Final answer:
The availability in the question is Energy availability, which is related to the synthesis and resynthesis of ATP in muscle fibers during contractions, utilizing stored ATP, creatine phosphate, and macronutrients like glucose and glycogen.
Step-by-step explanation:
The availability being referred to in the student's question is Energy availability, which is a function of the muscle fibers' ability to synthesize ATP from macronutrients and also resynthesize itself once it has been degraded to yield energy. During muscle contraction, ATP is first utilized from pre-existing stores within the muscle cell and then is quickly generated from creatine phosphate. However, when these initial resources are depleted, the muscles turn to other sources such as glucose and glycogen. Glucose can be metabolized to form ATP and pyruvate through glycolysis; then pyruvate can proceed through aerobic respiration to produce more ATP if oxygen is present, or it can be used in anaerobic respiration to form lactic acid when oxygen is scarce.
In the presence of oxygen, the breakdown of glucose during aerobic respiration primarily occurs within the mitochondria, providing approximately 95 percent of ATP for resting or moderately active muscles. Glycogen, a polymeric form of glucose stored in the liver and skeletal muscle, is an important energy reserve that can be converted back to glucose during intense exercise or when blood sugar levels drop.