Spino-pelvic-rhythm with forward trunk bending in normal subjects without low back pain
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- Hasebe, K., Sairyo, K., Hada, Y. et al. Eur J Orthop Surg Traumatol (2014) 24: 193. doi:10.1007/s00590-013-1303-1
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A strong correlation between low back pain and tight hamstrings has been reported. However, the effect of tight hamstrings on spinal biomechanics remains unclear. The purpose of the study was to investigate spino-pelvic-rhythm during forward bending of the trunk and to clarify the rhythm features with regard to hamstrings tightness. Eighteen healthy male adults with no history of low back pain volunteered to participate. First, we measured the finger-to-floor distance (FFD) in the upright position and set this parameter to 100 %. Using a spinal mouse, spinal alignment was measured in the following four positions: (1) upright posture—100 % FFD; (2) forward bending—50 % FFD; (3) forward bending—25 % FFD; and (4) forward bending—0 % FFD (fingers in contact with the floor). Changes of the angle of the thoracic and lumbar spine as well as the pelvis were calculated. As an indicator of tight hamstrings, we measured straight leg raising (SLR) angle. From positions 1–2 (phase I), the entire spino-pelvic angle moved in 104°. During this phase, the lumbar spine mainly moved. In the second phase (positions 2–3), it moved in 16°. Interestingly, all but 2 subjects showed a negative angle in the thoracic motion, meaning that the thoracic spine extended 4° during trunk flexion, thus exhibiting paradoxical motion. During this phase, lumbopelvic rhythm showed 2 patterns. In 7 subjects, pelvic motion was greater than lumbar motion, while the remaining subjects showed the opposite. In subjects without tight hamstrings, 83 % showed a pelvis-dominant pattern. Only 7 subjects were capable of position 4. During this phase, only slight motion was noted in the spine, and the majority of the motion occurred in the pelvis. Lumbar and pelvic motion correlated negatively in all phases. SLR angle and pelvic motion correlated strongly during phase III, indicating dominant pelvic movement in flexible subjects. The lumbo-pelvic-rhythm comprises 2 patterns—lumbar dominant and pelvis dominant. In flexible subjects, pelvis movement was dominant. In conclusion, improving tight hamstrings may reduce lumbar loading thereby reducing low back pain.
Understanding the range of motion of the lumbar spine is important for understanding low back pain [1–5]. Lumbo-pelvic-rhythm is another aspect for understanding spinal kinematics and spinal motion [6–8]. It was reported  that the lumbopelvic complex has a range of motion of 110°: 40°in the lumbar spine and 70° in the hip joint. During forward bending of the trunk, lumbar spine movement is dominant during the initial phase; lumbar spine movement and pelvic movement are similar during the middle phase; and pelvic movement is dominant during the ending phase . Esola et al.  measured lumbopelvic rhythm in healthy subjects and in subjects with a history of low back pain. In the back pain group, the lumbar segment moved more than the pelvis during forward bending of the trunk, suggesting that greater lumbar motion can induce overloading of the lumbar spine and consequently low back pain.
Tight hamstrings and low back pain correlate strongly with each other [6, 8–10]. The hamstrings attach to the pelvis and the lower leg; thus, it is not difficult to consider that tightness of the hamstrings affects pelvis, hip, and knee joint motion. Furthermore, hamstrings of subjects with a past history of low back pain are tight [6, 8], and tight hamstrings can restrict hip movement thereby increasing lumbar spine motion [11, 12]. This increased lumbar motion during trunk movement can cause loading of the lumbar spine and can be considered the pathomechanism of low back pain due to tight hamstrings.
We therefore hypothesized the existence of a specific spino-pelvic-rhythm in relation to tight hamstrings. As a pilot study, we measured spino-pelvic-rhythm during forward bending of the trunk in subjects with various degrees of hamstrings tightness.
Eighteen healthy male adults aged 35.6 ± 3.7 (range, 21–47) years old without low back pain volunteered to participate. Mean height was 170.8 ± 2.1 cm, and mean body weight was 67.0 ± 3.6 kg. After we explained the entire procedure of the protocol, all participants provided written informed consent. This study was approved by our institutional review board.
Changes in the angle of the thoracic and lumbar spine as well as the pelvis were calculated for each step. The change from positions 1 to 2 was designated as phase I, from positions 2 to 3 as phase II, and from positions 3 to 4 as phase III. A kyphotic angle was positive, and a lordotic angle was negative. In each phase, the ratio of lumbar motion to pelvic motion was measured to understand the lumbo-pelvic-rhythm. When the ratio was >1.0, the motion was considered lumbar dominant. Conversely, when the ratio was <1.0, the motion was considered pelvis dominant.
As a marker of tight hamstrings, we measured the straight leg raising (SLR) angle according to the method of Kutsuna and Watanabe . The SLR angle was measured twice per side, and the mean value was used. Kutsuna and Watanabe  measured the SLR angle in about 1,000 Japanese subjects and reported a mean value of approximately 79°. Thus, subjects with a SLR angle <79° were regarded as having tight hamstrings.
The Tukey–Kramer HSD method was used to assess differences between thoracic, lumbar, and pelvic angles in each phase. Correlations among the three angles and the SLR angle were evaluated using Pearson’s regression analysis. The chi-squared test was used to analyze flexibility and spino-pelvic-rhythm. All statistical analysis was performed using JMP version 9.0 (SAS Institute, Cary, NC). A p value <0.05 was considered significant.
Of the 18 subjects, all completed positions 1 and 2; 17 completed position 3; and only 7 completed position 4. The SLR angle revealed that the majority of those who completed the final step had flexible hamstrings.
