Muscle Contraction Steps: A Review

At the foundation of all movement is the humble muscle cell. If you remember from biology, there are quite a few steps to a simple muscle contraction, so we’ll do our best to simplify it in this article.

There are plenty of resources to explain the sliding filament theory and the makeup of a muscle cells and sarcomeres. Like here and here.  But if you’re not looking for a textbook style post, keeping reading this summary version.

A little muscle contraction fun 🙂

If you prefer a hands on learning experience, you might be interested in this giant sarcomere model on Amazon for $2,000. There’s only one available, so act fast haha. 😉 In all seriousness, this giant model of a sarcomere is kind of interesting to look at because it makes it easy to see how the components work together.

So let’s do a quick review of muscle contraction physiology:

  1. An action potential in a motor neuron causes acetylcholine to release in the synaptic cleft.

muscle contraction steps

  1. Acetylcholine binds with receptors on the cell membrane on the muscle fiber, opening Ca2+ -Na+ channels. Usually referred to as Calcium channels.
  1. Calcium is released from the terminal cisternae into the muscle fiber.
  1. Calcium binds to troponin
  1. Troponin shifts tropomyosin, which was blocking the active site on the actin.
  1. Myosin heads attach to actin by breaking down ATP to ADP and a phosphate via Myosin-ATPase
  1. The Myosin head forms a ‘cross-bridge’ on the active site of the actin filament.
  1. The cross bridge pulls actin, which slides over the myosin – known as the ‘Power Stroke.’
  1. The release of ADP completes the cross-bridge movement and ATP attaches to myosin, breaking the actin-myosin crossbridge.
  2. Every time ATP is split into ADP + P, the myosin head ‘cocks’ into place to form another cross bridge with actin.

This entire process shortens the sarcomere, which is functional unit of a muscle cell.

You can read a textbook, or even study the simplified steps above, but watching it in action is very helpful. Here’s a short snippet of a simulated muscle contraction showing the parts of a muscle cell and what happens during a muscle contraction.

muscle contraction steps
muscle contraction video


Movement Focus:

How can we explain an increase strength or force production with the sliding filament theory in mind?

Two ways: Frequency and Quantity

The faster a motor unit is stimulated, the greater strength and force it can produce.

Likewise, the more motor units activated, the greater your strength or force can be.

Remember, a muscle unit fires in an ‘all or none’ pattern, so that means no partial contractions. You’re either recruiting more or firing them faster.

Was this a good reminder of the basics?  Share your thoughts in the comments below.

image credit: Youtube

Tim Fraticelli DPT, MBA, CFP®

Tim Fraticelli is a Physical Therapist, Certified Financial Planner™ and founder of He loves to teach PTs and OTs ways to save time and money in and out of the clinic, especially when it comes to documentation or continuing education. Follow him on YouTube for weekly videos on ways to improve your financial health.

17 thoughts on “Muscle Contraction Steps: A Review”

  1. This is very helpful. I am interested in the frequency aspect. Is the frequency of the power stroke governed by a frequency in the action potential?

  2. You had the right ion as Na+, because the acetylcholine attaches to sodium gates, allowing the sodium to be released. This continues the action potential from earlier through the T-tubules of the muscles, which then releases the calcium or Ca2+.

  3. Jesus.
    Walking in knowing nothing just made me more confused to start with. The more I thought about it though, the more sense it made
    P.S. I’m attempting to synthesise muscle tissue… wish me luck

  4. If, after Myosin heads attach to actin by breaking down ATP to ADP and a phosphate via Myosin-ATPase, the pH of the muscle drops dis-enabling the enzyme, does the Myosin head remain attached and not released from the Actin?


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