Validity, Agreement and Reliability of the ForceFrame Dynamometer in Patients with Anterior Cruciate Ligament Injury.
Kamilla Arp, Thomas Frydendal, Troels Kjeldsen and 5 others
PMID 39229449WHAT IT FOUND
The ForceFrame reliably measures knee extension strength in ACL patients but underestimates absolute values compared to the gold standard.
A change of 22-27% is needed to confirm real improvement. It performs poorly for knee flexion, which requires cautious interpretation.
Key findings
01The device showed good day-to-day reliability for knee extension and flexion, and excellent inter-tester reliability, but agreement limits were wide.
02Concurrent validity against the isokinetic dynamometer was moderate for knee extension but poor for knee flexion in the injured leg.
03A change exceeding 22% for knee extension and 27% for knee flexion is required to distinguish true improvement from measurement error.
STILL TO COME
How it was doneWhat they foundWhat it means for PTs
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What it does not show
The study used a tailor-made stabilizing bar for knee extension that is not included in the standard commercial ForceFrame kit, so results may not apply to typical clinical setups. The sample was small (24 completers) and restricted to patients 6-24 months post-injury, limiting generalizability to acute or chronic phases. Test positions differed between devices (ForceFrame and handheld used a bench without hip fixation, while the isokinetic dynamometer used belts and back support), which may explain some validity differences. Knee flexion assessment showed poor validity and correlation, suggesting the tested position was suboptimal.
Declared interests
Funding was provided by the Research Council at Lillebaelt Hospital, Denmark, to purchase the test device and cover participant transportation costs. Authors declared no conflicts of interest.
The easy way to misread this
Do not assume the ForceFrame provides accurate absolute strength values comparable to a laboratory isokinetic dynamometer. It consistently underestimates force, particularly for knee flexion, and changes smaller than 22-27% are likely just measurement noise rather than true clinical improvement.