To me, impulse is something that is invaluable to understand. It’s a piece that fits really neatly into the puzzle of ‘what the fuck am I doing’ in a lot of rehab planning.

For people who have understood physics since they were back in school this might seem quite boring… of course Impulse = Change in velocity (and subsequently momentum), it’s just the Newtonian way to explain these things.

That is, J = Δp; where J is impulse, the triangly thing is change and of course, p is momentum. Because that just makes total sense seeing as there are, oh, well, NO J’s in Impulse and NO p’s in Momentum! It is precisely this type of language which is the reason it has taken so long for me to become interested in basic physics and how it applies to my practice as a physiotherapist; and I’m going to do my best to break it down in a way that makes sense clinically whilst respecting the background laws of physics!

Let’s begin with some simple definitions for the sake of this blog:

·Impulse = Is the force that would change an objects motion, either slowing it down or speeding it up. It is directly proportional to changes in velocity.

·Velocity = the speed of an object in a specific direction.

·Momentum = How much motion an object has based on its mass and speed. The formula is actually, Momentum = Mass x Velocity. So, whilst impulse is technically directly proportional to changes in velocity and not directly momentum, we can get away with it because of the clear formula where mass will be constant

You can also think of it of how hard something is to stop. A car and a truck slowly creeping towards you will be different in the sense of the truck has more momentum because of its mass and will be harder, or require more force in order to make it stop!

Okay, definitions out the way, why should we as physios / S+C’s / chiros / sports therapists e.t.c. care about this sort of stuff? … BECAUSE IT IS ALL AROUND US, ALL OF THE TIME!

Change in momentum is a necessary condition for movement, and that is kind of our bag. It applies to elderly people saving themselves from falling, long stayers on ITU trying to regain some independence, athletes trying to return to sport after a significant injury; the list goes on and on.

If we want to visualise impulse, we need to do this on a force time graph. This is important as Impulse can essentially be thought of as the total force applied over a certain amount of time. For the sake of keeping this simple let’s just think of a quads test where all force applied is isometric and positive; we don’t have to think about anything eccentric. See the graph below.

Here you can see the x-axis in the magnitude of force measured in Torque (Ft-Lbs)… but don’t worry about the units if you’re unsure what a Ft-Lb is, think of it just as force and make a note to check my previous blogs on torque (I talked about Newton-Metres in these blogs, Ft-Lbs is just the weird imperial, American metric for torque! Like inches instead of centimetres). On the y-axis is time in seconds.

The line going across is the force applied and everything underneath the curve is impulse / total force. You can see that the right side curve has a greater area under the curve and subsequently has more impulse! It’s measured in Newton-Seconds but that’s not that important for us really.

When quads are tested isometrically, the dynamometer or the belt attached the dynamometer applies just as much force back on the leg so there is no motion but it allows us to capture force capabilities of the quad, or the ‘efficacy’ of the quad. When we are doing this we are definitely more interested in peak force or peak torque rather than impulse, but it’s a nice easy graph to try and visualise what total force is.

So let’s move onto our next example, a squat jump (Hands on hips, start in a crouched or slightly flexed position, explode up). Have a look at the picture below.

Picture from Hawkin Dynamics @ https://www.hawkindynamics.com/blog/countermovement-jump-or-squat-jump

There is a lot more going on, and initially it can be a bit daunting but we are going to focus purely on the concentric portion after the dip (highlighted green).

If we recap everything so far, we know that impulse is directly responsible for changes in momentum. So let’s look at the point at the beginning of the concentric phase; we can say that the total area under the curve from this point to take off will dictate take off velocity (speed), and subsequently momentum, at which you move off the ground. More impulse or total force will lead to higher movement velocity and will dictate how displaced the centre of mass becomes and ultimately how high you jump.

Our goal in order to be able to jump higher then comes down to, how can we squeeze as much space underneath that curve in that short space of time between start and take off? Most of us will be interested in classical training targets; mainly peak force (what’s the most force you can apply into the ground to propel yourself up) and rate of force development (how quickly can you apply this force) which we can train through basics of developing muscular hypertrophy and targeting high loads for peak force benefits.

