Final answer:
Glycogen serves as the primary energy reserve for muscles during strenuous exercise, with significant stores found in the liver and skeletal muscles. It is broken down to glucose to provide energy for muscle contractions.
Step-by-step explanation:
The substance that provides an energy reserve for strenuous exercise by muscles is glycogen. Glycogen is a polymer of glucose and serves as a significant form of energy storage in animals. It is most abundant in the liver and skeletal muscle cells, where it is stored for energy demands such as fasting or strenuous physical activities. During exercise, muscle glycogen is broken down to produce glucose, which then undergoes glycolysis and the Krebs cycle, ultimately yielding ATP to fuel muscle contractions.
Liver glycogen plays a critical role in maintaining blood glucose levels, while muscle glycogen is directly used for energy within the muscle during vigorous exercise. As exercise intensity increases, the body may also begin to metabolize fats, but glycogen is the primary source of glucose during the initial stages of strenuous exercise.