Ways to assess different types of jumps have exploded over the past few years in the MSK rehab - strength and conditioning world! It wasn't that long ago that force plates were pretty much confined to a biomechanics lab; I was recently listening to a podcast episode with Jason Lake where he was talking about extracting raw data from an excel spreadsheet in order to pull out clinically relevant data! I'm sure that might still be the case in a lot of research settings but thankfully things have progressed for us mere clinicians to make life a lot easier!

Companies like VALD are paving the way in easily accessible force data via easy to use software, and jump tests are only some of the things that they do! It might actually be TOO easily accessible as there are genuinely **** tonnes of metrics collected in e.g. a countermovement jump which can leave you feeling a little bit overwhelmed when you first start out. That's not a fault of the ForceDecks (VALD's version of portable force plates and the system I have experience with) however, nor is it a fault of the software. It just is what it is - data.

The system does it's job of providing us with data exceedingly well and any confusion or frustrations we feel when being overwhelmed with the data will be due to our own lack of knowledge. Knowing how and when to apply this data clinically is going to be of vital importance for us as we start to integrate this technology into our practice more.

I've experienced it myself, and continue to do so (it will probably never end!), but what I have figured out I feel is worth sharing both to consolidate my own thoughts and to help out anyone else who might be a bit 'what the f***?!'.

The first thing I want to do is make a clear distinction in the things we are talking about:

1. Jump testing to inform rehabilitation strategies.

2. Jump testing to assess readiness / risk on returning to sport.

Jump testing is currently, in my opinion, very good at number 1. Number 2... meh, I don't know yet, I feel like if someone has massive deficits on a CMJ or SLDJ it's not ideal and they probably shouldn't have progressed to more end stage rehabilitation anyway (SLDJ is probably debatable but still isn't overly biomechanically demanding compared to some sporting movements); and with no screening tests currently demonstrating consistent risk of second injury (for any injury), I find it hard personally to buy into 'injury prevention' as much currently. This of course would change with convincing enough literature.

Regarding number 1 (informing rehab strategies) I just want to start by stating the obvious:

"Rehab needs to be focused on the end goal"

The reason that I mention this, is that I think it could be very easy for us to get 'too into' jump testing for the sake of jump testing. If a high standing vertical jump is part of the end goal then that is probably an exception; but for a lot of people this won't be the case. In these circumstances, rehabbing someone to be able to jump high is likely rubbish rehab.

A lot of people will just want to be able to return to running or hitting the gym after their leg injury (I'm going to try really hard not to make this about the ACL). For this population of people, perhaps jump testing doesn't really factor in massively compared to other rehab tests or strategies. For those who want to return to anything involving high levels of energy storage and release, sprinting, changing direction then jump testing can definitely help a lot more.

Let me explain, all movements have a biomechanical profile attached to them. Changing direction includes extremely high ground reaction forces (up to around 6x bodyweight) that in the deceleration phase will be quite posteriorly slanted and causing a lot of work for the whole leg musculature. Jogging at a casual pace can produce ground reaction forces around 2.5x bodyweight and the direction of the ground reaction force is a lot more vertical (if you don't know what I'm on about then check my previous blogs) which probably is why the calf does the most work (in the mechanical sense) when jogging.

You can probably see where I'm going with this... different end goals have different requirements of them! And for this reason alone, having a blanket battery of tests for everyone even if their original injury is the same, is pretty short sighted.

Once we have an understanding of the biomechanical profiles of the end goal, we then need to assess if objective testing provides us with any useful information relating to this; and that means understanding what it is exactly that the test tells us.

Let's talk about probably the most widely used jump test in the S+C world, countermovement jumps, be that double or single leg. As mentioned, we need to know what this test actually entails before we can translate any information from it to inform clinical reasoning. Here is a very quick and over-simplified breakdown:

• Initial unweighting phase (the countermovement) - this can be done slow or fast. One of the benefits of having a countermovement is that you can assess the eccentric capacities of the legs so it makes sense to do it faster than slower as applying breaks at higher speeds will show us more regarding potential deficits. Suggestions are to have eccentric peak velocity (>1.2m/s for bilateral and >0.6m/s for unilateral)

• Active braking phase. It is suggested that the difference in force between the 'lowest force' and 'force at the lowest point of the countermovement (when you're most dipped)' drives eccentric stiffness and elastic energy storage (energy is essentially the potential to apply force and is stored in the tissues temporarily).

