DSD is a common cause of disability and pain in the elderly [1]. Previous studies have highlighted the importance of the sagittal spine for pain and patient quality of life [2,3༌4]. This muscle system maintains posture balance, therefore we investigated key muscles involved in lumbar stabilization. Trunk muscles maintain normal arrangement and spine stability [6], and preliminary studies have investigated trunk muscles using histological analyses [21], electromyography [22], ultrasound [23], computed tomography scanning [24] or magnetic resonance imaging [25]. However, trunk muscle strength in DSD patients is an unknown area. Several devices have been developed to assess trunk strength, however the isokinetic dynamometer (IKD) is the gold standard [26]. Trunk flexion and extension force tests are typically performed in the sagittal plane. Isokinetic trunk strength assessments in flexion and extension, using the IsoMed-2000 dynamometer, are highly reliable according to Ralf et al. [14], therefore IKD is ideal for assessing trunk strength in DSD patients.
Our study observed that the DSD group exhibited lower trunk extensor PT, at all three velocities, when compared with the control group, although both groups were undifferentiated in terms of general condition, suggesting the trunk extensor muscle was compromised. Trunk extensor muscle mainly comprises multifidus and erector spinae, which are sensitive to pathological changes [27]. Several studies have shown extensor muscle degeneration in DSD patients; Shafaq et al. demonstrated significant smaller cross-sectional areas of multifidus in patients with degenerative lumbar scoliosis (DLS), when compared to those with degenerative lumbar stenosis (LSS) [21]. Seung-Jae et al. [28] observed that fat infiltration of the multifidus and erector spinae muscles in degenerative lumbar kyphosis patients was significantly higher than in healthy controls. Since back muscle radiological parameters are one of the most valuable indices for predicting back muscle strength [29], it is reasonable to speculate that a decreased size and increased fatty infiltration of trunk extensor muscle, may be associated with decreased trunk extensor isokinetic strength. Additionally, skeletal muscles tend to suffer with ‘disuse atrophy’ with lower activity levels and reduced muscle strength requirements [30]. DSD patients are often reluctant to perform trunk extension and strength training due to back pain, resulting in ‘disuse atrophy’ of trunk extensor muscles, culminating in a decline in muscle fiber recruitment. Thus, the ability to generate muscle strength is reduced, resulting in weakness of the trunk extensor.
Our findings showed that both groups exhibited similar trunk flexor and grip strength, suggesting the trunk extensor muscle is impaired exclusively in DSD patients. Yaji et al. [6] observed that muscular degeneration of the trunk extensor in DLS patients while the muscle strength and volume of the other body parts were normal, indicating that local myopathy rather than total degenerative loss of skeletal muscle which was called sarcopenia. A similar observation was recorded in lumbar degenerative kyphosis (LDK) patients [25]. Our study confirmed and extended the previous studies through muscle strength level, although the cause and effect relationship is still controversial.
Trunk extensor and flexor muscles interact with each other to maintain biomechanical stability of the lumbar spine [31], therefore evaluating the balance of trunk flexor, extensor muscle strength, and F/E ratios is an important evaluation index [32]. Spinal muscle balance is beneficial for F/E ratios < 1 in terms of equilibrium [32]. In this study, the F/E ratio of the control group was 0.9–1.1, which was within the ratio range of a normal population, whereas the F/E ratio in the DSD group, at 30°/s, 60°/s and 120°/s, were 1.1, 1.4 and 1.5, respectively. In addition, we observed higher F/E ratios at 60°/s and 120°/s, when compared with the control group, suggesting an imbalance in trunk flexor and extensor muscle strength in DSD patients. Although the PT of trunk flexor and extensor muscles in the DSD group was always lower than the control group, only extensor PT exhibited significant differences between groups. Therefore, we propose that impairments in trunk extensor muscles causes an imbalance of trunk muscle strength in DSD patients, and this imbalance is identified at speeds of 60°/s and 120°/s (fast) in isokinetic assessments.
Regular training of trunk muscle, and good core strength are important for daily life and physical activities [33]. Therefore, it is important to clarify the impact of trunk muscle strength changes on the quality of life of DSD patients. A previous study revealed that maximal muscle strength was observed in patients with higher body weight [34], therefore we adopted relative PT (N·m·Kg− 1) as the index, while accounting for body weight, to minimize the influence of individual differences on correlation analyses between trunk muscle strength and quality of life of DSD patients.
Our study revealed significant correlations between trunk extensor PT at 60°/s and disability scores, including ODI and RMQ scores. This suggested that decreased strength in the trunk extensor muscle reflected increased disability in DSD patients, consistent with previous reports. Keller et al. [35] observed that the correlation coefficient between trunk extensor PT at 60°/s and ODI was − 0.57 in LBP patients. Kudo et al. [36] suggested that when compared with the sagittal position of spinal and lower limbs, trunk extensor strength was the most reliable index of RDQ scores in the elderly. Seo et al. [34] showed that is negatively correlated with ODI score. A significant relationship was observed between trunk flexor PT at 120°/s and ODI scores in our study. Vieira et al. [37] showed that the strength of abdominal muscles in elderly patients with lumbar osteoarthritis, was directly proportional to their quality of life. Based on current data, we propose that both extensor and flexor muscles of the trunk are important for quality of life, while extensor muscle strength may be more important.
We also found that both trunk extensor and flexor strength predicted physical functioning and role-physical scores. Previous studies have shown that weakness in trunk muscle strength in the elderly, leads to increased fall tendencies, impaired mobility, impaired daily living activities and increased disability [38]. While strong trunk muscles can not only decrease the kyphotic, but also accelerate the recovery of normal physical activities [39]. Thus, trunk muscle strengthening should be considered a specific intervention in preventing spinal deformity.
Our data showed that isokinetic trunk extensor strength at 60°/s and trunk flexor strength at 120°/s predicted patient HQOL. This discrepancy in velocity may be related to variant muscle fiber recruitment, and pathological deterioration in strength at different speeds [15]. We propose that muscle contraction intensity of trunk extensor and flexor muscles coincide with the set compliant resistance of IKD at 60°/s and 120°/s, respectively. Therefore, it better reflects the true level of muscle and its impact on patient quality of life. As we did not evaluate patient muscle contraction intensity using electromyography, further research is required in this area.
Our study had some limitations. Firstly, participants were recruited from a single center, and the sample size was relatively small. However, this was the first study to report isokinetic trunk muscle strength in patients with degenerative spinal deformity, and its association with HQOL. Secondly, DSD is often combined with degenerative discs, endplate degeneration and other degeneration, however these factors were not considered here, and thus may affect some quality of life, potentially introducing some bias to our data. Thirdly, there were fewer males, and primarily scoliosis deformities in our sample. Therefore, in future studies, we will expand sample size and make comparisons between patients of different genders and deformities.