How We Get the Energy to Run

Energy System Characteristics


(rate of ATP production)

(total ability to produce ATP)

Fuels Used

Phosphagen System

Very High

Very Low

creatine phosphate
stored ATP




blood glucose
muscle & liver glycogen

Aerobic System


Very High

blood glucose
muscle & liver glycogen
adipose & intramuscular fat

Fat, which is stored as triglyceride in adipose tissue underneath your skin and within skeletal muscles (called intramuscular triglyceride), is the other major fuel for the aerobic system, and is the largest store of energy in the body. When using fat, triglycerides are first broken down into free fatty acids and glycerol (a process called lipolysis). The free fatty acids, which are composed of a long chain of carbon atoms, are transported to the muscle mitochondria, where the carbon atoms are used to produce acetyl CoA (a process called beta-oxidation). Following acetyl CoA formation, fat metabolism is identical to carbohydrate metabolism, with acetyl CoA entering the Krebs cycle and the electrons being transported to the electron transport chain to form ATP and water. The oxidation of free fatty acids yields many more ATP than the oxidation of glucose or glycogen. For example, the oxidation of the fatty acid palmitate produces 129 ATP. No wonder you can sustain an aerobic activity longer than an anaerobic one.

More: 3 Long-Run Principles to Remember

Understanding how energy is produced for physical activity is the basis for getting the most out of your workouts. So, next time you get done with a workout and think, "I have a lot of energy," you'll know exactly where you got it.

 Energy System Workouts

Be sure you warm-up and cool-down before and after each workout.

Phosphagen System:
Short, very fast sprints lasting 5 to 15 seconds with 3 to 5 minutes rest between each. The long rest periods allow for complete replenishment of creatine phosphate in the muscles so it can be reused for the next interval. 

  • 2 sets of 8 x 5 seconds at close to top speed with 3:00 passive rest & 5:00 rest between sets
  • 5 x 10 seconds at close to top speed with 3:00-4:00 passive rest

Fast intervals lasting 30 seconds to 2 minutes with an active recovery period twice as long as the work period (1:2 work-to-rest ratio).

  • 8-10 x 30 seconds fast with 1:00 active recovery 
  • 4 x 1:30 fast with 3:00 active recovery

Aerobic System:
While the phosphagen system and glycolysis are best trained with intervals since those metabolic systems are only emphasized during high-intensity activities, the aerobic system can be trained with both continuous exercise and intervals.

  • 60 minutes at 70 to 75 percent max heart rate
  • 15- to 20-minute tempo run at lactate threshold pace (about 80-85 % max heart rate)
  • 5 x 3:00 at 95 to 100 percent max heart rate with 3:00 active recovery


More: Should You Run More Miles?

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