Approximately 10-15 mins read

Do biomechanics confuse you? What does the word ‘biomechanics’ mean to you? It’s meaning has evolved for me over the years as I learn more and more about it. I tell you for sure what it is not – it is not a nebulous description that is dominated by upper or lower crossed syndrome, pelvic rotation or dynamic knee valgus, that you will typically get on Instagram. My favoured term for those types of posts is pseudobiomechanical bullshit! Don’t be fooled by them!

No, biomechanics is actually scientific, but I didn’t really know much about it even up to a few years ago! I have distinct memories of sitting with my mate over a beer and him mentioning a ‘moment’, to which I replied… what the F is a moment?! Clearly, I had glossed over GCSE physics as I swear I had never heard of this terminology before, but little did I know at the time how much my reasoning would be enriched by just getting to understand the basics of Newtonian physics.

Now if you are like I was, even the phrase ‘newtonian physics’ would fill you with some confusion and dread! But fear not, there is no judgement in this blog. It’s rare that we would get taught anything related to biomechanics (in the U.K at least) in undergraduate studies and as a result it isn’t really talked about in clinical practice either, so it would be strange if we were all experts in it.

The best we can do is take out heads out the sand and start to understand it a bit better, and this blog is here to hopefully help some of you out with the start of that journey! Then it will be much easier to understand some future blogs such as using ‘Biomechanics as a framework for exercise provision for strength training’, ‘Understanding the kinetics of mid-end stage rehab’, ‘The effect of external cues and training environment on running biomechanics’ and ‘Dynamometry and return to sport testing’. At times it might be a bit repetitive, but I know from personal experience of learning this area myself, and trying to teach others, that being painfully obvious with explanations is best!

Obvious disclaimer before we begin: I am a clinician, not a biomechanist. I have adopted the concepts that I have found useful to clinical practice with regards to human movement but by no means declare myself an expert in ‘biomechanics’. The field itself is HUGE and spans everything to do with understanding motion and forces in living organisms. Spanning from impulse conduction along our nerves, to things like fluid mechanics, which may be of quite significant importance to people like cardiac surgeons but of less importance to us in MSK!

What do we mean by Newtonian Physics? Newtonian physics, or, classical mechanics, refers to a set of laws developed by Isaac Newton that aim to describe how objects (we are going to be focusing purely on the body today) behave with respect to motion and how forces affect this motion. It’s important to note that we aren’t going to go anywhere near Einstein’s theory of relativity for a couple of reasons; the main reason is that it would take me about 5 words before I started spreading misinformation. But that is okay, because the second reason is that here on planet earth, Newtons laws seem to hold up very well!

You may or may not recall Newton had 3 laws of motion, I am going to write them out in quite over-simplified terms below:

· 1st Law: Things won’t change their state of motion unless acted up by the application of an unequal force (E.g. At the beginning of a cabled knee extension exercise, your knee will stay bent until enough force is produced by your quadriceps to overcome gravity’s effect on the leg AND the resistance provided by the machine to create motion into knee extension)

· 2nd Law: Force = Mass x Acceleration. The rate of a change in motion is proportional to the force applied. (E.g. at the start of the cabled knee extension exercise, the more force that is produced, the faster the leg / machine arm will move assuming the mass of the leg and external weight is constant. We will save going into what ‘power’ is a different time)

· 3rd Law: Every action has an equal and opposite reaction. (E.g. Ground reaction forces. Every time we step we hit the ground with a certain amount of force, and the ground hits back at us with exactly the same amount of force in the opposite direction. We call this the force vector as we are talking about magnitude and direction. So as you are standing still the GRF / force vector is directed up to the sky, when you come to decelerate the GRF / force vector will be angled more posteriorly)

We will run through some examples of everyday tasks below to hopefully bring these laws to life by application. But first, we need to run through what we mean by a ‘force’. Not from a philosophical standpoint, but a practical measuring one.

For the sake of this blog we are interested in two types of measurement for forces; linear (in one direction) and rotational:

· Linear force: Ground reaction force is a good example of this, you hit the ground and it hits straight back (pictures of this will come in a minute). Muscles produce force and pull in one direction. When we talk about linear forces we use the measurement of Newtons (N). This is just the same as measuring volume in litres or centimetres as a measure of distance so don't be put off by the terminology. If you have ever used digital in-line dynamometry you will notice you can record the force produced in Newtons.

