Sliding Filament Theory & Steps Explained

The sliding filament theory is a complex process, especially when it’s explained in an intricate way. In this article, I will break down the basics of this theory to help you understand the process of how it happens and what some key words mean.

What is sliding filament theory?

The sliding filament theory was proposed by Andrew Huxley in 1954 and has helped scientists understand how muscle contractions work at cellular level with proteins sliding against each other causing cross-bridges which then leads to muscle contractions – thought this may sound complex, this is how movement appears which is unique to the traits of an individual such as the flexibility or ballerinas or the strength of a powerlifter.

In this article, we will explain the sliding filament theory in a more simple nature, but first, to help our understanding on the sliding filament theory, it’s useful to have a quick recap on muscle structures and the three types of muscle contractions…

Muscle fibre structure – When muscle fibres (i.e. a muscle) are put under a microscope, we can see they contain smaller fibres called myofibrils. Through a microscope muscle cells form a striped-like pattern, with each unit called a sarcomere.

Sarcomeres – There are thousands of sarcomeres in each muscle cell, which contain filaments called actin (thin) and myosin (thick). These filaments slide in and out between each other causing muscle contractions, hence the name sliding filament theory!

Eccentric muscle contraction – the muscle is lengthening and is typically used to resist or slow motion (eg. lowering phase of a bicep curl or squat).

Concentric muscle contraction – the muscle shortens in length and is typically used to generate motion (e.g. upward phase of a bicep curl or squat).

Isometric muscle contraction – there is no change in muscle length, yet the muscle is still contracted. This is used for producing shock absorption and to maintain stability (e.g. plank or actively hanging from a bar).

Now that we know we’ve covered muscle structure and the types of muscle contractions, we’ll now use a practical example of a concentric and eccentric contraction when performing a bicep curl to explain the sliding filament theory…we’ll explain this through a 5 stage process:

Step 1:

The brain sends a message (nerve impulse) to the muscle it wants to contract. For example, the brain will send a message to the bicep brachii during a bicep curl. This will cause calcium to be released from the sarcoplasmic reticulum (note: calcium is essential for contraction mechanisms to take place).

Step 2:

With an increase in calcium ions now present, they attached to a part of the sarcomere called troponin. The binding of calcium ions to troponin results in it’s changing shape, which causes it to move tropomyosin towards actin. This causes a cross-bridge to be formed.

Step 3:

Myosin filaments must then slide over one another and pull on actin filaments to cause concentric contraction to occur. This happens across every sarcomere in the muscle! From our example of our bicep curl, this step would result in the dumbbell being lifted upwards.

An alternative explanation to steps 1-3:

Jacob Krans from Central Connecticut State University provides a great analogy for the sliding filaments when sarcomere shortening (i.e. steps 1-3) occurs, which we’ll include below:

Jacob uses a bookcase for his analogy, he says, “imagine you are standing between two bookcases, that are a couple of meters apart and each filled with books. You must bring the two book cases together, by only using your arms and two ropes, which you have one end in each hand and the other end tied to each end of the bookcases. You repeatedly pull each rope towards you, re-grip it, and then pull again. Eventually, as you progress through the length of the rope, the bookcases move together and approach you.

In this example, your arms are similar to the myosin molecules, the ropes are the actin filaments, and the bookcases are the z discs to which the actin is secured, which make up the lateral ends of a sarcomere. Similar to the way you would remain centered between the bookcases, the myosin filaments remain centered during normal muscle contraction.”

Step 4:

For the dumbbell to be lowered, myosin lets go of actin with the cross-bridge being broken. The stages are then reversed as tropomyosin returns to it’s original place.

Step 5:

As long as the human body has enough energy (and calcium) available, then this process can occur over and over – without it, we would not be able to function as humans.

Other Considerations:

Now that we’ve explained muscle contraction from a concentric and eccentric portion of movement, we must think about the sliding filament theory during an isometric contraction. During an isometric contraction, cross bridges are still formed (stage 1-2), however, force is equally distributed between filaments. Though it must be noted that force production is reduced during isometric contraction in comparison to the potential force production of eccentric and concentric contraction.

Though the sliding filament theory was proposed in the 1950s, it has been proven to be applicable to all muscle fibre types throughout the body. This process occurs over and over throughout the muscle during everyday life from performing bicep curls in the gym to simply standing up from a chair.

Summary

Now that you understand the 5 step process of muscle contraction, we must begin to think about applying this knowledge to different movements such as a squat or pull-up. Though the process is the same for every single muscle fibre, think about how the different muscles work that are involved.

Below I have referenced a few important sliding filament theory papers that will help give you an even better understanding as well as provide a reference point for your understanding.