How a muscle is stimulated to hypertrophy remains unestablished.
One theory is the energetic theory.
It's theorized that there is only a certain amount of energy available
to the muscle fiber, and this energy is used for two purposes.
One is to build protein that will be used by the muscle to enhance its structure,
and this is referred to as the athlete's adaptive reserve.
Now, adaptive reserve is a term used in cardiology to describe
the ability of the heart to increase muscle mass and cardiac function.
In its effort to protect itself from damage when under stress,
and it's being borrowed from cardiology,
the other use of energy is to perform muscular work.
And under resting and light exercise conditions,
there is sufficient energy available in a muscle's cell to
simultaneously perform both of these functions.
During heavy resistive exercise, however, a high proportion
of the energy in the cell is needed for performing muscular work.
And here is the work being done by the contractile actin and myosin proteins.
And a lot of ATP is needed to break the cross bridges here to
keep the actin-myosin contracting.
And this is the molecular motor of the muscle,
and when training is intense,
there's a lot of ATP needed for this task.
Okay, so here's a graph divided into workout and recovery cycle phases.
This is the workout phase, and it's shown here in blue,
and this is the recovery phase, this red dotted line here,
is the normal homeostasis in the cell.
Now, if the energy supply for the synthesis of protein
decreases when it's diverted to perform mechanical work,
then it follows that there is insufficient energy to resynthesize
proteins fast enough to match the damage being done to the actin-myosin,
protein synthesis will decline, as shown here by the green line.