Quantitative data on vertebral body movement in the body is essential to enhance our understanding of spinal pathology and improve the current surgical treatment of spinal diseases. As the most basic exercise mode, side bending exercise participates in the formation of daily movements, but it is rarely studied separately. At the same time, few studies have investigated the influence of different weights on changes in exercise patterns. In this study, we investigated the lumbar vertebral disc deformation in asymptomatic living subjects when they performed standardized lateral bending under different load-bearing conditions. In general, the measured tensile deformation of the L3-4 and L4-5 discs is very small, ranging from 15% to -40%, and the shear strain range is rather constant from 30–60%. The data demonstrated that during left bending, the upper intervertebral disc (L3-4) had larger ranges of deformation than the lower intervertebral disc (L4-5). The tendency of tensile deformation during left and right bending was approximately symmetric. During the functional bending of the body, there is a greater compression deformation behind the same side of the movement and a higher tension deformation in front of the contralateral movement. Meanwhile, it was not found that the small load had a significant effect on the tensile deformation of the intervertebral disc.
Many studies have investigated the biomechanics of the lumbar spine, including those that described the morphological features of human lumbar discs and examined the range of motion or biomechanical responses of the lumbar spine to external loads. Zhong et al.  Obtained in vivo morphological features of human lumbar discs according to Magnetic resonance images of the lumbar spine of 41 young Chinese. The data showed segment-dependent geometric features of the lumbar IVD. Notably, the disc height and length of L4-5 are significantly larger than the upper lumbar disc. In vitro cadaveric tests have examined the biomechanical responses of the disc to external loads using various mechanical loading equipment setups. Fu et al.  measured the segment-dependent changes in lumbar intervertebral space height during flexion-extension motion in a custom-made mechanical loading equipment set-up. The author found that the changes in disc height at L4/5 were different from those at the L3/4 during flexion-extension motion. The changes in anterior and posterior disc heights were similar at the L4/5 level from neutral to extension, but the changes in anterior disc height were significantly greater than those in posterior disc height at the L3/4.
Pearcy and Tibrewal et al.  investigated the ranges of lateral bending plus the accompanying rotations in the planes other than that of the primary voluntary movements in two groups of normal male volunteers using a three-dimensional radiographic technique. They reported larger bending ranges in the upper segments when compared with the lower levels of the vertebrae, which was basically in line with our results. However, Li et al.  found that the L4-5 had a larger range of left-right bending motion than the L3-4. In their study, an unrestricted lateral bending was performed by all subjects, which is different from our restriction on the position of the pelvis and hips to a certain extent. The differences between the data emphasize the importance of motion patterns when investigating vertebral kinematics.
Finite element studies have also simulated the biomechanical responses of the disc. Wang et al. created three 3D finite element models of the L3-4 disc using MR images. During the weight lifting extension, the L3-4 disc experienced a maximum shear load of about 230 N or 0.34 bodyweight at the flexion position and the maximum compressive load of 1500 N or 2.28 bodyweight at the upright position. Masni-Azian et al.  created a three-dimensional nonlinear finite element model of the L4-L5 functional spinal unit. At a moment of 10 Nm and compression of 500 N, the degenerated IVD obtained the maximum shear strain in the posterolateral area during lateral bending. Wu et al.  investigated in vivo motion of the lumbar spine during a weight-lifting activity. Their data showed that the lower lumbar motion segments L4/5 had larger anterior-posterior and proximal-distal translations than the upper lumbar segments. Considering the magnitude of compression load in different experiments, it explained that small load (10kg) had no significant effect on disc deformation. Moreover, our subjects were mainly lower lumbar discs of young asymptomatic patients.
To our knowledge, few previous studies have reported data on the geometric deformation of the lumbar intervertebral disc during lateral bending. Most of the researches concentrate on the changes of lumbar segments during flexion and extension and the coupled motion patterns. Wang et al.  measured the geometric deformation of lumbar intervertebral discs under in-vivo weightbearing conditions using DRIS. The average maximum tensile deformation was − 21% in compression and 24% in tension, and maximum shear deformation on the disc surface reached 26%. In general, the higher-level discs have higher deformation values. To sustain body weight during standing, lumbar lordosis increases which causes the anterior location of the L3-4 disc to be under tension and the posterior location to be under compression. To balance the tension of the L3-4 (left-anterior tension, right-posterior tension), the left portion of the L4-5 was compressed and the right portion was tensed. The movement patterns during lateral bending can be explained by physiological weightbearing conditions. From the supine to the standing, and then to the bending, the combination of weight and posture causes the diagonal deformation of the lumbar disc, in which the measured tensile deformation of the L3-4 and L4-5 discs range from 15% to -40%, and the shear strain range is rather constant from 30–60% during maximum lateral bending.
Although most of the subjects showed similar patterns of disc deformation, despite our efforts to standardize the experiment, differences between subjects can still be expected and observed. We found that the direction of some disc deformation is opposite to the direction of motion. When considering possible, to maintain postural stability, the vertebral body performs compensatory motion. This may be related to the phenomenon of compensatory scoliosis. [8, 24, 27, 28] In the future, we plan to increase the number of test subjects in order to improve statistical ability, discover more different movement patterns, and conduct statistical induction.
The present study has several limitations. First of all, the sample size is too small, which limits our ability to observe the differences in motion patterns. It also explains why some of the differences found are not statistically significant, and the large SD obtained. Although we revealed the difference in the changes in disc deformation during lateral bending motion under different load-bearing conditions, we could not analyze the effects of age and gender on vertebral kinematics. Also, the assistance provided to exercise is not sufficient. In order to complete the maximum side bending and maintain the posture, the body may need exercise compensation and cannot complete the typical and standardized exercise form. Hence, due to the size limitation of the fluoroscope, we only examined the end-of-motion status of the segment L3-5, which has a high probability of degenerative lumbar disease. We did not check the instantaneous position of the human body during dynamic movement of the vertebral body. Future work should overcome these limitations. Despite these limitations, this study systematically examined the changes in disc deformation during lateral bending motion under different load-bearing conditions.