The sliding filament theory is an explanation for the biomechanical and biochemical processes of muscle contraction. The theory involves a series of complex interactions between the proteins in muscle cells and the fuel used to drive each contraction. In basic terms, however, the theory can be described as follows:
• Under a microscope, muscles appear as bundles of protein fibers aligned parallel to one another. Before a muscle contracts (when it is in a resting state), the muscle is said to be at its “resting length.”
• In order for a muscle to contract, the brain has to send a message through a nerve impulse to the resting muscle. When the nerve impulse reaches the muscle, certain protein fibers in the muscle respond by grabbing onto and then sliding along their neighboring fibers.
• When enough muscle fibers receive the message to attach to and slide along their neighbors, the eventual result is that the whole muscle belly shortens or contracts. A muscle in this state is said to be at a “shortened length.”
• One substance that a muscle needs in order to fuel the sliding, and thus contraction, is called adenosine triphosphate (or ATP). Once the available ATP is used up and/or the brain no longer sends a message to the muscle to contract, the protein fibers let go of their neighborly attachments and slide back into place (that is, resting length).
Keep in mind that all of the above processes take place in a fraction of second. Pretty amazing, I’d say!