ATP-adenosine triphosphate- is the ultimate and only energy source used by the body. The body has three energy pathways for creating ATP. Around 100g of ATP and 120g of creatine are stored in the body. These are used in the ATP-Pcr energy chain which is the first one used regardless of intensity when you exercise. This is the most powerful of the three systems and can sustain all out exercise for 3-15 seconds. This period is when you have the greatest potential for maximum power.

The second energy system is the Glycolytic energy system. This system uses glucose for energy and there is two types. Fast glycolysis produces lactic acid as a byproduct. Slow glycolysis produces pyruvic acid which is used in the Krebs cycle. After 10 sec fast glycolysis starts (and there is a drop in maximal power output) taking more and more of the energy production and by 30 sec of sustained activity it is almost the exclusive energy provider. At around 45 seconds the Oxidative system starts taking over. Which corresponds with a second decline in maximal power output.

The oxidative system consists of 4 cycles.

Slow glycolysis is the same as fast glycolysis except its slower and produces pyruvic acid. The body changes pyruvic acid into acetly coenzyme A. Acetyl coenzyma A is then funneled through the Krebs cycle.

The Krebs Cycle is a series of complex reactions that procuses hydrogen. Hydrogen is also produced by the glycolytic reactions. This hydrogen is then used in the electron transport chain.

The electron transport chain is also very complicated and uses oxygen to make lots of ATP.

Beta oxidation is used to break down fat into free fatty acids, acetyl coenzyme A and glycerol which can all be used by the Krebs Cycle and the Electron Transport chain as well.

Fat produces more ATP than glucose during the Krebs cycle and the Electron transport chain but it also requires more oxygen.

Protein is metabolized by converting the amino acids to acetyl coenzyme A or glucose.

After 90 seconds the majority of energy will be supplied by the Oxidative system. The slow glycolysis can make up a significant proportion of that even in very long activities.

Training can increase the output of all three energy systems. The ATP-Pcr system and Glycolytic system can only be increased by about 10-20%. The Oxidative system is much more trainable but I don't have a number.

V02 max can be increased by as much as 50% but genetics play a very large factor.

ATP and creatine can be replinished quickly by the body. When lifting weights, especially doing power lifting this is the primary energy system used. This is why the rest between sets is important to build more ATP and creatine.

With distance running 10% of the activity is fueled by ATP-Pcr system, 20% from the Glycolytic system and 70% from the oxidative system. Carb intake can affect which system is used and how much is provided by slow glycolysis vs the others.

Carb loading can only extend the fast glycolysis phase by a few seconds. Carb loading will also not effect the ATP-Pcr phase.

Most athletes who switch to a low carb diet experience performance difficulties for about a month, afterwhich they perform equally with carb eaters.

So no, your muscles do not need carbs. The carbs used in fast glycolysis will be replenished by synthesis from dietary protein in the liver. Krebs cycle and etectron transport chain would simply be more utilized by a low carb eater than in a high carb eater.