While it is generally recognized that prolonged sitting periods at work can harm the locomotor system, little attention has been paid to the impact of sitting behavior on muscle stiffness. This study investigated the effect of sitting posture and postural activity on lower back muscle stiffness in a controlled experiment in which participants sat at a desk for 4.5 h. Lower back muscle stiffness was measured before and after the sitting period. In addition, continuous recording of kinematic data of the lower back using an eight-camera motion analysis system was applied to quantify sitting posture and the level of postural activity. The results show that the prolonged sitting period led to a significant increase in muscle stiffness. Further, all participants spent a substantial amount of time in a slumped sitting posture, and the level of postural activity varied significantly throughout the 4.5 h sitting period. Those results suggest that the increase in lumbar muscle stiffness is presumably related to the often-preferred slump sitting posture and may help to understand how prolonged sitting periods can increase susceptibility to common pathological conditions such as low back pain. However, the results also leave some uncertainties that need further investigation.
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