Weight Training 101: How Muscles Work

The Musculoskeletal System

We all have a basic understanding of how our muscles work.  They’re attached to our bones, and when they contract or elongate, the bones that make up our skeleton move one way or the other.  As a result, muscles are commonly compared to things like pulleys, springs, pistons, and basically anything that produces force by getting bigger or smaller.  For a crystal clear example, take a look at the image of the biceps curl.  

When the biceps contract they pull the forearm up towards the rest of the arm.  On the other hand, when the biceps relax and get longer, the forearm is lowered back to it’s resting position.  Our joints are the fulcrums, and our muscles are the pulleys.  Our muscles pull on our bones so that they move like levers around the joints. 

Before we really get into it, it’s important to note that muscles only produce force by contracting.  When a muscle relaxes to it's elongated state, the process is always passive.  As a result, a muscle cannot produce force without contracting.  To return to the biceps contraction example, it would be wrong to say that the elongation of the biceps produces the force that causes the forearm to move away from the upper arm, even though these two things are happening at the same time.  The force necessary to extend the arm is the result of the triceps, the muscles on the back of the arm, contracting. 

Even though our muscles can only do work by contracting, they can move our bones in multiple directions depending on where each muscle is attached to each bone.

Form and Function

Let’s play god for a moment and come up with a justification for why our muscles work the way they do.  We do so many things that require “pushing” forces.  Wouldn’t it be easier push things if our muscles could work both ways?

 If we we’re solely talking about physics, then yes.  It would be more efficient to have some muscles that pull, some that push and some that could do both.  But we’re talking about biology here, so we have to abide by its rules.  Like all things in biology, our muscles evolved into what they are today because they’re form perfectly matches their function.  Recall that muscles have two functions.  First, they hold our skeleton together, and second, they pull on it to make it move.  Imagine trying to hold two things together with something that can only push away at both ends, it’s just not going to happen. 

Our muscles only produce force by contracting because they have to do two jobs at the same time.  Every time our muscles make us move, they also have to hold us together

The Anatomy of Skeletal Muscle: A Picture's Worth A Thousand Words

In this picture, you can see how each of your muscles is made out of multiple bundles of fibers.  Each individual fiber is composed of a series of “myofibrils”, which all contain “sarcomeres”.  Don’t worry about these names; they’re not important for our purposes.  The important takeaway is that the “sarcomere” is the functional unit of any muscle.

I like to think of sarcomeres as the equivalent to pixels on a TV screen.  The screen only lights up because thousands of pixels are simultaneously producing only a little bit of light each.  In the same way, when a muscle contracts, millions of sarcomeres are microscopically contracting at the same time to create a macroscopic contraction.  Let’s zoom in on the sarcomere:

The sarcomere has three main components.  The “Z-Lines”, “Actin”, and “Myosin”.  In this picture, the dark blue Z-lines are held together by the red, actin filaments that are, in turn, held together by the light blue myosin filaments.  The light blue, myosin filaments produce the force that pulls the Z-lines together with each contraction.  The myosin molecules grab onto the actin strands and pull the two sides together. 

It’s kind of like Velcro.  The myosin heads act like hooks that grabs on to the actin loops.  It’s not the easiest process to visualize right away, but the next section takes you through the contraction step by step. 

Muscle Contraction: A Simplified Process

The steps below are a simplification of a very complicated process that involves many steps and signaling cascades.  In the interest of simplicity, a muscle contraction can be considered to take place in the following steps:


Step 1: Electrical signal from the brain

In order to contract, our muscles need to receive a signal from the brain telling them to do so.  If this weren’t the case, we’d all either be flailing around randomly or we’d never be able to move. 


Step 2: Spread of the electrical signal to the sarcomere

Once the electrical signal gets to the muscle, the signal has to spread down through the layers of the muscle tissue to reach the sarcomeres.  Remember, the sarcomeres are the functional unit of our muscles, so they need to be receiving the signal to contract.  This is a picture of the neuromuscular junction, the place where electrical signals are transferred from nerve cells to muscle cells. 


