Assessing how daily activities and seated kinematics influence passive lumbar spine stiffness throughout the week

To uncover a link between prolonged sitting and low back disorders, this study assessed how lumbar spine passive stiffness varied with activities and seated spine kinematics. Data were collected from twenty participants performing seated office work throughout a work week. The time spent sitting, standing/stepping, and moderate-vigorous physical activity (MPVA) were derived from a thigh-worn activity monitor. Thorax inclination and lumbar spine flexion and movement were calculated from low back tri-axial accelerometers. Lumbar spine flexion passive stiffness was measured on Monday morning and evening, Tuesday morning, Friday evening, and the following Monday morning. Forward-backward subset regression models were constructed between spine stiffness and activities and seated kinematics, then models were subjected to bootstrapping. The median adjusted R2 ranged from 0.10 to 0.65 for stiffness measures in low to moderate flexion ranges. Sitting time, particularly at work, was associated with earlier tissue engagement and reductions in the low stiffness zone slope, while MVPA demonstrated the opposite effects. More frequent and smaller spine micromovements while seated were associated with smaller changes in passive stiffness. Modifying workplace sitting behaviours and incorporating movement, through MVPA and in sitting, may help mitigate the mechanical changes in the spine that could contribute to low back disorders.