Increasing the level of PCr increases the total amount of immediately available energy to
fuel muscle contraction, and therefore increases the total short term work capacity of a muscle.
As these metabolites deplete (especially CP), muscle fatigue will set in as there is less energy to
fuel muscle contractions.
Glycogen is important because that's one of the energy sources that your body uses to
fuel muscle contraction (energy systems are a topic for another day)
The premise is that when you lift heavy weights, your body needs more glycogen to
fuel the muscle contractions.
As discussed by Robergs and Roberts [22], amino acid catabolism during exercise is important for three reasons: 1) for free energy during exercise to
fuel muscle contraction; 2) to increase concentrations of citric acid cycle intermediates and therefore support carbohydrate and lipid catabolism; and 3) to serve as gluconeogenic precursors.
Adenosine tri-phosphate (ATP) is the energy source used by muscle cells to
fuel muscle contraction.