· Rotational force: Now we are talking about ‘Torque’ or ‘Moments’, these are both interchangeable terms so don’t get too confused if you see them used together in books or journal articles! These are measured in Newton-Metres (Nm). Again, this measurement just quantifies how much rotational force is occurring. For clarity I am only going to use the phrase ‘moment’ moving forwards in this blog! When we talk about movement about a joint, we are generally talking about moments when we talk about force, as all of our movement that occurs at a joint is rotational movement caused by rotational force! (Ignoring the forces that occur in trauma / pathology)

The next few minutes are important to lay out the theory behind calculating moments, but I want to stress that it is more the concept that is important to us as clinicians. We aren’t in the game of accurately measuring force and movement outside of specific tests, we don’t have our labs kitted out with triaxial force plates and 3D markerless motion capture. Even if we did there is no chance we would have the time to actually analyse it all!

Calculating moments 

A moment is calculated as such; Moment = Force (N) x Distance (M). Here (N) represents Newtons and (M) represent metres, the result is a number with the measurement of Nm (Newton-metres).

You will be familiar with the pictures of see-saws from school textbooks I’m sure. We are going to go back to these as they do nicely illustrate how moments (forces) influence movement (Newtons 1st law).

So, we know that motion only occurs when there are UNBALANCED forces acting about a fulcrum or pivot point. In the picture we see a see-saw with two weights on it. One exerting 10N of force and the other exerting 20N of force. So we have the first part of our equation but we cannot calculate the moments just yet, we need to know what the distance to the fulcrum is. The 10N force is 2m from the fulcrum and the 20N force is 1m from the fulcrum. When calculating the moments we call the distance between the line of force and the point of rotation called the moment arm. This is a phrase you will hear a lot. So, remember it means the distance from the line of force / force vector to the axis of rotation and it always has to be PERPENDICULAR (at a right angle) from the line of force to the axis of rotation [examples to follow in a couple of paragraphs].

In the see-saw example [Force (N) x Distance (M)] results in the moments created by each weight being 20Nm.

Because the moments are balanced, there is no motion about the fulcrum and the see-saw stays balanced. Intuitively we know that if we increase or decrease either the weight OR the distance on one side it will change the moment that the weight is exerting on the see-saw and this WOULD create motion if the forces became unbalanced.

Easy, right?! It is a simple enough concept also when it comes to calculating moments with joints in the body.

The first thing to mention is that when we are talking about moments on the body we talk about them in very descriptive terms – for example, a quadriceps contraction will create an internal knee extension moment. At risk of being painfully obvious here we call it internal because the force is originating from within the body, knee because that is the joint we are interested in that has the force acting on it, and extension because that is the motion that would be created should the force go unopposed. Conversely, an ankle weight strapped to the bottom of the shin would exert an External Knee Flexion Moment. Should the external moment be greater than the internal moment, the knee will move into knee flexion and vice versa.

The second thing we need to discuss when talking about moments specific to the body is knowledge of where the external force is coming from and the direction of the force. There are generally two ways to do this, from the ground reaction force and from the centre of mass. Lets start with using the GRF.

Moments associated with ground reaction forces 

In the picture below we have a drawing of a skeleton walking and a big green arrow which represents the ground reaction force vector. It gives us an idea of the magnitude (how big) the force is (measured in Newtons); remember Newtons 2nd Law – Force = Mass x Acceleration, therefore if someone is holding a weight (increased mass) or is hitting the ground quicker (acceleration) the force will be greater. In diagrams like this, the arrow (force vector) would be taller and thicker!

Also of great importance is the direction of the ground reaction force vector, this is important as it will affect the distance from the line of force to the axis of rotation (we will focus on the knee for this example).

Remember that we need to have a moment arm or, the distance from the line of force to the knee that is at a right angle to the line of force in order to calculate the external flexion moment acting upon the knee joint (if there was no quads countering this force then the knee would go into flexion… think back to seeing very deconditioned people on a ward whose knees would give way on stepping e.t.c.)

The moment arm is absent in the picture above, try and picture where it would be if it had to branch off at 90 degrees off of this line heading directly to the centre of the knee. Let’s say that the GRF is quantified as 1000N and the moment arm is 0.2m. We could therefore calculate the external knee flexion moment as 200Nm [remember force x distance = moment]. We therefore know that in this position, the internal knee extension moment created by the quads HAS to be greater than this to bring the knee into extension and if it is less than that (e.g. from fatigue, atrophy and weakness, paralysis) then the knee will move into flexion. For an isometric hold there would be no unequal forces.

