Which substances are used by muscles as a source of energy for contractions?

Muscles primarily use glycogen and triglycerides as sources of energy for contractions. Glycogen, which is stored in the muscles and liver, is a polysaccharide that can be quickly broken down into glucose during periods of high energy demand, such as exercise. This glucose can then be metabolized to produce ATP (adenosine triphosphate), the energy currency of the cell, which fuels muscle contractions.

Triglycerides, on the other hand, are the main form of fat stored in the body. During prolonged or moderate-intensity exercise, muscles can utilize triglycerides for energy through a process called lipolysis. This process breaks down triglycerides into free fatty acids, which can also be converted into ATP.

The combination of glycogen and triglycerides allows the muscles to maintain energy levels during different types of physical activity. Other substances, such as proteins or vitamins, are not primarily used directly for energy during muscle contractions, making this answer the most accurate regarding muscle energy sources.