Assessing quadriceps strength in patellofemoral pain patients: A study on the reliability and validity of a low-cost load-cell for clinical practice

Background Patellofemoral pain (PFP) is a common knee complaint affecting diverse populations both acutely and chronically. Quadriceps muscle weakness is one possible aetiology, but current devices for measuring muscle strength (isokinetic dynamometer and hand-held dynamometers) are frequently too expensive for practitioners, especially in under-resourced settings. There is a need to evaluate a low-cost device to manage rehabilitation of people with PFP. Methods Isometric quadriceps strength of participants aged 18-35 years (total [n = 33], control group [n = 17] and PFP group [n = 16]) were evaluated on an isokinetic dynamometer and a low-cost load cell at baseline and after an 8-week non-standardised intervention for validity scores. Results The load cell showed high absolute and relative reliability (intraclass correlation coefficient = 0.89-0.99; typical error of measurement = 3.9-10.4%). Clinically meaningful difference scores (12.2-45 Nm) were greater than the typical error of measurement, implying sufficient sensitivity of the load cell to measure true changes in isometric quadricep strength. Strong to very strong correlations were evident between the load cell and isokinetic dynamometer torque measurements (r = 0.88-0.90, SEE = 0.05-0.07 Nm), but slope values (β = 0.65-0.77) indicated that torque from the load cell was typically lower than that obtained from the isokinetic dynamometer. An average systematic bias of 16.3-28.8 Nm was evident in favour of the isokinetic dynamometer, with no statistically significant between-group differences being noted between the baseline and follow-up testing. Conclusion The load cell is a reliable instrument, sensitive enough to detect clinically meaningful differences in quadriceps strength in healthy individuals and those with PFP. The load cell lacks validity and cannot replace isokinetic dynamometry. Given the low cost and excellent reliability, the load cell can be a valuable tool to assess quadriceps muscle deficits and track rehabilitation progress in people with PFP.