Creep deformation of the viscoelastic lumbar tissues: Effects of trunk flexion posture, exposure-recovery schedule, and individual flexibility

Abstract

In-vivo human experiments have previously shown significant effects of trunk flexion angle and exposure-recovery schedule on the creep response of the viscoelastic tissues of the lumbar spine. In the current study we explore the effects of trunk flexion, exposure-recovery schedules, and lumbar flexibility on this creep response. Sixteen participants, categorized into two groups based on lumbar flexibility (GROUP: low-flexible, high-flexible), performed four 30-minute protocols consisting of alternating periods of trunk flexion (exposure) and periods of upright standing (recovery). On four different days, combinations of two trunk flexion postures (POSTURE: Max, SubMax) and two exposure-recovery schedules (ER: Long (3 min:6 min), Short (1 min:2 min)) were tested. Trunk flexion–extension motions were performed before/after the 30-minute protocol to capture the changes in peak lumbar flexion angles and changes in the angles of flexion-relaxation in L3/L4 paraspinals that resulted from the protocols. The analysis of lumbar flexibility showed that the low-flexible group had significantly greater changes in the L4 flexion-relaxation angle in the Max posture (Δ2.2°) than in the SubMax posture (Δ1.0°), while the high-flexible group showed no POSTURE effect, indicating a POSTURE × GROUP interaction. In addition, the high-flexible group exhibited greater creep responses in the Short (1:2) condition (Δ2.5°) than in the Long (3:6) condition (Δ0.6°) while the low-flexible group showed no ER effect, denoting an ER × GROUP interaction. Collectively, these results support the complex, multi-dimensional nature of viscoelastic creep responses, and particularly note the impact of an individual’s lumbar flexibility in these responses.