Continuing with the theme of assessment, in this post we’re focussing on isokinetics. Quite a provocative title perhaps, but for some reason isokinetics appear to be held as the ultimate assessment method to which everything else should be compared. In this post I’m going to give you a few reasons to consider why isokinetic testing is not the gold standard.
Attention to Detail
Last time we looked at some of the key strategies to reduce measurement error associated with HHD (hand-held dynamometry) testing. From the post, it might look like HHD testing is fought with errors! And indeed there are a multitude of things that we need to consider and practice to enhance our ability to obtain quality and useful data.
But, that doesn’t mean that HHD is less useful than other modes of assessment. We need to have a rigorous attention to detail with any assessment that we’re conducting, whether that be HHD, isokinetics or functional tests like single-leg hop.
What is Isokinetic Testing?
So let’s get to it. “Iso” meaning same (derived from the Greek isos, meaning equal) and “kinetic” referring to motion, in this case speed. Hence the assessment of muscle performance is conducted through a specified range of motion at a set movement speed. The participant applies as much force as possible through the entire range and the machine applies the required resistance to maintain the speed.
Depending on what type of test you’re performing, you might get an output like this (below). These data represent a several concentric – concentric tests of knee extension and knee flexion in ACL-injured pre- (solid line) and 6-months post- (dotted line) ACL reconstruction.
Isokinetic testing is frequently used in sports and research settings to evaluate muscle performance. We can obtain data on indices such as:
- Strength / peak torque through a range of motion (like the above)
- Isometric strength / peak force
- Eccentric torque assessment
- Rate of torque development
- Angle of peak torque
Isokinetic Testing Is Not The Gold Standard
There’s a largely held belief that isokinetic testing is the gold standard to which other assessment methods should be compared. Why is this? Perhaps because of the range of data that we can obtain – see above list. Also I think in part because people view it as more transferable to real life because unlike isometric assessments, it involves movement.
But in my opinion (and it’s not just me) isokinetic testing isn’t the supreme assessment method and everything else is second rate. Isokinetic assessments can involve a great deal more variability, and thus error, by comparison to well-conducted isometric testing, as can be achievable with HHD.
Greater levels of variability means that we need to take more and more measures to obtain a level of measurement sensitivity to detect meaningful changes or differences in performance when we’re assessing the individual.
Measurement Error
We spoke about variability last time – the greater the error or variability of a measurement, the less confident we can be about the score reflecting the person’s true value and the less precise we can be about interpretation of change.
Why is this important? Well, are you using data to inform decisions about rehab? Refer to the literature and we may find recommendations for example that we need a >90% limb-symmetry index for thigh muscle strength to permit return to play in the ACLR athlete (Lynch et al.). To detect this, our assessment methods need to be really precise – capable of detecting a +/-5% (total 10%) difference between intra-individual scores.
Isokinetic Testing Requires Experience
You may have seen my video recently on social media, if not have a quick look below. These are all gallant attempts at isokinetic testing, however, each trial has a series of errors. These errors will all inflate the error of the measurement.
The point here is that there are a lot of technicalities with isokinetic testing and the experimenter needs to be very experienced and practiced. Furthermore, the participant needs to be familiar and practiced. It’s a very strange experience, especially if undergoing testing of eccentric strength and a number practice trials can be required to eliminate a steep learning effect.
Isokinetic vs. Isometric Testing
Let’s say we’re pretty experienced at conducting muscle force assessment and we’re charged with assessing an individual’s performance, which modality do we go for – assuming we have access to both? Let’s also add that this data is being use for RTP and we’re looking for a >90% ILS (I’ve spoken before about the lack of utility of ratio data, but save that for now), we need to be confident we can detect this.
Here’s some data that we’ve got on these assessment methods, that will help guide us.
Okay, what speed of isokinetic test do you want to use? Whilst fasted speeds might approximate better real-life situations, faster speeds involve larger errors. At the slower speeds, like 60°.s-1, you may need to conduct 3 trials of 5 maximal contractions per trial and take the average of the best 3 efforts in order to be able to detect a +/- 5% difference in thigh muscle strength (Gleeson & Mercer). So, that’s 15 maximal contractions and each set will need to be separated by at least 2 minutes to enable recovery. A lengthy protocol and perhaps difficult to also keep the participant motivated.
By comparison some of our data show that well-conducted isometric assessments might only require the average of the 2 best contractions within a single set of 3-5 maximal efforts to achieve a level of measurement precision of +/-5% (Minshull et al). Somewhat quicker.
For me, this is a key reason why isokinetic testing is not the gold standard. But let’s weigh everything up. I’ve summarised the main points below.
Summary
So what’s the take-home? Is isokinetic testing useless? Absolutely not! When well-conducted it can be a really useful mode to evaluate muscle performance. But that’s the key, cost aside, to acquire useful data significant practitioner expertise is required and a detailed knowledge of measurement error, especially if we’re assessing the individual.
By comparison, well-conducted isometric assessments can provide useful strength data, accrued with often fewer number of trials.
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