Influence of Posture, Spinal Level, Gender and Muscle Activation on Biomechanical Properties of Lumbar Erector Spinae in Healthy Young Adults

<i>Background and Objectives</i>: This study set out to better understand how posture, spinal level, gender and muscle activation influence the biomechanical properties of the lumbar erector spinae (LES) in healthy young adults. We aimed to measure how these factors influence LES tone, stiffness, and damping using a myotonometry device. <i>Materials and Methods</i>: Thirty healthy young adults (14 males, 16 females; aged 20-25 years) were evaluated at bilateral L3-L5 levels in prone, unsupported sitting, and standing positions, both under relaxed conditions and during submaximal isometric lumbar extension. The myotonometer measured LES tone (Hz), stiffness (N/m), and damping (logarithmic decrement). For each outcome, a mixed-model repeated-measures ANOVA was conducted with Gender as a between-subject factor and Posture, Level, and Action (relaxed vs. contracted) as within-subject factors (Bonferroni-adjusted α = 0.0167). <i>Results</i>: Posture produced the most significant and consistent effects on all properties-stiffness, tone, and damping (<i>p</i> < 0.0167)-with sitting and standing generally increasing stiffness and tone compared to prone, and sitting showing the highest values. Gender significantly impacted stiffness and tone (<i>p</i> < 0.0167), with males showing higher values. Spinal level also significantly influenced damping, stiffness, and tone (all <i>p</i> < 0.0167), with differences more apparent in females. Significant interactions included the influence of Posture × Gender on tone and damping (<i>p</i> < 0.0167), and of Posture × Action on stiffness and tone (<i>p</i> < 0.0167), alongside a strong three-way interaction for Level × Action × Posture across all outcomes, suggesting posture-related responses depend on activation state and spinal level. <i>Conclusions</i>: LES biomechanical properties are strongly affected by posture and further modulated by muscle activation, gender, and spinal level. These results support the creation of posture- and gender-specific reference values and underscore the value of dynamic, posture-specific myotonometer-based assessments for paraspinal muscle evaluation and clinical planning.