Step 3: Conversion of electrical signal to chemical signal

The sarcomere can’t contract with an electrical signal alone though.  The electrical signal has to be converted into a chemical one for the reaction that causes contraction can occur.  Think of this conversion from electrical to chemical signaling as analogous to the electrical signal turning on a microscopic "machine" that dumps the necessary chemical structures into the sarcomere.  In fact, this is exactly how it works.  The main chemical messenger that the sarcomere responds to is the calcium ion (Ca2+).  But again, this is mostly irrelevant for our purposes. 


Step 4: Chemical activation of the sarcomere

Once released into the sarcomere, the calcium has the effect of “unlocking the sarcomere’s contraction potential” by binding to the actin filaments.  Basically, the calcium changes the shape of the actin filaments (the Velcro loops), so that the myosin heads (the hooks), can grab onto them.  In a lot of ways, calcium acts like the hand grips on an artificial rock climbing wall.  In the same way that small protrusions on a rock climbing are what allow for people to hold themselves on its vertical surface, calcium bound to the actin filament allows for the myosin heads to hook onto the actin filament.  


Step 5: Myosin heads pull on the actin chain

When the myosin hooks grab onto the actin, they pull back on it.  This brings the Z-lines of each microscopic sarcomere together and, when millions of these microscopic contractions occur simultaneously, the result is a muscle contraction.  The sarcomere needs a constant supply of chemical energy and calcium to keep contracting.  Deeper contractions require the myosin to “walk” down the actin filaments in order to pull the Z-lines closer together.  By looking at the cartoon picture, you can see that the sarcomere system looks a lot like a twist-on cover system .  The spiral "walking" action of the myosin on the actin filaments is just like the spiral action of a screwing on a bottle cap and closing the space gap between cap and bottle.    


Step 6: Release of the myosin heads

As I mentioned in the last step, the myosin heads have to keep “walking” further down the actin filaments to create a deeper contraction.  It seems counterintuitive, but it requires chemical energy for the myosin heads to release their initial hold on the actin filament so it can walk further down.  To restore the intuitive nature of the process, think of the default state of the myosin/actin system as being "always connected".  To cause the spiraling, walking action of the myosin heads on the actin filaments, energy is required to temporarily break the connection between actin and myosin.  If this connection is broken while the neural, calcium signal is present, the result will be a deeper contraction of each sarcomere.  If the calcium signal is no longer present when the myosin heads release their hold on, each sarcomere will elongate to a state of rest.  

A lack of chemical energy in the sarcomere results in weaker contractions because slowing the rate at which actin and myosin disconnect from each other slows down the progress of the myosin heads on the actin filament.  To return to our rock climbing metaphor, you can never proceed to another hand hold if you are unable to let go of the peg you're currently holding onto.  In our muscles the presence of chemical energy is necessary for the myosin heads to "let go". 


Here’s an animation of the process:

Sound isn't necessary for our purposes but it will enrich your understanding of how muscles contract

Key Takeaways

Now that you know what your muscles do and how they do it, you’ve built yourself a great knowledge foundation.  With this foundation, you can start to truly understand how weight training works to make our muscles grow and get stronger.  Those are topics for the next Weightlifting 101 articles though.  For now, let’s go over a few key points from this article

  • Muscles only produce force by contracting.  This is because our muscles have to hold us together while they create the force that drives movement. 
  • All of our skeletal muscles are comprised of many muscle bundles, which are then comprised of many fibers, which are then comprised of many myofibrils.  Each myofibril contains a series of sarcomeres, the functional unit of the muscle.
  • Sarcomeres are to muscles just like pixels are to TV screens.  Millions of microscopic contractions gives rise to a macroscopic contraction. 
  • Sarcomeres are made of Actin, Myosin, and Z-lines.  Actin holds the Z-lines together and myosin holds the actin together.
  • Myosin “walks down” actin filaments just like a rock climber climbs a wall and a bottle cap screws onto a bottle.
  • The brain sends an electrical signal to the muscle that needs to be converted to a chemical signal (Ca2+) at the sarcomere.  Presence of the electrical signal encodes the message "actin and myosin keep pulling each other closer".  Absence of the electrical signal encodes the message "actin and myosin can't hold each other right now". 
  • Muscle fatigue is the result of low chemical energy availability in the sarcomeres of a given muscle.  Chemical energy is required for the myosin filaments to release their hold on a given section of the actin filament.  Without this release, the myosin filament can't continue to walk down the actin filament.  The result is a failure to contract any further. 

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(Disclaimer: Images and Videos are not my own)