Understanding how your body creates and uses energy is critical to understanding how to most effectively workout to see results through both building muscle and also burning fat.
Your body uses a substance called adenosine triphosphate (ATP) for all energy. ATP is structurally composed of a nitrogen-based compound, adenine; a five-carbon sugar called ribose; and three phosphates. The energy your body creates by using ATP is the energy necessary to make your muscles contract. There is a limited amount of ATP in each cell, so for your body to be able to create energy on an ongoing basis, each cell must be able to either create more or get more from another location. The three main ways this happens is through the phosphagen system and glycolysis, both anaerobic systems, and the oxidative system- an aerobic system.
The Phosphagen System
ATP and CP (creatine phosphate) are both phosphagens, thus their usage is called the phosphagen system. This system gives your muscle the energy needed for mainly high-intensity, short-duration bouts of exercise or activity. Your body will use ATP-CP to create ATP for up to about 10 seconds until switching over to using glucose for energy.
Glycolisis uses the breakdown of carbohydrates (glucose) to rapidly produce ATP. During glycolisis, one glucose molecule (from glycogen stored in the muscle) will produce 2 ATP through anaerobic glycolysis. This system can produce double the energy from ATP-CP, and is used for muscle activity lasting approximately 30 to 50 seconds. Many workouts will focus on the usage of glucose, as a typical set of 8-12 reps falls within this time frame.
The Oxidative System
The oxidative system produces ATP from carbohydrates and fats stored in other areas of the body. This is the slowest to produce energy because it takes the energy stored in other areas of your body and uses oxygen to convert those to ATP. During this process, one glucose molecule will produce 36 to 38 ATP! This means that while burning fat stored, the amount of energy you must use is much higher than the energy needed for other types of energy. This could help to explain why circuit training is so much more effective at burning fat than simply doing cardio, because you’re using a combination of different things to burn energy. The oxidative system becomes involved in energy consumption after 30 seconds, and is the primary method to create ATP for movement lasting longer than 2 to 3 minutes. You can not burn fat unless you are doing energy-burning movements that last longer than 30 seconds, and average between 2 to 3 minutes.
So now that you understand how your body creates and uses energy, you can focus more on the things necessary to making that happen: taking deep breaths while working out, eating healthy foods to get GOOD carbohydrates that are low in sugar that won’t cause your body to crash after using them, and making sure that you have an appropriate amount of fat (not too much, but also not too little) stored so that you can have the energy necessary to work and appropriately fatigue your muscles.