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

Lumbar disc herniation (LDH) is one of the most common causes of lumbar radiculopathy and may result in lower limb motor weakness due to nerve root compression. While radicular pain often improves with conservative treatment, motor deficits may persist, recover slowly, or require surgical intervention. Despite the clinical importance of paresis, objective quantification of muscle weakness remains challenging because no standardized, practical, and validated measurement method is routinely available for clinical use.

Current clinical assessment primarily relies on manual muscle testing (MMT), which uses an ordinal grading system and is inherently subjective. MMT is limited by examiner experience, strength, and the inability to detect small but clinically meaningful changes in muscle function. Isokinetic dynamometry is widely regarded as the reference standard for objective muscle strength assessment because of its high accuracy and reproducibility. However, the equipment is expensive, non-portable, requires specialized training, and is therefore unsuitable for routine outpatient assessment or repeated follow-up measurements.

This study aims to validate a portable isometric ankle strength measurement method using a handheld dynamometer secured with an external fixation strap, minimizing examiner-related variability. The ultimate goal is to develop a clinically feasible method capable of objectively quantifying ankle dorsiflexion and plantarflexion strength in patients with LDH-induced paresis.

The study is designed as a prospective observational validation study conducted at Helsinki University Hospital. It consists of two consecutive phases.

During the first phase, healthy adult volunteers will undergo repeated ankle strength measurements using both the investigational handheld dynamometer method and an isokinetic dynamometer. This phase will establish criterion validity by comparing the handheld measurements with the reference standard. In addition, intra-rater and inter-rater reliability of the handheld method will be assessed through repeated measurements performed by different examiners.

If adequate validity and reliability are demonstrated, the second phase will recruit patients diagnosed with lumbar disc herniation presenting with clinically verified lower limb paresis. The objective of this phase is to evaluate whether the measurement method performs across the full spectrum of muscle weakness encountered in clinical practice and to determine its clinical relevance.

Primary measurements include maximal isometric ankle dorsiflexion and plantarflexion force and time to peak force. Secondary assessments include manual muscle testing according to the Medical Research Council grading system, patient-reported weakness using an 11-point Numeric Rating Scale, Lower Extremity Functional Scale (LEFS), and functional performance tests consisting of repeated toe raises and heel raises.

Criterion validity will be evaluated by comparing handheld dynamometer measurements with isokinetic dynamometer measurements using Pearson or Spearman correlation coefficients, linear regression analyses, and Bland-Altman plots to assess agreement and systematic bias. Construct validity will be examined by correlating objective strength measurements with patient-reported weakness, LEFS scores, and functional performance tests. Reliability will be assessed using intraclass correlation coefficients for both intra-rater and inter-rater agreement, together with coefficient of variation, standard error of measurement, and minimum detectable change.

Approximately 35 healthy volunteers and 35 patients with LDH-induced paresis will be recruited to allow for participant dropout while maintaining adequate statistical power based on previous validation studies.

The study involves no therapeutic intervention and poses minimal risk. Participants will undergo standardized muscle strength testing and complete patient-reported outcome measures. Any discomfort, such as muscle cramps or transient fatigue during testing, will be documented. Participants demonstrating clinically significant deterioration in muscle strength during the study will be referred for further clinical evaluation according to standard practice.

By establishing a valid, reliable, and practical method for objective ankle strength assessment, this study aims to improve the evaluation and longitudinal monitoring of patients with LDH-induced paresis. The validated methodology may facilitate future clinical trials, improve outcome assessment, enable earlier recognition of neurological recovery or deterioration, and support evidence-based treatment decisions in patients with lumbar disc herniation.