Optimization of Lumbar Sensory Stimulation Parameters in Office Workers With Chronic Low Back Pain (LSS-OPT)

Phase II is an experimental laboratory study designed to optimize the neurophysiological parameters of lumbar sensory stimulation in office workers with chronic nonspecific low back pain. The primary objective is to determine which combination of vibration frequency, amplitude, and direction produces the most favorable cortical response in the bilateral 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- and post-intervention comparison.

Visit 1: Baseline clinical and sensorimotor characterization During the first visit, participants will undergo clinical and neurophysiological assessments to characterize their baseline clinical and sensorimotor profiles. Clinical assessments will include pain intensity, 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 magnetic stimulation (TMS), with motor-evoked potentials recorded from the lumbar multifidus and erector spinae muscles. Somatosensory cortical and postural responses will be assessed during unstable sitting using functional near-infrared spectroscopy (fNIRS) over the bilateral S1 together with inertial measurement units (IMUs). Participants will also perform a repeated loaded forward-bending task while lumbar multifidus motor-unit behavior is recorded using decomposition electromyography (dEMG) and trunk movement is recorded using IMUs. Finally, lumbar multifidus morphology and contractile response will be assessed using rehabilitative ultrasound imaging (RUSI) at rest and during contralateral arm lifting.

The measurements obtained during Visit 1 are intended to characterize individual baseline sensorimotor function and to permit exploratory analyses of associations between clinical characteristics, corticospinal excitability, cortical responses, postural control, motor-unit behavior, lumbar multifidus contractile response, and responses to lumbar sensory stimulation. These measurements are not intended to serve as pre-intervention values for an acute pre-post comparison with Visit 2.

Visit 2: Randomized within-participant vibration-parameter optimization During the second visit, participants will undergo the vibration-fNIRS parameter-optimization experiment. Participants will lie prone in a relaxed position while an eight-channel fNIRS system records hemodynamic responses over the bilateral primary somatosensory cortex. A customized vibration apparatus will be positioned over the bilateral lumbar multifidus region.

Each participant will receive all eight vibration conditions generated from a 2 × 2 × 2 factorial combination of two vibration frequencies, two vibration amplitudes, and two stimulation directions. The order of the eight conditions will be randomized by computer for each participant to minimize systematic order effects. Randomization therefore applies to the sequence of stimulation conditions within each participant rather than allocation of participants to separate study arms.

Each vibration condition will be applied for 1 minute. A 5-minute passive rest period will be provided between successive vibration conditions to minimize fatigue and potential carryover effects and to allow the hemodynamic response to return toward baseline before the next condition.

The primary neurophysiological outcome will be the baseline-corrected change in oxygenated hemoglobin concentration (ΔHbO) in the bilateral S1 measured using fNIRS during each vibration condition. Secondary fNIRS outcomes will include changes in deoxygenated hemoglobin and total hemoglobin concentrations, peak oxygenated hemoglobin response, time to peak response, and area under the oxygenated hemoglobin response curve.

The effects of vibration frequency, amplitude, and direction, including their interactions, on the primary fNIRS outcome will be evaluated using a linear mixed-effects model with participant treated as a repeated or random effect. Condition order, testing period, baseline response, and preceding condition may be considered to evaluate potential sequence or carryover effects.

The optimal vibration parameters will not be selected solely on the basis of the largest cortical response. Parameter selection will integrate the magnitude and consistency of the bilateral S1 hemodynamic response with predefined criteria related to participant comfort, biomechanical safety, technical stability and performance of the stimulation apparatus, and suitability for subsequent incorporation into a wearable sensory stimulation belt. The selected parameters will subsequently be used in the Phase III randomized controlled trial.