If you don't know what ground reaction forces or moments are before reading this blog, then I strongly recommend you read this blog first, otherwise I may as well have written this in wingdings!

Exercise 'prescription' was always something that I found difficult.

I would find myself giving out the same exercises to a lot of patients with varied problems and goals just because I felt comfortable with just a few. Something wrong with your knee, hip, ankle or back? You were getting a squat regardless!

Issues with your shoulder? Yep, still getting a squat under the kinetic chain theory alongside some banded rotation and loaded long lever movements!

Now, you might argue (if you are anything like me) that the level to which an exercise needs to be specific to help people in pain is unknown, and likely to differ from person to person... so what does it matter if you just give out the same exercises again and again?

Frustratingly (and countering my purpose for writing this blog), I believe there is MAYBE some truth in this critique. Exercise is quite often likely to be the vessel in which we deliver the important features of our care. It gives us a way to demonstrate tailored care to individual needs with the right explanation, it gets the body part in question stressed to encourage adaptation, and it can help build self-efficacy of this body part that patient would likely describe as torn, injured, 'completely ******'.

However I would argue it is still lazy to just fall back on some familiar exercises because we ultimately don't really know the ins and outs of exercise based treatment. I would also argue that it is way more nuanced depending on the goals of the exercise.

[This is another thing that I probably didn't think about for a while also... exercise based treatment is meant to have a specific goal! I'll be honest, at the start of my career sometimes just getting through the day with patients all seen on time, notes finished and hopefully having at least one or two coffees to stave off the withdrawal headaches was my main goal]

At this stage in my career, I have found that splitting the goals of exercise therapy into two categories works quite well for me. Am I targeting pain / disability reduction? Or am I targeting 'function'? (I've written about this previously)

If it's pain and pain related function, the majority of the time I probably don't need to worry myself too much with the biomechanics of it all. From what we know to date, it is most likely psychometric variables that carry the most 'oof' with exercise for these problems. Sure, I might think an individual could do with having higher movement capability or force production in a certain area, but my 'compass' to help me navigate the types of exercise for the individual in front of me lies mostly in their clinical history, preferences and beliefs! (Hopefully by now this is not a controversial thought, but please comment if it is!)

If I'm targeting function then I am specifically thinking about movement options for the person and their individual constraints or action capacities related to their end movement goals (e.g. being able to do something in work, run better, get back to basketball). These phrases might sound a little confusing to you, and if they do then I would direct you to read up more on ecological dynamics and maybe start with this blog which introduces the idea of Ecological Dynamics!

This is where we get into the basic biomechanics of an exercise because if I am looking to improve aspects of function by targeting things (action capacities) like peak force or even just 'strength' then there are such things as bad, good and better exercises and it helps to know which!

Hold on though... 'there are no bad exercises, there is only load' is something I have echoed from Erik Meira in the past, so how does that fit? IN GENERAL there are no such things as bad exercises, meaning they don't inherently carry risk. However when we have a specific goal such as strength or hypertrophy of a certain muscle group in mind there certainly are bad exercises.

A way too obvious example I use in my course 'The Force Awakens' is to ask, what is a bad exercise when targeting the quadriceps?

... Well, duh. Hamstring curls, stiff leg deadlifts, bicep curls!

Because to stimulate the quads you need an external knee flexion moment that the quads need to counteract with an internal knee extension moment! (If you skipped the blog recommendation at the start, park this one here and read this blog instead!)

You don't need me to tell you this, obviously, but it serves as a nice example to demonstrate that if an exercise can't provide the mechanical stimulus to the tissues you've reasoned need that stimulus, it isn't fit for purpose. However, it's a sliding scale rather than a 'fit for purpose/not fit for purpose' as some exercises can still target the tissue you want but maybe not as effectively as another!

RIGHT, enough of the background setting. We've now arrived at the whole point of this short blog... how do we know if an exercise is targeting what we want it to? Or perhaps more importantly, how can we tell if our patients are being crafty self-organising systems again and finding a way to cheat around it? (Patients will do that from time to time)

If you go to a biomechanics lab and hook your patient up to motion capture and 3D force plates, and have the time to go through and clean all the data before settling down and creating your clinical report then you can be pretty accurate about this. Alas, I doubt there are many people in the world who either can do this or want to do this. So what can we do with our caveman brains and eyeballs in a quick clinical assessment?

We can try to understand exactly how forces act on the body and use this theory in our reasoning, and there are two fairly quick and simple ways to go about this:

• Reasoning the rough direction of the ground reaction force and making an educated guess on the magnitudes of this force on 'Exercise A' relative to 'Exercise B'

• Making an estimated guess as to the location of the centre of mass.

This gives us an idea of the line of external force acting on the body. We then can draw the forces to give us an idea of the length of the moment arm to the joint we are interested in (we are going to keep it to the knee because it's easier).

Right, last reminder, if you have no idea what I have just said in the last 3 lines go read that first blog!!!

The Biomechanical Bits

Quick reminder of the definitions:

Ground Reaction Force: The line of force that is a reaction to the foot hitting the ground. It is equal in the magnitude or size of the force of the foot hitting the ground and in the exact opposite direction.

Centre of Mass: The average position of all mass of the body (if looking at the entire person) or of the 'segment' such as the lower leg if we are thinking of exercises where there isn't any contact with the ground. This is how we would think about upper limb exercises as there aren't any reaction forces... but you aren't gonna get that in this post!

