: This system kicks in when you're out of breath but still moving fast. It produces ATP at a high rate for about two minutes, using a combination of creatine phosphate (a naturally occurring compound chemical stored in muscles) and muscle glycogen. Lactic acid is a by-product.

ENERGY SYSTEM: Creatine phosphate
USED FOR: Explosive energy required for resistance training and repeated sprints
HOW IT WORKS: This system rapidly produces ATP from creatine phosphate. It yields about six to eight seconds of fuel for explosive efforts. You can take creatine supplements to bolster the system, but it helps you do only a few extra reps during power building or gives you an additional boost for repeated sprints.

Five Habits of Highly Successful Eaters
(1) EAT A GOOD BREAKFAST. It jump-starts your metabolism, provides energy for mid-morning workouts, and keeps you from losing steam before lunch.
(2) SNACK OFTEN. Skipping meals, or waiting more than five hours between them, can slow your metabolism, cause energy dips, and lead to overeating. Bring fresh fruit and vegetables to work so you never resort to the candy machine.
(3) PLAN AHEAD. Look at your schedule for the coming week, plan meals, and allow time for grocery shopping. You make time for working out; you should do the same for cooking healthy food.
(4) BE RESTAURANT SAVVY. It's fine to eat out; just don't treat it like a nutritional vacation. Avoid supersize portions, ask about ingredients, and make smart substitutions (like a baked potato instead of fries).
(5) GET EIGHT HOURS OF SLEEP. Inadequate rest alters levels of appetite-regulating hormones, leading to increased appetite (but not increased activity).