Kinematic and temporal interactions of the lumbar spine and hip during trunk extension in healthy male subjects.

Kinematic properties of trunk extension are considered sensitive differentiators of movement between asymptomatic and low back pain subjects. The aim of this study was to quantify the continuous interaction of the hip and lumbar spine kinematics and temporal characteristics as a function of direction during the task of trunk bending backwards and returning to the upright position in healthy young subjects. The sagittal hip and lumbar spine kinematics during the extension task were examined in 18 healthy male subjects. Five trials of trunk extension were recorded for each subject and paired t-tests were then used to determine significant differences (P < 0.05) between the mean lumbar and the hip time-normalized kinematic and temporal variables. The data from the full cycle of trunk extension was analyzed with respect to movement initiation, time to reach peak velocity and peak angular displacement during the full cycle of trunk extension. Three distinct phases of movements were identified based on the continuous movement trajectories of velocity and angular displacement in the lumbar spine and hip; that of extension, return and, a terminal overcorrection phase. There were significant differences identified in the respective mean peak angular velocities of the lumbar spine (21.7 +/- 8.6, 37.0 +/- 14.7, 8.3 +/- 5.0 deg/s) when compared with those of hip (14.6 +/- 6.1, 21.7 +/- 8.5, 5.4 +/- 3.5 deg/s) in each of these three phases. The lumbar spine initiated the movement of trunk extension when bending backwards and returning to the upright position significantly early than that of the hip. These results highlight that in normal healthy adults there is the tendency for the lumbar spine to dominate over the hip during the task of backward trunk bending in terms of the amount and velocity of movement. At the end of extension the kinematics of the lumbar spine and hip kinematic are characterized by a terminal overcorrection phase marking the completion of the movement.