As for the global pattern of the spinopelvic rhythm, the present data are in good agreement with the previous reports. However, the absolute motion angle was not the same. The lumbar-hip ratio for each phase was 4.0, 1.0, and 0.4 for phases I, II, and III, respectively (Fig. 2). These ratios differed from the findings of Esola et al.  who reported 1.9, 0.9, and 0.4, respectively, and may be due to differences in the type of machine used for evaluating movement: They used a three-dimensional optoelectric motion analysis system, whereas we used a spinal mouse. Mannion et al.  found excellent reliability of the spinal mouse. Reliability was also evaluated by comparing radiographs , and it was concluded that the spinal mouse is a useful device for in vivo and noninvasive measurement of spinal curvature. On the other hand, a three-dimensional analysis system with skin surface markers is not suitable for investigating spinal curvature, since skin surface marker is likely to move with spinal motion. Thus, a spinal mouse is preferable for obtaining reliable spinal motion data.
Thoracic paradoxical motion
During phase I, the thoracic spine, lumbar spine, and pelvis moved 28.7 ± 6.7, 60.5 ± 4.0, and 17.2 ± 2.9°, respectively. To achieve forward bending to the 50 % FFD position, all three segments moved positively. On the other hand, during phases II and III, the thoracic spine showed −5.0 ± 2.6 and −5.6 ± 9.2 degree, respectively. This negative motion during phases II and III indicated that the thoracic spine extended during trunk flexion, in other words, paradoxical motion. During the initial phase, the pelvis moves only slightly; thus, the thoracic spine needs to flex to achieve bending. During phases II and III, the pelvis rotated further. In such a situation, to complete the task, a subject should move their arm perpendicularly toward the floor (Fig. 1), which means that the shoulder joint must extend. Edmondston et al.  found that the thoracic spine extends in line with shoulder extension. The paradoxical rhythm observed in the present study may be due to this coupling motion with shoulder joint extension.
We also examined the relationship between paradoxical motion and flexibility. During phase III, a significant negative relationship was observed (Fig. 7). When the SLR angle is large with flexible hamstrings, a greater paradoxical angle is observed, suggesting that flexible hamstrings produce greater pelvic rotation. In such a situation, the thoracic and lumbar spines do not need to bend as much; thus, the thoracic spine shows greater paradoxical motion and less lumbar flexion.
Correlation of lumbar motion and pelvic motion
During forward bending, lumbar and pelvic motion showed significant negative correlations in all three phases (Fig. 6). Sugawara  reported similar results—subjects with greater pelvic motion show less motion in the lumbar spine. Another previous study revealed that subjects with a history of low back pain exhibited greater motion of the lumbar spine  and less motion of the pelvis . Thus, low back pain can be reduced by ensuring greater pelvic rotation.
In this paper, we focused on the relationship between flexibility of the hamstrings and pelvic motion. We defined tight hamstrings as having an SLR angle of 79° based on the findings of Kutsuna and Watanabe . Phase II revealed two types of spino-pelvic-rhythm—lumbar dominant and pelvis dominant. The majority of the subjects (83.3 %) showed pelvis-dominant rhythm during phase II. Furthermore, during phase III, we noted a strong correlation between SLR angle and pelvic movement (Fig. 8). These findings support the hypothesis that subjects without tight hamstrings have a high pelvic rotation angle.
Patients with low back pain often have tight hamstrings [6, 9, 10] and poor pelvic motion . Thus, it is logical that overcoming hamstrings tightness and improving flexibility will ensure greater mobility of the pelvis, leading to reduced lumbar motion and mechanical loading during the trunk motion. The previous studies (6, 8, 18) indicated that during the trunk forward bending motion, lumbar angle and pelvic angle showed the significant negative correlation each other. These studies support our statement. From the standpoint of the tight hamstrings, it has been reported that  the pelvic forward bending angle increases after obtaining flexible hamstrings by active stretch and concluded that the flexibility of hamstrings would decrease the mechanical stress of the back during activity. This report is also in good agreement with our statement.
Active stretching is effective for reducing hamstrings tightness, since this technique efficiently utilizes muscle reciprocal inhibition [19, 20]. Sairyo et al.  demonstrated jack-knife stretching as an effective form of active stretching. They reported that jack-knife stretching for 4 weeks improved the mean FFD of 8 healthy subjects from 14.1 ± 6.1 to −8.1 ± 3.7 cm, indicating a gain of 22 cm. If the spino-pelvic-rhythm with tight hamstrings is changed from lumbar dominant to pelvis dominant by improving flexibility through effective stretching, our hypothesis will be proved. We have since initiated research along this line.
In this study, we define the tight hamstrings as the SLR angle being <79°, since the mean value in our nation is 79°. Finger-to-floor distance would be another parameter indicating tight hamstrings. In our study design, only flexible subjects could complete phase 3. To complete the task, one must touch the floor meaning FFD to be a negative value. Only 2 subjects out of 12 (SLR < 79) could touch the floor. On the other hand, 5 out of 6 (SLR more than 79) could touch the floor. Thus, it seems that the value of 79° in SLR angle may be correlated with tightness of the hamstrings evaluated by FFD value. However, there is no definition of the tight hamstrings in the literature. Low back pain and tight hamstrings are reported to be in the vicious cycle each other. We need to establish the definition with clinical relationship with low back pain.
The present findings suggest that achieving pelvis-dominant movement through overcoming tight hamstrings will reduce lumbar motion and consequently improve low back pain.
Conflict of interest