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. Compounding the problem is that amino acid entry into the muscle cell is depressed during exercise, and this means that amino acids needed to build proteins will be in short supply. The protein damage exceeds the ability of the cell to replace it, because it does not have the necessary building supplies. So let's now take a look at what happens during the recovery phase. During recovery, energy is not being diverted to muscular work, so there is more energy available for protein synthesis. The red line here indicates how much energy for protein synthesis increases when the athlete is in the recovery phase. During recovery, there's more amino acids entering into the muscle's cell, and this allows a higher rate of protein synthesis, as indicated by the green dotted line. This repeated process of high destruction of contractile proteins and low energy during the workout, followed by enhanced protein synthesis and energy during recovery, is thought to result in a super compensation of muscle protein synthesis. And this is similar to the way muscle glycogen is supercompensated in response to endurance training. Muscle glycogen is drained to a low level during training, and this stimulates the brain to supercompensate the reserve of glycogen in the muscle's cell, and this same activity happens in protein synthesis of a muscle fiber, and this is how it supercompensates or gets bigger.
