Fixed vs. Unfixed Dynamometry Explained

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: 

• Field Screens: Quick assessments on the sidelines where time is a premium. It is important to note, however, that many portable dynamometers do come with attachments that allow them to be fixed out in the field.  

• Low-Force Populations: Testing with youth or in post-operative rehabilitation, where the force produced is relatively low. 

• Space Constraints: Environments in whereby setting up a fixed system is challenging. 

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: 

• Anchor When Possible: Use rack mounts, base plates, brackets, etc., to anchor your device.  

• Standardize Positioning: Maintain consistent joint angles and limb positions for every test (this is easier to standardize when fixed). 

• Use “Make” Tests: Have the athlete push against a stable, fixed resistance rather than resisting a force applied by the tester. 

• Ensure the Tester’s Capability: When unfixed assessments are necessary, confirm that the tester is sufficiently strong and trained to stabilize the device.  However, be cautious when having multiple assessors, as there may be variability within the readings (inter-rater variability)  

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! 

References 

1. Wikholm JB, Bohannon RW. Hand-held dynamometer measurements: tester strength makes a difference. Journal of Orthopaedic & Sports Physical Therapy. 1991;13(4):191-198.
2. Stone CA, Nolan B, Lawlor PG, Kenny RA. Hand-held dynamometry: tester strength is paramount, even in frail populations. Journal of Rehabilitation Medicine. 2011;43(9):808-811.
3. Kim WK, Kim D-K, Seo KM, Kang SH. Reliability and validity of isometric knee extensor strength test with hand-held dynamometer depending on its fixation: a pilot study. Annals of Rehabilitation Medicine. 2014;38(1):84-93.
4. Toonstra J, Mattacola CG. Test-retest reliability and validity of isometric knee-flexion and-extension measurement using 3 methods of assessing muscle strength. Journal of Sport Rehabilitation. 2013;22(1)
5. Beshay N, Lam PH, Murrell GAC. Assessing the reliability of shoulder strength measurement: hand-held versus fixed dynamometry. Shoulder & Elbow. 2011;3(4):244-251.
6. Thorborg K, Bandholm T, Hölmich P. Hip-and knee-strength assessments using a hand-held dynamometer with external belt-fixation are inter-tester reliable. Knee Surgery, Sports Traumatology, Arthroscopy. 2013;21:550-555.
7. Bohannon RW. Hand-held dynamometry: A practicable alternative for obtaining objective measures of muscle strength. Isokinetics and Exercise Science. 2012;20(4):301-315.
8. Deones VL, Wiley SC, Worrell T. Assessment of quadriceps muscle performance by a hand-held dynamometer and an isokinetic dynamometer. Journal of Orthopaedic & Sports Physical Therapy. 1994;20(6):296-301.
9. Lipovšek T, Kacin A, Puh U. Reliability and validity of hand-held dynamometry for assessing lower limb muscle strength. Isokinetics and Exercise Science. 2022;30(3):231-240.
10. Lu Y-M, Lin J-H, Hsiao S-F, Liu M-F, Chen S-M, Lue Y-J. The relative and absolute reliability of leg muscle strength testing by a handheld dynamometer. The Journal of Strength & Conditioning Research. 2011;25(4):1065-1071.
11. Katoh M, Yamasaki H. Test-retest reliability of isometric leg muscle strength measurements made using a hand-held dynamometer restrained by a belt: comparisons during and between sessions. Journal of Physical Therapy Science. 2009;21(3):239-243.
12. Katoh M, Yamasaki H. Comparison of reliability of isometric leg muscle strength measurements made using a hand-held dynamometer with and without a restraining belt. Journal of Physical Therapy Science. 2009;21(1):37-42.
13. Martins J, Da Silva JR, Da Silva MRB, Bevilaqua-Grossi D. Reliability and validity of the belt-stabilized handheld dynamometer in hip-and knee-strength tests. Journal of Athletic Training. 2017;52(9):809-819.
14. Florencio LL, Martins J, da Silva MRB, da Silva JR, Bellizzi GL, Bevilaqua-Grossi D. Knee and hip strength measurements obtained by a hand-held dynamometer stabilized by a belt and an examiner demonstrate parallel reliability but not agreement. Physical Therapy in Sport. 2019;38:115-122.