Hopefully this makes sense so far and I haven’t lost anyone. Remember, it is not about the numbers for us! We can say with some certainty that movements that have larger EXTERNAL moment arms than others are going to require a much bigger INTERNAL moment to counter it. Given that the moment arms for our muscles to our joints don’t change (our structure is what it is) it means that to create a bigger internal moment, the force needs to be greater from our musculature and other passive restraints (again remember, a moment = Force x Distance so it the distance can’t change then the force has to, to increase the magnitude of the moment!)

Right let’s put you to the test a bit here (not as casual a read as you were hoping for I bet); have a look at this picture below of a front barbell squat and a low bar and high bar squat. The dashed line from the middle of the foot is the GRF vector, it is vertical and the moment arm needs to be at a right angle from this line pointing to the knee. From using just the distance from the GRF vector to the knee (the moment arm) can you tell which would have a greater external knee flexion moment requiring an even greater internal knee extension moment to create knee extension movement and get out of the hole?

Typically, it is a front squat that is considered more demanding on the quads as the moment arm is the greatest out of the 3, and if you didn’t really know before, hopefully now you have a bit more insight!

Calculating moments from centre of mass 

Lets talk about a bicep curl and show the upper limb some love!

The same principles we have talked about above still apply. This time however there is no ground reaction force vector that we can use as our ‘line of force’ in order to calculate a moment arm. We therefore use the line of force generated from the centre of mass of the limb / trunk we are interested in; in this case the elbow (Labelled ‘W’ in the picture). We also need to calculate to the line of force generated by gravity acting upon the weight in the hand (labelled ‘P’ in the picture). Add these together and we have a total external elbow extension moment that an internal elbow flexion moment would need to overcome in order to flex the elbow and lift the weight. See if you can figure it out from the picture below. Can you think why it is easier to hold a dumbbell in angles greater than 90 degrees elbow flexion compared to 90 degrees elbow flexion?

Another thing I would like to mention here is compensation programmes for massive rotator cuff tears. If you are familiar with exercise programmes of having people on their back and then (with assistance from their other hand if they need it) ‘punching’ up to the ceiling; in this position they can then do forwards / backwards movements, or small circles. Over time you would ask them to hold a small weight (e.g. bottle of water) and/or you would progressively sit them up.

The picture below is a picture in the starting position followed by a snapshot of the exercise where you move the weight forwards and backwards, you can see the more you ask them to move the weight forwards / backwards, the greater the moment arm (in red) is leading to a larger external shoulder extension moment! When you ask someone to increase the distance they move the weight, this is part of the biomechanics behind it!

Now consider if you start to sit them upright. In this picture there is no weight, but at 90 degrees there is now a much bigger external moment arm to the force vector created by gravity on centre of mass of the arm than there was just lying down. By getting the person more vertical, we are progressively overloading the arm by increasing the external shoulder extension moment (by increasing the moment arm)! Now there are likely other competing mechanisms behind why people improve with this type of intervention, but this is the biomechanical explanation!

Wrapping it up 

The main concept I wanted to communicate here is that we can easily manipulate external moments acting upon the body by either increasing the weight that someone is holding (by increasing the force, remember Force = Mass x Acceleration) or by increasing the moment arm.

An easy example would be a lunge driving knees over toes compared to a backwards lunge; knees over toes has a much greater external moment arm to the knee relative to a backwards lunge, therefore we can say that assuming mass is the same, a knees over toes lunge has greater quadriceps demand to overcome the external moment acting on the knee. This concept will form a basis for a future blog on using kinetics as a framework for strength training or early stage rehab!

But we need to be mindful this is very simplistic. We are looking at static moments and obviously human movement is more complex than this, but it provides a great baseline for our understanding to develop.

There we have it

I think that is probably enough for one blog. The next step is to talk about the 3 main branches of biomechanics and how they relate to each other! Those being kinematics, kinetics and spatiotemporal variables and how to use these variables to understand movement better.I hope this was helpful, please do reach out and let me know if it was and if you have any questions!

Until next time,

Jeff

[If you didn't catch the pun in the title to begin with, I hope you can now!]

Last couple of pictures adapted from https://providephysiotherapy.org.uk/wp-content/uploads/2021/10/Anterior-Deltoid-Programme.pdf?x56893