• Concentric phases. Now you make use of the elastic energy and force you have piled into the force plates and countdown to take-off whilst pushing through the legs like your life depends on it!

• Flight and landing phases. We aren't going to really mention these, sorry. They can be important but we should really be calculating jump height from impulse-momentum measures rather than flight time and landing can be so variable / dictated by the prior steps that it's probably a discussion point for another time!

So we could summarise that a CMJ gives us insight into the braking and subsequent propulsive variables that are associated with a jumping movement that is:

a) Relatively low velocity compared to 'higher level activity' (e.g. with sprinting you can be looking at 6-9m/s for non-elite athletes compared to the ~2ish m/s max you will get from the countermovement)

b) Made up of a more vertical resultant GRF.

c) Pretty evenly split in terms of work contributions of the ankle, knee and hip. This isn't the case when it comes to other activities such as decelerating, and is velocity dependent.

Does it sound like it should be a 'return to sport' test for someone who wants to get back to rugby or hockey?

It doesn't to me, but I think it could serve as a good interim test to evaluate whether or not someone is ready for tackling the higher magnitudes and more posteriorly directed forces associated with running and stopping! If they can't demonstrate good symmetry on the CMJ then I really doubt they will be able to cope as well when we get more into the dynamic movements! This comes back to the principles I hold with muscle strength testing: if a muscle group can't produce force in an isolated movement then it lacks efficacy and then why would I trust it to do it in a more dynamic movement?

The person might still be able to do the next movement, but I would theorise that they are self-organising their way out of using that muscle group as much either by altering movement patterns, starting to stop earlier or other context specific strategies to unload. It's the same with CMJ.

If, for example, someone can't match their uninjured leg in deceleration impulse, phase duration or mean eccentric force on single leg CMJ's (assuming they have familiarised to it enough) then I just don't trust they will be able to manage well, or without significant compensations on landing from a horizontal hop, nevermind deceleration steps! And in this scenario maybe I need to give it a bit more time with the current rehab strategy before pushing things forward.

This comes back to seeing any tests as measures rather than targets.

Once they can come up with equal metrics and good speed on a CMJ, for some, it might have served it's purpose specifically within a rehabilitation setting. If the overall goal includes performance to the highest level physically possible for them which includes quick and reliable measurements of power e.t.c. then it probably still has a role! But improvements in overall jump height don't really tell me much about what to do next or what might still be lacking from an overall rehabilitation programme, that is designed around the end goal!

So then what would be next? Here we would move on to tests of horizontal performance including deceleration and change of direction. Unfortunately, portable force plates like ForceDecks can't help us with this as they are a measurement of vertical force only, so we need to branch out to things like timed 5-0-5, qualitative analyses of CoD and deceleration strategies e.t.c.

So, my advice to you is as follows:

• If you're going to test biomechanics, make sure you know the basics of biomechanics. There are tonnes of books, youtube videos, BBC GCSE revision programmes (yes, they really would help!) to help you do this. I've also written blogs on the basics if you haven't checked them out yet!

• Try to understand the biomechanical profile associated with the end rehab goal! Both this point and the last are what I cover in my course 'The Force Awakens' which is specifically for physios, S+C's and other MSK rehabilitators who don't have a clue about this sort of stuff!

• Understand the biomechanical profile and the demands of the test that you are actually applying - how does that relate to the end goal... and if it doesn't, is the information you get from it still useful to help measure where you are throughout the rehab process? If the answer to both of these is no, then don't feel like you have to do it even if it is published somewhere or other people say it's a really good idea; we need to have a strong biomechanical rationale above anything else.

• Be able to let go once it has served its purpose, or at least modify it to make it more biomechanically challenging (e.g. adding mass to a CMJ to test the force production capacities more!)

And finally, jump testing in the rehabilitation world is very new. A lot of the research out there is from an S+C focus which is usually performance based rather than addressing asymmetries which is a common theme in rehabilitation. This requires us to go back to the fundamentals and see if they actually match up with our specific goals! And then you need to add on things that the athletes in performance based studies don't usually have (e.g. trauma / surgery, kinesiophobia, pain, poor self-efficacy, loss of motivation / arousal) - but let's not even get into that with this blog!

Just some quick thoughts, but I thought useful and hopefully can help you out if you're struggling with this yourself.

Til next time!

Jeff