What’s the best way to measure strength?

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I’m asked this question: what’s the best way to measure strength quite a lot and a more specifically what’s the best strength testing position?  Given that these recent posts are focussed on dynamometry, I’ll focus on the specific question of strength testing positions in this post. Clearly any answer here would relate to the muscle group and joint that you’re testing, and there are some positions that are ‘better’ than others.  However, I would argue that the having absolute control over the measurement situation is more important per se than the joint position. Let me explain what I mean and give you some tools.

Strength Testing For Rehab

Muscle strength is a very common objective outcome measure in rehabilitation and conditioning. It’s also an important metric in older populations as it’s associated with falls risk, ADL dependency and all cause mortality (see here).  

A common way to assess strength is through isometric (or static) muscle contractions using a strain gauge, hand-held dynamometer (HHD) or specially-positioned force platforms.  With the arrival of 100s of affordable dynamometers on the market it’s great to see practitioners start to collect strength data as an objective outcome.

Using Data To Inform Decisions

The utility of objective data, like muscle strength, is that it can inform decisions:  

  • Pre- post- strength testing can tell us if our intervention has achieved the desired level of change, or if we need to continue the training.  
  • Side-to-side, or inter-limb strength testing can enable the calculation of the inter-limb symmetry (ILS) index and inform decisions about return to play (RTP)*.  
  • Data may even be obtained to compare to reference, or normative values to inform decisions about a person’s risk of a certain health event. 

*Caution here, in isolation the ILS is not a great marker of RTP readiness. It misses absolute performance and as such an individual’s real strength*

Now this is all great if the data that is used to inform decisions is accurate and reliable.  If it’s not, we can run the risk of making poor decisions – like concluding someone is ready to RTP, when in fact the scores obtained were compromised by measurement error.  Read all about that here.

Which Testing Position?

The position used to test strength can influence accuracy, but rather than focus on that – there are a lot of joints in the body to cover, let’s focus on 3 characteristics of a good testing position and let’s pick a knee extension assessment of quadriceps muscle force as an example. 

SECURE & STABLE

Our testing position needs to yield a valid result; that is to say that the test score is a measure of (in this example) quadriceps force and not of something else.  Selecting a seated testing position and an open-kinetic chain knee extensor test helps to isolate the knee extensor muscles and as such is the most commonly-used configuration. 

Other positions and assessments like an instrumented squat or leg press will involve contributions from other muscle groups and will require a rather blunt calculation based on EMG data to estimate the relative contribution of the quadriceps to the test score. So this isn’t ideal if we just want to know about the quads.

In this seated position, we also need participants to be secured and to minimise extraneous movement.  What does this mean? Well, when the person extends their knee with maximum force they don’t shift around on the seat. Why? Because it’ll compromise the amount of load going through the load cell and result in an inaccurate underestimation of force production. 

Also, and a critical factor is to remove any slack in the measurement system. What does that mean? That any strapping securing the dynamometer to the leg is inextensible and is taught prior to the test. If it’s not, it’ll result in a massive artefact in the data – i.e. an overestimation of force production. Imagine a person kicking out as hard as they can and the first part of that manoeuvre is taking up the slack of the belt – they’ll hit the end hard. See the red line in the figure below, this is called an artefact and we want to avoid this.

There are lots of other things that we could mention here too, like having the load cell – or line of pull, perpendicular to the limb, but let’s leave it there, for now.

Compare these three pictures (screenshots from Instagram videos). All are describing an assessment of the knee extensors. Review the pictures and make your assessments of the good points of stability and what could be improved. You may find it helpful to go back to the previous post and to come back here having read the next bits to get some more info. Look out for the next post, where I’ll give my critique. **Just to say here thank you to all of the individuals for posting these and hats off for taking objective measures**

CONSISTENT

Now you’ve established a great testing position, is it consistent?  And by this I mean within each testing session? Do you have rigorous control of a person’s seating, are they holding on the the sides of the chair on each occasion, or not, is the back of the knee secured on the bench/chair, or is hanging off the end, how about the rest of the body, is that secure? For dynamic testing is the axis of joint rotation in line with the axis of rotation of the machine?  All of these things are important and inconsistencies add error into your measurement.

Have a look at this this isokinetic assessment. My intention here is most definitely not to be critical. It’s fantastic that these guys are testing and I commend them in doing so. But, there are a few key things in this test that could be improved.

See how the back of the knee is unsupported? This should be flat to the chair. Partly because of this, the joint axis of rotation is way off the rotation point of the lever arm (too far forward and too high up). Stop the video at 3 seconds. Do you notice that the ankle cuff isn’t tight? This will cause artefacts in the data – as mentioned above. Thank you to Focus Physio Performance for posting this (post here on instagram. Sterling effort and I hope these pointers are helpful.

REPRODUCIBLE

Finally, is this testing position reproducible, over time? Can you replicate the the exact position on repeated tests? So, if your patient/client comes in for their second assessment after having engaged in their rehab will you be assessing them in the exact same way?  We need to consider all that’s mentioned above, plus  things like joint angle – is it the same? How do you know, did you measure it? The position of the dynamometer or strapping on the leg, is that in the same position too? How do you know?  Instructions, were you as clear and as encouraging on both occasions. Research show us that all of these things influence performance.

What’s the best way to measure strength??

To start with ensure the position: is secure and stable; consistent and reproducible and delivers valid and reliable results.  Once you have these things nailed, then we can start to think about other things perhaps like optimal position for force production (the length-tension relationship, see here), or more ecologically-valid approximations. But until then, let’s master the basics.

When you generate a number, be sure to know what it means.

What Next?

Thanks for reading. If you’re keen to learn more about improving your assessments, check out my new page and be sure to put your name on the waitlist! Oh, and look out for more posts like this coming up soon

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How to Use Dynamometry

in Clinical Practice