In my experience, most people think about the centre of mass (CoM) because it's probably what we are most familiar with. It's also pretty much somewhere in and about the pelvis in most weight bearing exercises. But remember it is the average position of the mass of the body and therefore will change with things like arm position and trunk lean. If I'm in a lunge position and I lean backwards and swing my arms backwards, my CoM will move posteriorly relative to the start position.

If I am leaning over to grab something on the floor just a bit too far out of reach, my CoM will travel anteriorly. If it travels so far anterior that it goes outside my base of support I for sure am going to lose balance and likely fall flat on my face!

It's really quite intuitive and easy to think about. And if you haven't read about the development of the high jump technique over the years, then go do this to understand more about centre of mass! (It's really cool)

So CoM is quite easy to think about; look at your patient doing an exercise in the frontal and sagittal planes and make an educated guess at where their CoM is. If someone is massively leaning forwards then their centre of mass is likely more anterior than when they are upright. If you see someone squatting with their arms out infront of them, you can edge the CoM backwards by getting them to stick their hands on the head to bring the CoM posteriorly (great adjustment in a spanish squat by the way).

By comparing our guesstimation of technique A to technique B, we can reason what happens to the moment arms. See how the moment arm lengths are different below.

Easy, right? It's lacking something though... what is it?

It's an indication of HOW MUCH FORCE each leg (and joint) is contending with. Obviously my mass hasn't changed in those two pictures as they were taken very close together, but is the force going through the front foot the same in both pictures?

... No chance! I'm putting a lot more force through the lead leg on the left picture compared to the right so the knowledge of the moment arm is only part of the picture.

This is one limitation I've found when thinking about the basic kinetics of static exercises, I don't have a way to think about the magnitude of the forces. And when it comes to more dynamic things like hopping, landing and changing direction I really don't have a way to think about relative differences in external moments acting on joints. However for more static movements, this could be countered in one way by looking at the position of the patient in the frontal plane. By looking at how they position themselves for the exercise you can then suggest adjustments to position in order to nudge the CoM over one limb more than another giving that leg a bit more force to contend with.

I think this is a really nice, simple way to start to think about the kinetics behind exercises and help you realise execution of these can be altered to achieve the goal you set out to achieve in the first place instead of what the patients body wants to do! And definitely worth getting some reps in thinking this way if you currently only think about hip hitching without understanding why, or just not thinking about forces in any form.

But I don't think about it this way...

I prefer thinking about the kinetics by the way of ground reaction forces. This is how a lot of the biomechanists do it! It's directly measurable with force plates and motion capture and the external moments acting on a specific joint can be calculated through a process called 'inverse dynamics' - it's mostly done by computer programmes now but the hardcore biomechanists used to do this with trigonometry, calculators and I'm guessing a f**k tonne of coffee.

The benefits of this approach in clinical practice, to me, allows us to think about both the magnitude of force (holding more weight (mass) or travelling faster will increase the magnitude of the GRF) and also the direction of the GRF! This is hugely important when thinking about different tasks including accelerating and decelerating and how to manipulate exercise to increase the external moment acting on joints. It also allows us to justify certain environmental constraints to help nudge loading towards the limb AND the joint that we want.

For the knee, generally speaking if we complete exercises with a larger posteriorly directed horizontal force component, there will be a more posteriorly directed resultant force meaning a greater external moment arm to the knee, and greater demand on the quads!

This isn't the time to dive into how we calculate resultant forces, but in brief, force plates collect the force of movement in 3 axis (frontal, sagittal and transverse), and in the sagittal plan we collect the vertical and horizontal components separately, before converging them to produce an overall or 'resultant' force arrow. Here is a very quick graphic I created (you can also go back to GCSE physics).

Here's an example:

In the suspended single leg squat, my foot is generating a large horizontal component, there is a high amount of friction where my shoe hits the floor because my CoM is very posterior and without the upper limb support I would fall flat on my backside!

The resultant ground reaction force (red line) is therefore angled far more posteriorly than in a standing single leg squat where the resultant force would be pretty much vertical because there is no horizontal component to the movement. So the suspended single leg squat makes the external moment arm (perpendicular distance from line of force to the knee) relatively large compared to a single leg squat, making the quads work extra hard!

Try this if you never had, it's bloody difficult! (We will conveniently ignore the forces occuring at the upper body...)

Bilateral Exercises and Environmental Constraints

Thinking about ground reaction forces I find especially helpful compared to thinking about CoM when it comes to bilateral exercises. We know people with an injury will sub-conciously load their other leg more through self-organisation and just because a squat looks normal, does not mean that the forces are equal! That has been demonstrated quite nicely by Dimitriou et al.

With this in mind, we can be crafty and give people things like foot elevated squats or toe float exercises to nudge the magnitude and direction of the ground reaction forces in the ways we want to minimise this compensation!

Wrapping it all up

Hopefully this has provided a brief introduction to different ways to think clinically about how forces act on the body, and how compensations from our patients can actually undermine our efforts to stress the body part we want, as much as we want it to be.

There are multiple work arounds for this including exercise dosage, speed of exercise and the mass that is lifted all of which are valid ways to go about it. But I personally feel there is real value in having an understanding of the kinetics in order to deliver environmental and task constraints, getting people to move in the way WE want them to! After all, we are meant to be movement experts.

Until next time,

Jeff

If you have learnt something, or even just enjoyed reading this blog then I would be eternally grateful if you could drop it a like, a comment or share it on your social media to spread the message!

Additionally, if you want to learn more about this, you can purchase the day 1 materials of my course 'The Force Awakens: How to REALLY use biomechanics in clinical practice' and have unlimited access to this, and myself with any questions! Check it out HERE for links to the online and in-person events.