Final answer:
Proteins are pivotal in both muscle repair and, when necessary, can act as a fuel source. Initially, muscles use creatine phosphate for quick energy bursts, then switch to glycogen, and if the energy demand persists, fatty acids or even proteins may be utilized.
Step-by-step explanation:
The components used to build and repair muscles that can also serve as a fuel source include proteins and their derivatives. Proteins are essential macromolecules that are vital for the structure, function, and regulation of the body's tissues and organs, particularly muscles. Muscular energy demands can be met through different metabolic pathways depending on the duration and intensity of exercise.
Initially, creatine phosphate provides a rapid energy burst for muscle contraction that lasts about 8-10 seconds. When this supply is depleted, muscles turn to glycogen. Glycogen metabolism, which involves approximately 12 chemical reactions to convert glucose into ATP (adenosine triphosphate), takes over, although it supplies energy at a slower rate than creatine phosphate. This provides enough energy for activities lasting approximately 90 seconds and does not require oxygen, leading to the production of lactic acid as a by-product.
For prolonged activities such as a marathon, carbohydrate loading is often used to maximize the storage of glycogen in muscles. This glycogen can then be mobilized quickly to meet sudden energy needs. During extended low to moderate intensity exercises, fatty acids become the primary fuel source. In situations where carbohydrate and fat resources are inadequate, the body may resort to breaking down protein for energy, which can lead to tissue breakdown and is not considered ideal.
Endurance athletes may utilize supplements like erythropoietin (EPO) to enhance oxygen delivery for aerobic respiration, or human growth hormone (hGH) to facilitate muscle repair and recovery, thus improving overall endurance performance.