Fast Twitch muscles have always been associated with speed. The central nervous system sends the action signal to the muscles. The fuel already stored inside the muscle cell reacts to the nerve signal creating intense kinetic energy for a fast twitch muscle response. Creatine Phosphate, an amino acid, releases the phosphate that was synthesized in the liver to activate ATP, the main energy supply for the muscles. Like the starter in a car, the reaction ignites glycogen stored inside the muscle cell and can fuel the muscle without an external oxygen supply, This anaerobic fast twitch energy burn in a high fuel, high speed, high output reaction. The remainder of glycogen inside the muscle cells and glucose in the blood supply continue to burn over the next 20 minutes with access to oxygen from the red blood cells.
It is important to note that fuel must be inside the muscle cells before the ATP can be activated for movement. Carb loading is a recovery process replacing expended muscle fuel. Creatine and glucose are delivered to resting muscle cells after exertion through the blood system, and insulin works to store the fuel inside the muscle cells to prepare for the next adventure.
The 20 minutes of fast twitch sugar burn allows time for the cardio fat burn to ramp up production of slow twitch energy, named for the slow processing time to perform the many steps required to convert fat into ATP.
Once the fuel inside the muscle cells is depleted and activity continues past 20 minutes, the slow twitch process takes over primary production of energy for endurance activity such as running long distances to hunt food or migrating to the next food supply. The pancreas emits the hormone, glucagon, to pull stored glycogen directly from the liver, and corticosteroids from the adrenal glands start hunting simultaneously for fat and protein supplies which are broken down into glucose. Corticosteroids are also a natural anti-inflammatory and pain reliever which can keep the body moving through intense activity.
Endurance or migration mode in the body is a low grade stress response to a shortage of glucose. The body shifts into a conservative slow twitch low fuel, high output fat burn that allows for long-term high performance. The muscle cell fuel is depleted, and the fascia of the muscles takes over the majority of movement fed by the delivery of systemic fuel provided by the glucose from the liver and scavenger corticosteroid in bio fuel conversions of easily accessible fats and proteins as well as the tapping of deep storage fuel locations such as belly fat.
The central nervous system (CNS) consisting of the brain and spinal cord controls the fast twitch motor phasic action line of communication in a high sugar fuel, high performance, anaerobic manner.
The cerebellum in the brain receives sensory system information from the skin such as heat and pain from the peripheral nervous system (PNS). The fascia in the body has sensory proprioceptive neurotransmitters, and fascia tissues in the body are innervated by sensory nerve endings controlled by the PNS. The sensory component in the fascia of muscles regulates posture, balance, coordination, equilibrium and muscle tone. Muscle tone is the form left in the muscle after a contraction takes place, the residual postural function of muscles.
I would argue that fascia manages the utilization of slow twitch fuel supply of aerobic fat and protein burning for endurance activity and posture. The PNS is the primary system for the fight or flight stress response. It makes sense that a low grade modified stress response to mobilize fuel in a glucose shortage to support endurance activity happens in the PNS controlled by sensory spacial awareness guidance of fascia.
The Vagus Nerve, Cranial Nerve X, is part of the PNS. It sends a parasympathetic calm down message to the organs after the sympathetic emergency surge begins to subside. The PNS orchestrates the yin and yang of stress with the goal of seeking balance, much like the fascia controls the physical balance of the body.
What happens when Fast Twitch muscle contractions run out of fuel? The PNS and fascia takes control of movement using stress processes to mobilize and distribute energy to muscles in a systemic delivery plan for endurance using low fuel, high output resource. What happens when the need for endurance energy lasts for years?