Diurnal variation in trunk kinematics during a typical work shift

Diurnal variation in trunk flexibility, defined as changes in range of motion, has been previously observed and quantified. Prior studies have shown that decreased disk height occurs as the day progresses and allows the spinal ligaments to slacken, resulting in an increase in the range of motion (flexibility) and a possible reduction in the risk of injury. This risk may be accentuated under dynamic motions of the spine. This study was conducted to observe the change in dynamic trunk mobility as a function of time of the day. Trunk motions of 21 men were observed at three specific times of the day using a triaxial electrogoniometer. No variation in trunk range of motion in any of the cardinal planes was observed. However, velocity and acceleration in the sagittal plane showed significant variations, suggesting the reexamination of the "slack ligaments" hypothesis. This study asserts that identifying flexibility by only its static component, range of motion, gives only partial information about the diurnal variation experienced by the spine. Industrial injuries occurring in the early morning hours may be a result of insufficient trunk mobility. This study indicates that risk associated with diurnal variation is far more complex than originally thought.