Portable dynamometry has transformed strength testing in sport, rehab, and research. No longer confined to labs or bulky isokinetic machines, practitioners can now assess force output practically anywhere—from the gym floor to the sidelines. But here’s the catch: portability alone doesn’t guarantee quality data. The way the device is set up (fixed vs. unfixed) can make or break your results.
What is portable dynamometry?
Dynamometry has long been a cornerstone of strength assessment. In the 1970s and 1980s, large lab-based isokinetic systems were (and often still are) considered the gold standard for quantifying muscle torque. But these devices have several limitations— they're expensive, immobile, time-consuming, and require extensive training.
Hand-held dynamometers (HHD) gained popularity for their portability and lower cost, but early adoption revealed key limitations, particularly the influence of tester strength and inconsistent positioning during high-force assessments (e.g., knee flexion and knee extension) 1,2.
Portable fixed dynamometers (PFD - like the Hawkin TruStrength) are small, wireless, and versatile tools that measure muscle force. They allow for quick assessments of strength in a range of positions and muscle groups without requiring expensive, lab-based isokinetic systems, and don’t have the limitations of the HHDs.
What you need to understand is that the quality of your data depends heavily on how you use it, and whether the dynamometer is handheld (unfixed), semi-fixed, or fixed (see Figure 1).
Figure 1: Versions of portable dynamometry - A) hand-held/unfixed; B) semi-fixed; C) fixed to rack.
Fixed versus Unfixed: What’s the difference?
Associated with getting accurate data is reducing the biological and technological variability. Biological variability refers to the person being tested and includes factors such as familiarization or learning effects, motivation, internal temperature, etc. However, in the case of unfixed dynamometry, it can also refer to the assessor/tester and how familiar they are with the assessment, their strength, and their instructions. When we use our technology, we assume that the readings will be the same; however, this is not always the case. For example, if we collect at high sample rates, there may be drift, which is a type of technological variability. Other ways technological variability can be affected are by the way you hold or attach the device.
So, which of the three types of dynamometry depicted in Figure 1 do you think would provide the most accurate and reliable information, i.e., minimize technological variability? Let’s break it down…
Unfixed (hand-held) Dynamometry
Hand-held dynamometry is popular among many therapists and clinicians for its ease of use, low cost, minimal training, and assessment efficiency. It offers practitioners a quantitative option compared to the traditional manual muscle testing. Though it offers a lot of benefits, there are limitations to this equipment that should be considered when deciding what the best option is for you. As alluded to previously, this type of assessment is heavily influenced by the examiner's strength (biological variability), especially when it comes to larger muscle groups such as the hamstring complex and knee extensors. Because of this, HHD has been shown to have poorer validity when compared to an isokinetic dynamometer, as compared to other methods (semi-fixed and fixed) 3,4. It has also been reported to have lower inter-rater reliability 1,5,6, which can be problematic for those in team environments where athletes and patients may have several different individuals assessing them. Lastly, unfixed dynamometry has been reported to underestimate strength in stronger individuals and larger muscle groups, likely due to the lack of stability provided by examiners 7-10. This lack of stability can also create issues for getting a stable baseline force and a steady pre-tension before the onset of the assessment. One thing to note is that unfixed hand-held dynamometry is typically used in the measurement of compressive forces.
Semi-fixed Dynamometry
To address concerns around the biological variability associated with unfixed dynamometry (tester strength and position), researchers began experimenting with semi-fixed setups to minimize human tester error and improve the consistency of their data. The most common method that has been investigated is the use of a belt strap 6,11-14. This type of assessment lowers the biological variability that is associated with unfixed/hand-held dynamometry and has been shown to be reliable, as well as have lower error and produce greater strength values compared to unfixed 14. However, this setup can still have a lot of variability given the movement of the chain/tether, holding the device (technological variability). Lastly, semi-fixed dynamometry is typically used for measuring tensile forces.
Fixed Dynamometry
True fixed dynamometry uses external fixtures such as racks and base plates to anchor the device securely. If you have a device that can zero, such as the Hawkin TruStrength, you can eliminate all technological variability by attaching it to the rack and attaching any accessories such as handles, straps, pads, etc., and then zeroing the device. Fixing your dynamometer has also been shown to improve inter-rater reliability 1,5,6 (decrease biological variability), and tends to have stronger agreement with isokinetic dynamometry, i.e., superior validity 3,4. Having your dynamometer fixed allows for a stable surface for the athlete to push or pull against, allowing them to produce maximal forces. Because of the increased stability compared to unfixed and semi-fixed, it is easier for the athlete to hold a steady baseline force and set accurate pre-tension thresholds. Additionally, dynamometers that can be fixed, like the Hawkin TruStrength, have the ability to measure both compressive and tensile forces.
When Unfixed Might Still Be Used
There are circumstances when an unfixed approach might be acceptable:
However, it is crucial to be aware that small changes in testing angles can have a large effect on range of motion and muscle lengths, which in turn can have large effects on force output. Therefore, unfixed setups tend to be less reliable, especially when high forces are involved, and may underestimate strength due to examiner limitations.
Best Practices for Portable Dynamometry
To maximize both the convenience of portability and the quality of your data, consider these best practices:
Conclusion
Portable dynamometry has revolutionized strength testing by bringing it out of the lab and into the field. However, to ensure that the data collected is valid and reliable, fixing your dynamometer is key to producing the most accurate and reliable results. Whether you’re monitoring athletic performance or guiding rehabilitation, reducing tester bias and standardizing your setup will lead to better, more actionable insights. With a system like Hawkin TruStrength, you have the flexibility to deploy the best testing methods in any environment.
Curious how to best set up your Hawkin TruStrength for your chosen assessments? Reach out to our team for a chat!
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