In the past, researchers studying LBP have comprehensively assessed the vertebrae, discs, and intervertebral joints of the back. Nevertheless, the importance of the lumbodorsal muscles in stabilizing the lumbar spine should not be underestimated(28, 29), and this viewpoint was excellently illustrated in a study that provided quantitative data about the stabilizing effects of muscles with regard to the mechanics of the spine(29, 30). Therefore, in this study, we focused on the changes in the back muscles of patients with LBP.
Previous studies have shown that the lumbar muscles often degenerate by different degrees when patients suffer from LBP(31, 32). This degeneration manifests as decreases in the cross-sectional areas (CSAs) of muscles(31, 33–37), increases in fat content(32, 38–40) and changes in the proportions of muscle fiber types (more type II fibers, fewer type I fibers)(41). Although the above structural changes of the paraspinal muscles have been reported frequently in the existing literature, there is no consistent conclusion on the changes in muscle morphology and structure in LBP patients, and many of the results are inconsistent. This inconsistency is probably due to different etiologies leading to different pathological changes, such as fatty infiltration itself, which increases muscle thickness. Moreover, there are many other related factors; for instance, the pathology of the same disease varies at different stages(42). On the other hand, other studies have confirmed that pain is not associated with structural changes in paraspinal muscles(43–47).
Changes in structure may lead to changes in muscle fiber elasticity, which inevitably affect muscle function. Even without changes in muscle structure, pain causes pain-related nerve suppression, and the level of activity of lumbar muscles decreases to prevent tissue damage(48), thereby affecting muscle contractile function. Therefore, it can be assumed that patients with LBP exhibit changes in muscle contractile function to a certain extent, regardless of whether there are morphological structural changes or how the structure changes.
As actin and myosin filaments overlap more during muscle contractions than during relaxation, the muscle generally becomes thicker and shorter(49, 50). Real-time ultrasound can be used to observe dynamic changes in muscle thickness from relaxed to contracted states. The contraction ratio can be calculated according to the muscle thickness in the two different states to quantitatively evaluate muscle contractile function(51, 52).
In the existing literature on lumbodorsal muscles, the posture in which patients were examined differed among studies. Some evaluations were performed in an upright position, while others were performed in a prone or supine position. In the supine position, which is often used for MRI and CT scans, the back muscles are often compressed and deformed due to the patient’s body weight. In the upright position, the human body needs small levels of muscular activity to maintain the pose, which might affect the thicknesses of lumbar muscles(42). Furthermore, in the standing and supine positions, it is difficult to observe and measure changes in lumbodorsal muscles with US. Therefore, the prone position was chosen for our study. With subjects in the prone position, we can ensure that there are no additional contractions of the muscles in the relaxed state, and it is convenient for taking measurements.
Yoga has been widely accepted by clinicians and patients with LBP(18–25). However, yoga contains a variety of postures(18, 53), and we assume that each posture targets different muscles. According to our experience in clinical practice, the locust yoga pose is an effective exercise to relieve LBP. We chose the locust yoga pose as the contracted state because of the following three primary reasons. First, the locust yoga pose is effective in relieving LBP. Second, the action is simple and easy to perform. Third, it is convenient for us to observe and measure muscles during this pose.
In our study, the data objectively showed that the locust yoga pose can be used to exercise the lumbar back muscles, especially the longissimus. Therefore, the movement can be prescribed for patients with LBP. According to the measurements of the thickness of lumbodorsal muscles taken in a uniform manner during the locust pose, the contraction ratio of each muscle (C/R) was calculated to uniformly quantify the contraction function of each muscle.
However, our study has some limitations. First, yoga involves various poses; in this study, we investigated only the locust pose. Second, the sample size was relatively small, so a study with a larger sample size needs to be conducted to determine the normal range of the contraction ratio of each muscle. Third, the subjects of this study were normal people without LBP. Some patients may not be able to perform the exercise mentioned above and therefore may not be eligible for future studies including patients with LBP.