Validity, Agreement and Reliability of the ForceFrame Dynamometer in Patients with Anterior Cruciate Ligament Injury.

Background: Restoring maximal muscle strength of the knee extensors (KE) and knee flexors (KF) following anterior cruciate ligament (ACL) injury and ACL reconstruction is of great importance to reduce the re-injury rate after ACL reconstruction and to reduce the risk of knee osteoarthritis. Therefore, it is essential that clinicians and healthcare providers use valid and reliable measures to assess knee muscle strength to ensure a safe return to sport.

Purpose: To evaluate the reliability (test-retest reliability, inter-tester reliability and test-retest agreement) and validity (concurrent validity, convergent validity and ForceFrame (FF) vs. isokinetic dynamometer (ID) agreement) of the ForceFrame (FF) dynamometer during isometric testing of the knee extensors and flexors.

Study design: Cross-sectional study.

Material and methods: Twenty-seven participants with ACL injury or reconstruction were recruited for participation in this study. maximal voluntary isometric contration (MVIC) of the knee extensors and flexors was tested on two separate days. Day one included validity assessments with FF, a gold-standard ID and a handheld dynamometer (HHD). Day two included reliability assessments with FF performed by two assessors. Main outcome measures were day-to-day test-retest reliability and agreement and inter-tester reliability of FF, and concurrent validity (FF vs. an ID and a HHD). Reliability was tested as test-retest and inter-tester reliability using interclass correlation coefficient (ICC), while agreement was tested using Bland & Altman plots with limits of agreement (LOA), standard error of measurement (SEM) and smallest detectable change (SDC). Concurrent validity between FF, ID, and HHD was assessed using Pearson's correlations and mean difference was evaluated by Bland & Altman plots.

Results: Twenty-seven participants (10 females, 17 males) with a median age of 25 years (range 19-60) were included in this study. There was a good day-to-day test-retest reliability for MVIC of KE (ICC=0.77, CI95:0.48-0.90) and KF (ICC=0.83, CI95:0.61-0.92) and excellent inter-tester reliability for MVIC of KE (ICC=0.97, CI95:0.94-0.98) and KF (ICC=0.93, 95CI:0.85-0.97). Standard error of measurement (SEM) was 8% and 9%, while the smallest detectable change (SDC) was 22% and 27% for KE and KF, respectively. FF showed fair concurrent validity compared to ID for KE (r=0.56), poor concurrent validity for knee flexors (KF (r=0.24) and compared to HHD a moderate correlation for KE (r=0.74) and poor correlation for KF (r=0.12). Bland & Altman plots between FF and the ID showed a mean difference of -0.51 Nm/kg for KE and -0.32Nm/kg for KF.

Conclusions: FF can be used to obtain reliable and valid results to assess MVIC of the KE, but not the KF. It should be noted that absolute results produced by the FF may be considered an underestimation of actual MVIC. The test position to assess KF in FF does not appear to be optimal, and different test-positions may be considered.

Level of evidence: Level 3.