The thick filament, myosin, has a double-headed structure, with the heads positioned at opposite ends of the molecule.

[6] The axon terminal of a motor neuron releases the neurotransmitter, acetylcholine, which diffuses across the synaptic cleft and binds to the muscle fiber membrane.

Active transport moves calcium ions back into the sarcoplasmic reticulum of the muscle fiber.

While the exact mechanism of myofilament alteration in response to exercise is still being studied in mammals, some interesting clues have been revealed in Thoroughbred race horses.

This study provides evidence of the mechanisms for both immediate and delayed myofilament response to exercise at the molecular level.

Again, researchers are not completely clear about the molecular mechanisms of change, and an alteration of fiber-type composition in the myofilament may not be the answer many athletes have long assumed.

[10] This study looked at the muscle specific tension in the quadriceps femoris and vastus lateralis of forty-two young men.

Researchers report a 17% increase in specific muscle tension after a period of resistance training, despite a decrease in the presence of MyHC, myosin heavy-chain.

While the research on muscle fiber remodeling is on-going, there are generally accepted facts about the myofilament from the American College of Sports Medicine.

Recent studies suggest that these conditions are associated with altered single fiber performance due to decreased expression of myofilament proteins and/or changes in myosin-actin cross-bridge interactions.