Validation of a Methodology Using Isometric Dynamometer for Ankle Strength Assessment to Quantify LDH-induced Paresis

Lumbar disc herniation (LDH) is a common condition that may cause lower limb motor weakness (paresis), particularly affecting ankle dorsiflexion and plantarflexion. Although motor deficits are clinically important and may influence treatment decisions, objective and standardized methods for quantifying the severity of paresis are lacking. Current clinical assessment mainly relies on manual muscle testing, which is subjective and has limited sensitivity, while isokinetic dynamometry, the reference standard for muscle strength assessment, is expensive and impractical for routine clinical use.

This prospective validation study aims to evaluate a clinically feasible isometric ankle strength measurement method based on a handheld dynamometer fixed with an external strap to eliminate examiner-dependent resistance. The study will be conducted in two sequential phases. First, validity and reliability of the method will be established in healthy adult volunteers by comparing measurements with those obtained using an isokinetic dynamometer. Second, the method will be applied in patients with LDH-induced lower limb paresis to determine its performance across a broader range of muscle strength values and its clinical applicability.

Outcome measures include maximal isometric ankle dorsiflexion and plantarflexion force, time to peak force, manual muscle testing (Medical Research Council scale), patient-reported weakness using a Numeric Rating Scale, Lower Extremity Functional Scale (LEFS) scores, and functional ankle strength tests. Validity will be assessed using correlation analyses and Bland-Altman agreement analysis, while intra- and inter-rater reliability will be evaluated using intraclass correlation coefficients, coefficient of variation, standard error of measurement, and minimum detectable change.

The study seeks to establish a reliable, valid, and clinically practical method for objective assessment of ankle muscle strength in patients with LDH-induced paresis. Such a method could improve clinical evaluation, enable objective monitoring of recovery, facilitate future clinical research, and support treatment decision-making.