The Sliding-Filament Theory is the process of muscle contraction in which muscle proteins slide past each other to create movement and force. This process happens at a very small level in myofibrils which as smaller than a human hair. Below are the steps of how the sliding filament theory works as muscles contract during strength training.
To Activate a muscle the brain has to send a signal to the muscle through a nerve. The nerve transmits a signal from axon to axon using the neurotransmitter Acetylcholine. Acetylcholine then targets the muscle by reaching the neuromuscular junction.
1. The Acetylcholine will stimulate the release of calcium. The actual calcium and the amount of it is what regulates muscle contraction.
2. Calcium binds to Troponin which moves Tropomyosin that is roped around the Actin to expose binding sites on the Actin. The Tropomyosin essentially moves out of its groves to expose Actin binding sites for the cross bridge to occur for the Myosin to connect with Actin.
3. The Myosin head binds to the Actin filament to form a crossbridge.
4. The breakdown of ATP (Adenosine Triphosphate) releases energy which enables the Myosin to pull the Actin filaments inwards contracting the muscle. This occurs along the entire length of every myofibril along the muscle cell. When an ATP molecule binds to the Myosin head the Myosin detaches from the Actin and the crossbridge is broken. When the ATP is then broken down the myosin head can again attach to an actin binding site further along an actin filament.
5. The H Zone (Myosin only) and I Band (Actin only ) shorten to create muscle contraction from a lengthened state to a shortened state. The H zone shortens in this process. This repeated pulling of the actin over the myosin is known as the ratchet mechanism. This process can last for as long as there are adequate amounts of ATP and Calcium. When there is no longer an action potential present and the muscle is relaxed the calcium moves back to the sarcoplasmic reticulum.
references
Haff, Gregory. Triplett, Travis. Essentials of Strength Training and Conditioning. Pg 6-8. Human Kinetics, 2016.