Have you ever read or heard something with the words kinematics, kinetics or spatiotemporal variables and not had a fucking clue what any of it meant? This is the blog for you!

Let’s say you have a runner in clinic who is struggling with what you think is patellofemoral pain. And let's say we are fairly happy with that diagnosis so we can cut straight to the chase! We know that we need to help the joint calm down a little bit as part of the rehab process for the vast majority of patients, and one way to do this is to try and reduce the ‘load’ going through the joint.

Now we all know the most effective way to reduce the cumulative ‘load’ going through the joint… and we all know how ineffective giving this advice to a runner is!! Asking someone to stop running altogether is generally a no-go because A) it is not what the patient is looking for as they are mostly still able to do it, but with pain, B) with a diagnosis of PFP it isn’t really a necessity as it would be for say, a bone stress injury and C) we are in the game of facilitating people to do what they want to do, not taking it away completely!

So we can’t get them to stop running completely but we still might want to reduce ‘load’. Easy go-to options include discussing an alteration in running distance, pace, terrain e.t.c., but the likelihood is that this has already been tried; you don’t need a formal education to realise reducing your running a bit may help your knee pain! Of course, if they haven’t considered this, these are by far the easier variables to try and change before interfering with the biomechanical variables, so I would go for these!

We are now at a point in our clinical reasoning where terms like ‘load’ and ‘biomechanics’ are maybe a bit too nebulous to be of any real benefit. Great for patients, as chances are they aren’t too bothered about the nitty gritty details of it all, but not great to reason which direction to go down.

Let’s delve into the different terminology that this blog is all about! Believe me when I say that these terms and principles are important for us who are in rehabilitation. You can debate the importance of certain biomechanical variables with clinical presentations and how much we need to focus on them, but in order to debate them we need to first understand them!

Kinematics

This category in the umbrella of ‘biomechanics’ relates purely to the observation of movement WITHOUT considering what caused that movement and for this runner we may talk about degrees of peak knee flexion or hip adduction, or excursion of knee flexion.

We love to talk about kinematics as physios! We can observe movement extremely easily and tend to make strong inferences off of what we see and what that must mean from a force perspective or a kinesiopathological standpoint. This is dicey at the best of times apart from commenting on gross movements where we can clearly state there has been motion into a certain plane and guess at how much this is. When it comes to estimating degrees of pelvic tilt, scapula dyskinesia or alignment/position of the knee on dynamic tasks then we need to admit there is probably a large degree of measurement error occurring because our caveman eyeballs just aren’t that finely tuned or able to see through clothes / soft tissues to observe what is actually going on. Add in to the mix that we then have to perceive what signals our caveman eyeballs are giving us, which can be influenced by our prior expectations, and it really doesn’t seem like an overly desirable or reliable thing to base a bunch of our reasoning off!

We can increase the granularity (i.e. the level of detail in capturing data) of our assessments by utilising technology. We can get a rough idea of uni-planar kinematics by using motion analysis apps on our phones in clinic; it may not be 100% reliable if you aren’t doing it in a standardised way, and even if it is reliable the measurements may not strictly be valid to what is actually occurring… But I would still argue it is better than just looking and guesstimating as you can get better at practicing the way to collect videos to make it more reliable!

When it comes to research, the capturing of kinematic data is obviously a lot more sophisticated. Sticking big markerballs onto different segments of the body for cameras to pick up, or using accelerometers is probably most common practice, but newer methods such as markerless motion capture and biplanar X-Ray imaging are emerging which is great and offers up much greater insight to looking at kinematics in more ecologically valid environments and getting to within a couple of millimetres measurement error for joint kinematics!

Anyway, starting to ramble and get dangerously close to other topics here so we best move on!

Kinetics

When discussing kinetics, we are talking directly about the forces. A lot of the time, we are talking about forces that cause a change in kinematics. If you are unfamiliar with this and how it ties in with joint moments / torques and Newtons laws of motion then please go back to read my previous blog ‘Taking a moment to discuss biomechanics’.

I underlined the word ‘cause’ above as I feel it is really important to hammer home. Changes in kinematic variables / movement will only ever come about from unbalanced forces acting upon an object, and in our example, this is internal forces from our musculature / passive restraints and external forces from e.g. ground reaction forces or direct contact. Therefore, if we are observing a change in movement, we can be sure there are unbalanced forces acting ‘behind the scenes’.

Kinematic changes are always driven by kinetic variables / ‘force’, but it is also important to highlight that you can have a change in kinetics WITHOUT having a change in kinematics. Confusing, right?! This is actually a common thing in clinical practice though. For example, providing someone with an unloader knee brace if they have uni-compartmental tibiofemoral osteoarthritis affecting the medial side – the straps on the brace create a knee abduction force which can reduce joint reaction force in the medial compartment, but the knee alignment doesn’t actually change! Or think of isometric exercise or testing… increases in force but no change in movement!

