Effects of external trunk loads on lumbar spine stability.

Stability of the lumbar spine is an important factor in determining spinal response to sudden loading. Using two different methods, this study evaluated how various trunk load magnitudes and directions affect lumbar spine stability. The first method was a quick release procedure in which effective trunk stiffness and stability were calculated from trunk kinematic response to a resisted-force release. The second method combined trunk muscle EMG data with a biomechanical model to calculate lumbar spine stability. Twelve subjects were tested in trunk flexion, extension, and lateral bending under nine permutations of vertical and horizontal trunk loading. The vertical load values were set at 0, 20, and 40% of the subject's body weight (BW). The horizontal loads were 0, 10, and 20% of BW. Effective spine stability as obtained from quick release experimentation increased significantly (p<0.01) with increased vertical and horizontal loading. It ranged from 785 (S.D.=580) Nm/rad under no-load conditions to 2200 (S.D.=1015) Nm/rad when the maximum horizontal and vertical loads were applied to the trunk simultaneously. Stability of the lumbar spine achieved prior to force release and estimated from the biomechanical model explained approximately 50% of variance in the effective spine stability obtained from quick release trials in extension and lateral bending (0.53<R(2)<0.63). There was no such correlation in flexion trials. It was concluded that lumbar spine stability increased with increased trunk load magnitude to the extent that this load brought about an increase in trunk muscle activation. Indirectly, our data suggest that muscle reflex response to sudden loading can augment the lumbar spine stability level achieved immediately prior to the sudden loading event.