Beyond Voluntary Effort: Reliability and Validity of Magnetic Femoral Nerve Stimulation for Assessing Quadriceps Strength in Critically Ill Patient

Intensive care unit-acquired weakness (ICU-AW) is a common complication of critical illness and can negatively affect recovery and long-term physical functioning. Early and reliable assessment of muscle strength is essential for detecting muscle weakness and monitoring changes during an intensive care unit (ICU) stay. However, conventional strength assessments, such as manual muscle testing, require voluntary muscle contractions, making them unsuitable for patients who are not yet cooperative and limiting the possibility of assessing muscle strength during the early stages of critical illness. Magnetic femoral nerve stimulation offers an alternative, non-volitional approach to assessing quadriceps muscle strength. Nevertheless, the reliability and validity of this technique in critically ill patients remain insufficiently established. This prospective methodological study aims to investigate the reliability and convergent validity of quadriceps twitch force measurements obtained using magnetic femoral nerve stimulation in critically ill patients. A total of 33 cooperative adult patients admitted to medical or surgical ICUs will be included. Participants will undergo repeated quadriceps twitch force assessments over two consecutive days, performed by two assessors. Measurements will be conducted using a custom-developed frame designed to facilitate standardized quadriceps strength assessment at the bedside. Both single and paired magnetic stimulation techniques will be evaluated. The primary outcome is the intra-rater reliability of paired twitch force measurements. Secondary outcomes include the inter-rater reliability of paired twitch force measurements, the intra-rater and inter-rater reliability of single twitch force measurements, and the convergent validity of twitch force measurements assessed through their association with maximal voluntary contraction force and potentiated twitch force. Reliability and agreement will be evaluated using intraclass correlation coefficients and complementary measurement error analyses. Establishing these measurement properties could support objective, non-volitional muscle strength assessment in critically ill patients, including those unable to participate in conventional strength testing. This could facilitate early baseline strength assessment, longitudinal monitoring of muscle recovery, and the evaluation of interventions aimed at preventing or treating ICU-AW..

Inclusion Criteria: * Voluntary written informed consent of the participant has been obtained prior to the initiation of any study-related measurements * Adults (≥18 years) * Expected to remain in the ICU for more than an additional 2 days starting from study enrolment * Fully cooperative, as assessed daily during routine clinical care by ICU physiotherapists using five standardized commands ("open/close your eyes", "look at me", "open your mouth and stick out your tongue", "nod your head", and "raise your eyebrows when I have counted to 5") * Quadriceps muscle strength of at least 3/5 on the Medical Research Council (MRC) scale for knee extension. Exclusion Criteria: 1. Medical instability A: Cardiorespiratory instability * Mean arterial pressure \< 60 mmHg or \> 110 mmHg * Fraction of inspired oxygen (FiO₂) ≥ 60% * PaO₂/FiO₂ ratio \< 200 * Respiratory rate \> 30 breaths·min-¹ B: Neurological instability * Intracranial pressure \> 20 mmHg * Cerebral perfusion pressure \< 70 mmHg C: Surgery within the past 24h D: Body temperature \> 40 2. Contraindications to magnetic stimulation A: Implanted metal or electronic devices, including * Lower limb prostheses * Internal and external fixation devices of the lower limb * Vascular stents * Cardiac pacemakers or defibrillators * Implanted nerve stimulators B: Lower limb structural, skin, or soft tissue conditions * Acute or chronic injuries * Wounds preventing supine positioning * Lower limb amputation C: Neuromuscular conditions * Pre-existing central or peripheral neuromuscular disorders * Neuromuscular blocking agents in the preceding 48h D: Morbid obesity (body mass index \>35 kg/m2) E: Pregnancy 3. Palliative care