Assessing isometric ankle abductor strength with a novel device: Reliability and electromyographic validation.

Abstract

Assessing ankle abductor (ABD) strength is clinically relevant because the peroneal muscles are key contributors to lateral ankle stability. However, no previous study has examined whether isolated ABD strength can be measured reliably and reflects peroneal muscle function. This study evaluated the intra-session test-retest reliability of a novel device for isometric ABD strength assessment and examined construct validity using surface electromyography (EMG). Two independent samples were included: n = 24 for test-retest reliability and n = 15 for EMG construct validity. Strength testing was performed using a custom-built, examiner-independent device that standardized positioning and stabilization while applying resistance through a load cell. Participants executed maximal isometric ABD efforts. Reliability was assessed using ICC (2,1), SEM, coefficient of variation, and minimal detectable change (MDC₉₅) values. EMG activity of the peroneus longus, peroneus brevis, and tibialis anterior was recorded during ankle ABD to examine muscle-specific activation patterns across force levels (30%, 50%, and 70% of maximal voluntary force). The device showed good-to-excellent reliability, with ICC values from 0.82 to 0.94 and acceptable SEM and MDC₉₅. The most stable results were obtained using the highest value across five or six attempts. EMG analyses demonstrated a clear graded, force-dependent activation pattern in the peroneal muscles, whereas tibialis anterior activity showed greater variability and a less consistent response. These findings indicate that the device provides consistent ABD strength measurements and elicits muscle-specific activation patterns consistent with peroneal function, supporting clinical assessment of lateral ankle stabilizers and guiding rehabilitation.