Optimization of Lumbar Sensory Stimulation Parameters in Office Workers With Chronic Low Back Pain

Phase II is an experimental laboratory study designed to optimize the neurophysiologicalparameters of lumbar sensory stimulation in office workers with chronic nonspecific lowback pain. The primary objective is to determine which combination of vibrationfrequency, amplitude, and direction produces the most favorable cortical response in thebilateral primary somatosensory cortex (S1), while meeting predefined safety, comfort,and technical criteria.Participants will complete two laboratory visits separated by approximately 48‐72 hours.The two visits have different purposes and are not intended to constitute a pre‐ andpost‐intervention comparison.Visit 1: Baseline clinical and sensorimotor characterization During the first visit,participants will undergo clinical and neurophysiological assessments to characterizetheir baseline clinical and sensorimotor profiles. Clinical assessments will include painintensity, low back pain‐related disability, work‐related musculoskeletal discomfort,movement‐control testing, tactile acuity, and pressure pain threshold.Corticospinal excitability will be assessed using single‐pulse transcranial magneticstimulation (TMS), with motor‐evoked potentials recorded from the lumbar multifidus anderector spinae muscles. Somatosensory cortical and postural responses will be assessedduring unstable sitting using functional near‐infrared spectroscopy (fNIRS) over thebilateral S1 together with inertial measurement units (IMUs). Participants will alsoperform a repeated loaded forward‐bending task while lumbar multifidus motor‐unitbehavior is recorded using decomposition electromyography (dEMG) and trunk movement isrecorded using IMUs. Finally, lumbar multifidus morphology and contractile response willbe assessed using rehabilitative ultrasound imaging (RUSI) at rest and duringcontralateral arm lifting.The measurements obtained during Visit 1 are intended to characterize individual baselinesensorimotor function and to permit exploratory analyses of associations between clinicalcharacteristics, corticospinal excitability, cortical responses, postural control,motor‐unit behavior, lumbar multifidus contractile response, and responses to lumbarsensory stimulation. These measurements are not intended to serve as pre‐interventionvalues for an acute pre‐post comparison with Visit 2.Visit 2: Randomized within‐participant vibration‐parameter optimization During the secondvisit, participants will undergo the vibration‐fNIRS parameter‐optimization experiment.Participants will lie prone in a relaxed position while an eight‐channel fNIRS systemrecords hemodynamic responses over the bilateral primary somatosensory cortex. Acustomized vibration apparatus will be positioned over the bilateral lumbar multifidusregion.Each participant will receive all eight vibration conditions generated from a 2 × 2 × 2factorial combination of two vibration frequencies, two vibration amplitudes, and twostimulation directions. The order of the eight conditions will be randomized by computerfor each participant to minimize systematic order effects. Randomization thereforeapplies to the sequence of stimulation conditions within each participant rather thanallocation of participants to separate study arms.Each vibration condition will be applied for 1 minute. A 5‐minute passive rest periodwill be provided between successive vibration conditions to minimize fatigue andpotential carryover effects and to allow the hemodynamic response to return towardbaseline before the next condition.The primary neurophysiological outcome will be the baseline‐corrected change inoxygenated hemoglobin concentration (ΔHbO) in the bilateral S1 measured using fNIRSduring each vibration condition. Secondary fNIRS outcomes will include changes indeoxygenated hemoglobin and total hemoglobin concentrations, peak oxygenated hemoglobinresponse, time to peak response, and area under the oxygenated hemoglobin response curve.The effects of vibration frequency, amplitude, and direction, including theirinteractions, on the primary fNIRS outcome will be evaluated using a linear mixed‐effectsmodel with participant treated as a repeated or random effect. Condition order, testingperiod, baseline response, and preceding condition may be considered to evaluatepotential sequence or carryover effects.The optimal vibration parameters will not be selected solely on the basis of the largestcortical response. Parameter selection will integrate the magnitude and consistency ofthe bilateral S1 hemodynamic response with predefined criteria related to participantcomfort, biomechanical safety, technical stability and performance of the stimulationapparatus, and suitability for subsequent incorporation into a wearable sensorystimulation belt. The selected parameters will subsequently be used in the Phase IIIrandomized controlled trial.