(*I have no affilitation with Ossur, and I also think symptomatic improvement comes about from a lot more than just a change in force! But the biomechanical principles are valid in theory!)

A great ‘normal’ example of this I first heard from Ian Griffiths (@sportspodiatryinfo) regarding a see-saw. If one person is sat on the end and nobody on the other side the see-saw will be leaning to one side; it is in a static state. We could theoretically measure the compressive force acting on the ground from the end of the see-saw using some force plates. If a second person then jumps on the SAME SIDE as the first person, there is no change in movement (kinematics) but there will be a significant increase in the compressive force measured on the force plates!

When it comes to measuring kinetic variables, physios are notoriously BAD! We only need to look at recent literature that demonstrates the vast majority of us use our hands to evaluate muscle strength (this isn’t technically the right phrase to use but for ease of reading I will keep with it! But mostly we are talking about peak force or peak torque [do you know the difference? Insert plug for my previous blog!] and sometimes about rate of force development).

Again, this becomes an issue of granularity as a quick screen of muscle strength during an assessment is a perfectly valid thing to do, if there is a great big whopping difference from side to side then you don’t need a dynamometer to tell you what type of treatment could be needed! But because we don’t have an internal gauge to let us know how many newtons someone has just unleashed into our hands it isn’t anywhere near reliable AND just like our vision, our interpretation is likely to be influenced by our prior expectations!! Therefore, if strength is something we think is important and need to track, we need to be more granular with our methods and invest in some technology such as increasingly affordable in-line dynamometers!

When it comes to other kinetic variables such as impulse, peak ground reaction force, power e.t.c. Then we haven’t stood a chance quite frankly. It is only in recent years that we can get access to affordable (for some) force plates from companies like VALD, Hawkins Dynamics and K-Invent, to name just a few. These are sophisticated bits of kit that can give you literally hundreds of data points per hop / jump; the difficulty here is know what to pay attention to… but again dangerously close to going off track so I will stop at that!

In this patient scenario we will be discussing ‘work’ at the knee (the integral of force over distance / excursion of ROM) and patellofemoral joint reaction forces.

Spatiotemporal variables

This is a much quicker section to get through you will be glad to know! These are variables that relate to space (Spatio) and time (temporal) such as step rate whilst running, stride length or stroke rate when rowing.

Physios are aware of spatiotemporal variables, but in my opinion don’t appreciate them enough!! They can be so helpful and are much easier to influence as they can act as an external focus for the patient, rather than attempting to get someone to change their movement from an internal focus.

Want to know what your runners cadence is? Pull up their garmin watch or whatever it is they are using to track their activity. If they aren’t using anything, then get them to! Knowledge is power and can help you make informed clinical decisions!

They come as a package

The great thing about all of these variables for us as physios is that they all influence each other, by changing one you will probably change them all for the most part!

Lets get back to our runner with PFP; we are looking to reduce their ‘load’ whilst running and for this we can focus specifically on patellofemoral joint reaction forces AND/OR patellofemoral joint stress where ‘joint reaction force’ is the force that the joint surfaces of the patella and the trochlea exert on each other and joint stress is the magnitude of force divided by the contact surface (smaller contact surface between the two bones increases the stresses on the cartilage; think what happens to the femoral head cartilage in hip dysplasia as a similar scenario). It is important to acknowledge at this point that I am not claiming there is a linear relationship between joint stress and pain! But for some people reducing the patellofemoral joint reaction force, and in turn joint stress, will help reduce nociceptive input!

It has been well researched, and summarised in a systematic review and meta-analysis by Anderson et al (2022) that changing step rate (a spatiotemporal variable) influences a change in the kinematics AND the kinetics of the knee joint.

Specifically, increasing the step rate significantly reduced the stride length (another spatiotemporal variable), peak patellofemoral joint stress (kinetic variable), peak knee flexion angle (kinematic variable which will in turn affect the joint contact surfaces and subsequently influence patellofemoral joint stress) and peak knee extensor moment and braking impulse (both kinetic variables).

Conversely, decreasing the step rate increased breaking impulse and negative knee work (essentially these boil down to increased quadriceps force output which is likely to increase patellofemoral joint reaction force).

So when it comes to clinical recommendations, why not get your patient on a treadmill and cue them to take smaller strides OR stick on a high cadence playlist on spotify and run to the beat? Or get their cadence from their watch and multiply it by 1.05 for a 5% increase target or 1.1 for a 10% increase target? It is a simple enough intervention but with a complex and quite clever biomechanical background in reducing patellofemoral ‘load’!!

Signing off

Hopefully now you may have a bit more of an idea of these variables than 15 minutes ago, if these terms were new to you. Remember that this blog is purely about the biomechanical principles and not meant to be a narrative on how to treat patellofemoral pain. Biomechanical rationales can certainly be helpful, but it would be wrong to assume that they are the be all and end all for the vast majority of patients… more like a small portion in a complex jigsaw!!

Let me know if you have found this helpful or have any queries!

Thanks for reading,

Jeff