Investigation of a Low-cost Portable Inline Dynamometer for Measuring Isometric Knee Extensor Strength.
Amantej Sran, Elira Leake, Sydney Yott and 3 others
On average a low-cost inline dynamometer reads knee strength within 0.1 Nm/kg of a lab-based one, but a single patient's reading can be off by about 25% of their peak torque.
Don't base a return-to-sport call on one number from the cheap device.
Key findings
1At 45° and 90° of knee flexion the average (bias) difference between the low-cost Tindeq inline sensor and the lab-based Humac was less than 0.1 Nm/kg, but the range within which 95% of individual readings fall (limits of agreement) was roughly ±0.75 Nm/kg, about 25% of a typical person's peak torque.
2At 15° of knee flexion both inline devices (Tindeq and MLP) read 0.16 Nm/kg higher than the Humac, a statistically significant difference (p < 0.01).
3The two inline force sensors (wireless Tindeq and wired MLP) agreed almost perfectly with each other (ICC 0.94–1.00), showing the measurement error comes from the inline setup itself—straps, chain, lever-arm measurement—not from the quality or cost of the force sensor.
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What it does not show
Only 14 healthy young adults; the study was underpowered (58% power) to detect a 0.2 Nm/kg bias, which is the difference reported between operated and non-operated limbs after ACL surgery. No reliability or repeatability testing: each person visited once, in a fixed order, with one researcher. You cannot tell from this paper how much the reading changes day-to-day or between therapists. Only healthy participants were tested. A previous study found wider limits of agreement in the uninvolved limb of ACL-reconstructed patients, so the error in a real rehab population may be different. Voluntary activation was confirmed only at 90°, not at 45° or 15°, so effort differences at those angles cannot be ruled out. The inline setup required the patient to pull slightly to remove chain slack at 45° and 15°, which may have inflated those readings relative to the Humac. The study was conducted in a controlled lab with the same chair and straps for both systems; in a busy clinic or at home, strap placement and lever-arm measurement will be less precise, and the error will likely be larger.
Declared interests
No funding source or conflicts-of-interest statement appears in the supplied text.
The easy way to misread this
Do not read the small average bias (<0.1 Nm/kg) as permission to treat a single inline reading as equivalent to a lab-based measurement. The limits of agreement of roughly ±0.75 Nm/kg mean that for any one patient the cheap device could be a quarter of their peak torque high or low, and a 10% limb asymmetry you are trying to catch may fall entirely within that noise. The authors themselves say clinical decisions should be based on multiple consistent readings alongside other functional tests, not on one number.
Summarised by AI from the full paper, without a clinician reviewing it. Check it against the source before it changes what you do. Read it on PubMed →