Sliding Filament Theory
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Before we begin: what’s the point?
Ultimately, the sliding filament theory is not just a scientific theory for the nerds of the fitness industry; rather, it is a scientific theory that explains how muscles contract, why your strength may vary in different exercise ranges and body positions, why certain exercises are physiologically different than (or the same as) others, and why electrolyte minerals are so important for muscle contraction and relaxation.
Don’t care about the science but want the value? Learn more about why it matters.
The sliding filament theory is our best understanding of how muscles actually contract; it provides detail on the phenomenon of muscle contraction at the microscopic level. Originally published in 1954 by two separate research teams, the sliding filament theory describes overlapping fibers—or filaments—that shorten the muscle. Despite being published ~70 years ago, it is still the most widely accepted description for the mechanisms of muscle contraction.
To understand the summary below, you should first understand some basics about muscle structure. A skeletal muscle (e.g., the biceps brachii) is comprised of many smaller bundles called fascicles. The fascicles are bundles of muscle cells, called myocytes, which are the individual muscle fibers. All muscle fibers consist of myofibrils which are comprised of the individual myofilaments (actin and myosin, described below) identified in the name, sliding filament theory (are you already confused? Don’t worry. It should make more sense after seeing some images).
Thus, muscular anatomy from the smallest to largest components: myofilaments → myofibrils → muscle fibers (muscle cells) → fascicles (bundles of muscle fibers) → muscles (e.g., biceps brachii)
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Brief summary
The sliding filament theory explains how a muscle is able to contract or shorten. Below is a list of definitions (use the pictures above and below to follow along) to refer to when reading about the sliding filament theory and its moving parts.
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A quick review of the process
When you consciously contract a muscle, your brain sends a signal to that muscle; this signal is called an action potential. The action potential drives into the muscle cells where it causes the release of calcium ions from the sarcoplasmic reticulum. Calcium ions will bind to troponin, shifting tropomyosin strands to expose myosin binding sites on the actin filaments.
The heads of the hinged rods of the myosin filaments are then able to take hold of the actin filaments and pull them inwards. When this occurs, the Z-lines come closer to each other; this occurs for each sarcomere from one end of the muscle to the other, and thus, the entire muscle becomes shorter. For a more detailed description, read below.
It is important to understand that every skeletal muscle contracts inwards, pulling both ends of the muscle towards its center point. From the outside of your body, it may appear that muscles are only pulling on one end (e.g., the biceps brachii pulling your forearm during all variations of biceps curls).
However, if you perform a chin-up exercise, the forearm is fixed and the upper arm is the part that appears to move. All muscles pull inwards from both ends regardless of how it may appear from outside your body.
It is also important to understand that muscles cannot push; they can only pull (shorten) to move a joint, as described by the sliding filament theory.
Sliding filament theory explained
Bold terms in the following description refer to the definitions listed above.
Every muscle contraction begins in the brain; your motor cortex will deliver a signal through the nervous system to a target muscle to trigger a contraction. This nerve impulse to and into the muscle is also called an action potential.
When the nerve impulse reaches the muscle, the t-tubules drive the action potential to the interior of the muscle cells. Each t-tubule in human skeletal muscle sits adjacent to two terminal cisternae* of the sarcoplasmic reticulum allowing immediate release of calcium for the muscle cells.
*Cisternae are reservoirs; terminal simply refers to its ends. Thus, terminal cisternae are the ends of the sarcoplasmic reticulum—the reservoirs of calcium ions.
On each actin (thin) filament are strands called tropomyosin which cover binding sites at rest. Also at rest, the actin and myosin (thick) filaments overlap but are not bound to each other regardless of their positioning.
However, when a muscle contraction is initiated by the action potential, calcium is released from the sarcoplasmic reticulum and begins binding to the troponin molecules on the actin filaments. When calcium binds to troponin, the tropomyosin strands shift to expose the binding sites for the myosin filaments.
With the binding sites exposed, the myosin filaments can link to the actin filaments. Myosin filaments are comprised of many rods, each of which has a head that can bind to the actin filaments; these rods are also hinged.
The heads of myosin also bind with ATP, and when ATP is hydrolyzed (broken down to ADP + Pi), the myosin heads can then bind to actin forming a cross bridge. ADP and Pi are then released causing the myosin filaments to bend, pulling the actin filaments inwards; myosin rods bending at their hinges to pull actin is called a power stroke. (Tap/click to enlarge images below)
Another molecule of ATP is then required to release the myosin heads from the actin and to straighten the hinged rods (ATP binding to the myosin heads removes the cross bridge). The myosin rods can then attach again and repeat the cycle. If calcium is bound to troponin and ATP is available, the process continues.
Through the entire process, neither the myosin nor actin filaments change length and instead appear to slide across each other, thus the name, sliding filament theory.
Relaxation occurs when the nerve impulse (action potential) is diminished, and calcium will then be sequestered back into the sarcoplasmic reticulum. Without calcium binding to troponin, tropomyosin strands cover the myosin binding sites, and contraction is no longer possible.
Why does it matter?
Art of Anatomy‘s program design utilizes variable muscle lengths, or training ranges, to target each muscle for maximum hypertrophy. When training a muscle in a lengthened or shortened training range, your strength will generally be diminished in comparison to training a muscle in the neutral training range. This strength discrepancy can be explained by the sliding filament theory and is most often described as the length-tension relationship.
Length-tension relationship
When a muscle is neither fully lengthened nor fully shortened, the actin and myosin filaments have an optimal overlap to create the strongest contraction. When the muscle is lengthened, the actin and myosin fibers have minimal overlap and the muscle contraction tends to be slightly weaker. Lastly, when the muscle is shortened, the myosin binding becomes crowded, and again, the muscle tends to be slightly weaker.
Despite the changes in strength, each of these three training ranges are invaluable in creating an exercise program that will maximize muscle growth. As you will learn in following sections, your muscles are not strong, individually. This simple fact is also unimportant because muscle strength is not the most important factor for muscle growth; muscle strength is not directly correlated with muscle mass!
The sliding filament theory can also help you to understand how exercises targeting the same muscle are similar or different from each other. For example, a standing biceps curl with dumbbells (with the forearms supinated) is no different than a standing biceps curl with a barbell other than the equipment.
At the microscopic level, the filaments of your muscle fibers slide within the same ranges for both version of the biceps curl. For exercises to be physiologically different than others, the target muscle needs to be trained through a different range (length) or trained at a different angle (position).
For example, a standing hammer curl (with the forearm neutral, as in when swinging a hammer) is different than the supinated curl because the muscle changes position (and length, minimally). Preacher curls are also different than standing curls (both supinated grip and hammer curl variations) due to a notable change in muscle length throughout the movement. The preacher curl shortens the biceps brachii fully where the standing curls do not.
The sliding filament theory also provides a look into the basics of biochemistry and the necessity of electrolyte minerals (sodium, potassium, calcium, and magnesium). These minerals are directly involved in nerve impulse (namely, sodium and potassium) and muscle contraction (calcium) and relaxation (magnesium).
Although not described earlier, magnesium can also bind to troponin molecules but will not cause a shift in tropomyosin. Thus, magnesium can act as a natural calcium blocker which helps muscles to relax (note: this mechanism is not part of the process of conscious muscle contraction; normal levels of blood magnesium will not cause weaker or inhibited contractions during exercise).