Both of these are great, because if you can see how if the peak of the curve is higher then you can squeeze more space underneath it, and same for the slope of the curve on the left hand side of this curve, if it’s steeper there is more room underneath the curve.

And breath for a second. Take a second to read back over the past few paragraphs to make sure you grasp it….

Alright, let’s get back into it.

We have used an example of a squat jump so far because there are force time graphs available for us to look at and it is quite an easy thing to wrap your head around.

Let’s bring it back to common example we have in rehab, returning someone to sport. For a lot of people, rapid changes in momentum are going to be needed to be any good and/or to do the sport successfully. Whether this is side stepping in rugby / American football or exploding in basketball / gymnastics or changing direction 180 degrees in cricket (the list could go on forever). It’s important to note especially for things that involve changes of direction, decelerating and reducing whole body momentum is a KEY COMPONENT without which it would be impossible to execute the movement… especially for things involving 180 degree turns where you literally have to stop before going back the other way.

By understanding Impulse, we can reason that to improve someone’s ability in these areas we need to first build back the efficacy of the muscle (that is, in laboratory / clinic testing conditions can it produce enough force) because if it can’t, asking that muscle group to produce force to slow someone from sprinting is pointless. If they physically can’t produce a high peak on that force time graph, then they cannot produce sufficient impulse to change momentum, and it doesn’t matter what else you do to try and improve their performance, they won’t be able to do it well.

This is why we say things like knee extensions are functional, they help build back muscle force through progressive overload of stress applied DIRECTLY to the quads so they can grow big and strong for whatever they need to do in the real world. It isn’t just the quads either, replace them with calf after being non-weight bearing or Achilles injury, pecs / deltoids after shoulder injury e.t.c!!

Once you’ve got enough peak force in controlled conditions we need to transition to force expression in the tasks similar to what the person will be returning to. This could be practicing deceleration so that the person can get used to expressing this type of force quickly (rate of force development) so that they can express more total force in real life situations because more total force means quicker stopping or speeding up (which usually leads to better performance!)

‘Yeah but I already do that’ – great if you do. But what framework are you working in? Are you just shooting in the dark or applying the same method to everyone you see? Rehab should be purposeful and reasoned, and understanding the background laws that dictate how we move is arguably the best framework to go from. It can help you problem solve in tricky cases where getting someone just jacked as fuck in compound movements hasn’t really translated to where you hoped it would (from a movement / physiological perspective). Conversely, it might also explain why the approach of getting quads strong as hell probably works for a good amount of people. Importantly though, blanket approaches will never work for everyone.

Principles versus methods

The great thing is that impulse DOES apply to everyone, as I mentioned at the start it can even be applied to inpatient and neuro settings because these patients are governed by the same laws of physics! Knowing we need to get someone better at expressing total force over a short amount of time opens up many doors – whether you choose to deconstruct people into looking at individual muscle group efficacy to look for deficiencies or think more about practicing force expression during specific tasks, both are principles to guide your rehabilitation plan, it’s part of a ‘needs analysis’.

Now the methods you use to deliver these principles is varied and a spicy discussion. One for another time.

Key points to end on:

• Changes in momentum are important for everyone we see, in all treatment settings.

• Changes in momentum are governed by Impulse, or, ‘total force’ that can be applied over a certain amount of time.

• Understanding impulse can guide us on whether we need to focus on muscle efficacy or force expression / movement execution type rehab more.

• By understanding these treatment principles better, equipment such as force plates and dynamometry make much more sense and can become invaluable to us in our decision making.

• Force is everything when it comes to movement and shouldn’t be trumped by ‘functional rehab’ with no consideration to the forces the body can produce.

I hope this has been helpful, I’m thinking next up is probably something around deceleration and change of direction but reach out if you have any other thoughts!

Thanks and until next time,

Jeff