The Basic Physics of Exercise

Very few people discussing exercise publicly - so-called “influencers” - can talk about exercise.

I mean this on a technical level.

Few “influencers” understand the basic physics principles that would allow for informed, objective opinions about resistance training.

People instead use emotional attachment and physical sensation as proxies to guide their decision-making progress.

And this is why we end up seeing so many crazy-looking, nonsensical exercises on the internet…

Today, I’m going to teach you the foundational principles of physics that underlie exercise - force, moment arm, and torque - so that you can start to guide your training with the nuances of physics rather than your emotional whims.

Force

We all intuitively know what force is, but to describe it is more abstract than one might imagine.

Force is physical influence - a “push” or “pull”.

Tom Purvis - one of my mentors - defines force as “the agent of change”. This beautifully captures the essence of force and what we need to understand about it.

Force is not something that you can see and touch, at least not directly - we see motion or lack thereof (which is not the same as force).

We can measure force through calculations of mass and acceleration (shoutout to Newton and F=MA), but by definition, force is an abstract concept.

Force = the physical influence of a push or pull.

Force Properties:

All forces have the same properties, and there are three major properties worth considering in the context of exercise: contact, direction, and magnitude.

Contact

We encounter force through physical contact.

Unless you’re telepathic (I’m working on it), forces need to do this by touch. I like to call this the point of contact or location of a force.

Direction

A force between two objects pushes or pulls in a single direction (a line).

This direction is subject to change as motion occurs, but if you freeze time, the influence of any given force is represented by a line. 

This does not mean that force only creates motion in straight lines, but rather that every force itself acts in a straight line. 

Remember - force is distinct from motion.

Motion occurs when forces in either direction are unbalanced.

To understand how force differs from motion, picture the following scenario:

• Person A pushes a box “to the right” with 50 pounds of force.

• Person B pushes the same box “to the left” with 40 pounds of force.

The box will move toward the right because the forces on the box are unequal.

Now imagine that person B pushes with 60 pounds of force “to the left”. Which way does the box move, and why?

It moves “to the left” because of a change in magnitude.

Magnitude

Magnitude refers to the amount of something.

In the first example above, the magnitude of a force pushing a box “to the right” (50 pounds) was greater than the magnitude of a force pushing a box “to the left” (40 pounds).

In the second example, magnitude of the 40-pound force changed to 60 pounds - so the total force on the box did, too.

In the context of exercise, magnitude is represented by the number on a dumbbell or a machine.

But other influences determine magnitude, too.

What are they?

Torque & Moment Arm

A force’s contact point and its direction should always be considered first.

But people aren’t boxes and we can’t simplify human motion in the same way.

Imagine I’m picking up a dumbbell to do curls or a barbell off of the floor to do deadlifts. 

What determines the difference between a dumbbell curl and a barbell deadlift?

It’s intuitively obvious, but why? 

The answer is that our experience of resistance tells us that these things are different.

Our experience of resistance is torque.

If you’ve been around the fitness scene for any length of time, you’ve probably heard the word torque used erroneously by coaches and therapists.

Many of these coaches and therapists use the word torque as if it means “a twisting force” or a “force that twists”. 

But…as we now know, forces don’t twist!

Forces only act in straight lines.

So, torque is not a twisting force.

This is a very important concept that needs to be drilled into your brain.

Torque is a property of force that determines how effective a force is.

Let’s go over some examples of what I mean:

Imagine that you’re holding a cup of water (or beer if you’re into that) at a party.

Aside from the fact that it might look weird - would you rather hold your cup out to the side - 90 degrees away from your torso - or down by your side?

The answer to this is hopefully obvious, but why it’s obvious is another question entirely - much like the distinction between curls and deadlifts.

So far, we’ve established that:

• Force is a physical influence.

• Torque is the effectiveness of that influence.

• Torque is not a twisting force because twisting forces don’t exist.

• Torque is our experience of resistance.

Torque is - more technically - a product of force and that force’s distance to our body (torque = force x shortest distance).

Torque takes into account a distance, which force alone does not.

Back to our cup-holding example:

You have the choice to hold your drink 90 degrees away from your torso, like you’re doing a lateral raise, or down by your side, under your hand in your fingertips.

Which would you choose?

Besides the obvious downside of looking like a lunatic when you hold a cup 90 degrees from your torso, everyone intuitively understands that holding the cup out to the side is a lot harder than holding it down by your legs.

Why?

Torque.

If we know that torque is the product of force and distance, the changing variable between these two cup-holding scenarios is that distance.

In other words, the amount of force within the cup itself didn’t change, unless you were actively chugging your drink between positions (and fair enough, if so).

We can measure force by identifying the weight of the drink and distance by how far away from our bodies the cup is. 

So how do we measure this distance?

Isn’t the length of the arm the same the entire time?

What kind of distance are we measuring?

This distance is technically called a moment arm - the shortest distance between a direction of force and a pivot point. 

A moment arm is an intangible distance, meaning that it’s not something we can see or touch (like force).

So, in this specific cup-holding scenario, the object is the drink, and the pivot point is the shoulder (glenohumeral) joint.

You could also look at the cup’s influence toward any other pivot point in the body, but its influence on the shoulder joint is most direct.

Let’s create a picture of what we’re looking at so that we can put this into practical application.

Below are the two positions I’ve described above, with the red line representing the first two force properties - the red line is drawn from my hand downward in a straight line to represent the action of the cup against me.

Note that the arrow extends upward (or “backward”, depending on how you look at it) to the level of my head - this is necessary in many cases because of what we’ll look at next.

Now, to identify moment arms in each of the above positions, draw a line that is 90º from the red arrow to the shoulder joint, like this:

Now imagine that, in the left image, I hadn’t extended the red line upward toward my head. I wouldn’t be able to draw a 90º line from the force direction to the shoulder joint.

What do you notice between the two photos?

The moment arm in the bottom position (left) is a fraction of the length compared to the top position (right).

What does this imply?

Torque is force times distance. Torque shows a force's effectiveness. So, the longer the distance, the higher the resistance.

In other words, the top is much "heavier" than the bottom. This is because the moment arm (green line) is much longer.

Did the weight of the cup change?

No.

But the distance did.

Let’s look at another example, which is taken directly from my online biomechanics course:

So there we have it - an introduction to the basic physics that underlies exercise.

Summary

• Force is physical influence - a push or pull.

• Force has three primary properties: contact, direction, and magnitude.

• Torque is a force’s effectiveness, not a “twisting force”. Forces only act in straight lines.

• Torque is the product of force and moment arm.

• Moment arm is the shortest distance between a direction of force and a pivot point.

• Changes in torque within an exercise are what determine where an exercise is “lighter” and where an exercise is “heavier”.

Many other factors can influence exercise and its forces. But, these properties are the bread and butter of how we can describe exercise.

Assess your learning by answering the 5 questions below (answers at the bottom of the page):

If you want to learn more from me, check these out:

• My online courses - where you can find my highest-value content that dives into learning anatomy and physics and how you can apply it to lifting immediately.

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Answers to learning comprehension:

1 - false.

2 - contact, direction, magnitude.

3 - a force’s effectiveness.

4 - contact.

5 - more torque.