Strapped for Strength: A Comparison Study of Dynamometry Techniques to Evaluate Knee Extensor Strength in Inclusion Body Myositis.

Abstract

Introduction/aims: Accurate measurement of knee-extensor strength in people with inclusion body myositis (IBM) is vital to track disease progression and provide a standardized outcome for clinical trials. Isokinetic dynamometers are the current gold standard tool for measuring knee-extensor strength. A less costly, more portable tool would have more widespread clinical and research applications. Current practice is to use handheld dynamometry, but there are concerns around the accuracy of this method as it relies on precise operator technique. This study investigates whether stabilization of the handheld dynamometer with a strap (SSHD) improves agreement with the isokinetic dynamometer (IKD) for measurement of knee-extensor strength in IBM participants compared to operator-stabilized handheld dynamometry (OSHD).

Methods: Fifteen IBM participants had bilateral knee-extensor force measured using three methods of dynamometry on the same day: the isokinetic dynamometer, operator-stabilized handheld dynamometry, and strap-stabilized handheld dynamometry. A crossover design was used to account for fatigue bias.

Results: Intraclass correlations (ICC) indicated poor absolute agreement of the IKD with OSHD (Strong leg = 0.240, Weak leg = 0.328), which was better for the SSHD method. Using the SSHD reduced the bias (i.e., there was less underestimation of force) between SSHD and the IKD compared to OSHD and the IKD (p < 0.05), indicating that strap-stabilized handheld dynamometry improved agreement and intraclass correlations with the IKD compared with operator-stabilized dynamometry.

Discussion: Strap-stabilized handheld dynamometry of knee extensor strength is feasible in IBM patients and may correlate better with isokinetic dynamometry than operator-stabilized handheld dynamometry, but larger studies are needed to confirm this finding.