Between-day Reliability Of Measures Indirectly Assessing Human Reticulospinal Tract Properties

INTRODUCTION: Muscular force production is mediated by descending pathways including the reticulospinal tract (RST), which originates in the brainstem’s reticular formation. Though inaccessible in humans, techniques involving voluntary reaction time or evoked responses preceded by startling auditory stimulus (SAS; >110 dB) have been used to indirectly assess RST. However, the between-day reliability of these RST measures remain scarce. PURPOSE: To assess the absolute (coefficient of variation, CV) and relative (intraclass correlation coefficient, ICC) reliability of key measures evaluating human RST function. METHODS: 20 neurologically healthy adults (27 ± 5 years old) completed 3 main identical sessions at a consistent time of day for isometric elbow flexion (EF, n = 10) or dorsiflexion (DF, n = 10). Force and surface electromyography (biceps brachii or tibialis anterior) were recorded to quantify maximum voluntary force (MVF), maximal compound action potential (Mmax, peripheral nerve stimulation) and RST function via three approaches: [1] StartReact protocol (RST gain ratio); the ratio between conditioned (prior SAS) and control [2] cervicomedullary evoked potentials (CMEP ratio, EF) or thoracic evoked potentials (TMEP ratio, DF) and [3] motor evoked potentials (MEP ratio). RESULTS: Measures were not different between test days (p > 0.05). CVs for MVF (EF: 4.8 ± 2.0%; DF: 6.3 ± 3.1%), Mmax (DF: 11.1 ± 7.0%) and RST gain ratio (EF: 7.6 ± 5.6%; DF: 8.6 ± 4.7%) indicated acceptable between-day absolute reliability (< 12%), whilst intermediate scores (12-20%) were evident for Mmax (EF: 17.8 ± 11.3%), SAS-conditioned CMEP or TMEP ratio (EF: 17.5 ± 4.2%; DF: 12.1 ± 5.0%) and MEP ratio (EF: 12.9 ± 6.1%, DF: 13.0 ± 4.1%). Between-day ICC exhibited excellent relative reliability (> 0.90) for MVF (EF: 1.00 ; DF: 0.99) and Mmax (EF: 0.96; DF: 0.95); moderate reliability (0.50 - 0.75) for RST gain ratio (EF: 0.69), CMEP or TMEP ratio (EF: 0.57; DF: 0.60) and MEP ratio (EF: 0.51; DF: 0.66); and poor ICC value (<0.50) for RST gain ratio (DF: 0.34). CONCLUSION: Between-day absolute and relative reliability of key RST function measures varied, careful interpretation is required when considering within-participant changes (i.e. pre to post intervention) or between-group differences in cross-sectional studies. Supported by: Graduate Excellence Programme, Malaysia