The relationship between transversal abdominis (TrA), trunk stability and diastasis recti abdominis (DRA)
During a drawing-in action, the TrA contracts bilaterally, resulting in an increase in muscle thickness and a decrease in length. This leads to a significant decrease in the cross-sectional area of the abdomen, resulting in a smaller abdominal cavity and increased intra-abdominal pressure [25-26]. The increased intra-abdominal pressure, combined with high tension in the thoracolumbar fascia, enhances the stiffness of the lumbar spine, reducing the translation of the lumbar segment and strengthening trunk stability [35]. DRA affects the integrity of the abdominal wall and an anterior pelvic tilt with or without lumbar hyperlordosis is common in most pregnancy cases. The insertion angle of the pelvic and abdominal muscles can be affected by these changes, hence influencing postural biomechanics. The mechanical changes and functional strength of abdominal muscles decrease result in reduced trunk stability and abdominal wall dysfunction, such as low back pain, incontinence, et al[3,15]. It can be seen that DRA is closely related to decreasing trunk stability. If the deep core stabilization exercise program is implemented based on the above evidence, it may alleviate the symptoms of patients with DRA. Some author [1,3,36]used intensive deep core stabilization exercise for DRA and found that deep core stabilization exercise can significantly reduce in inter-rectus distance. The above results suggest that DRA is related to the decline of trunk stability, and trunk stability exercise can promote the rehabilitation of DRA. The TrA is the most important deep trunk muscle with the most unique structure and function, so there is a theoretical basis as rehabilitation exercise for DRA.
The morphology of transversal abdominis (TrA) force generation and transmission
During the exercise of abdominal drawing-in maneuver, the TrA muscles thicken and shorten while sliding laterally, thus causing elongation of the transversal aponeurosis of the abdomen [25,27].
Jourdan[18,28]found that the lateral muscles of the abdomen were significantly deformed and displaced during the exercise. Although their study was not consistent with our goals of concern and did not explore TrA deformations with abdominal drawing-in maneuver, deep exhalation also activates TrA [37-38] as does abdominal drawing-in maneuver, which actually represents the expiratory phase of abdominal breathing. TrA belongs to the deep muscle group of the lateral wall of the abdomen, and its deformation is also influenced by the displacement of the muscle group. It is reasonable to explain TrA sliding in relation to its muscle group displacement. During the maximum contraction of the lateral muscles, the average radial displacement of the deep muscle is greater than that of the superficial muscle. As the most important part of the deep muscles, TrA also plays an important role in the displacement of the abdominal muscles during maximum exhalation[28]. It can be seen above that, from the spatial and temporal quantification of real-time dynamic MRI and the general imaging measurements, both aspects can indicate that the lateral group muscles, especially the TrA, are obviously sliding laterally after activation. This provides a morphological basis for the generation and transmission of TrA force.
The ventral layer of the linea alba and the anterior rectus sheath consist of the aponeurosis of the external oblique and of the anterior part the internal oblique, and its transverse deformation is obvious, the longitudinal stress is large. It is suggested that the linea alba of ventral layer and the anterior rectus sheath mainly transmit longitudinal force, which has nothing to do with transverse transmission, that is, the contraction of rectus abdominis and external oblique abdominis has no effect on the tension of the linea alba[29]. The dorsal rectus sheath consists chiefly of transverse fibril bundles and the stiffness of the dorsal rectus sheath is significantly greater than that of ventral rectus sheath[7]. Based on the data presented above, it is evident that the dorsal rectus sheath experiences significantly less deformation compared to the ventral rectus sheath[30]. This indicates that the force generated by TrA is primarily transmitted through the dorsal rectus sheath. This conclusion is further supported by the fiber distribution, stiffness, and deformation degree of the dorsal rectus sheath.
The mechanical characteristic of the linea alba which has more transverse fiber and lower compliance and higher stiffness and higher stresses at the infraumbilical part of the female linea alba are conductive to the lateral transfer of force. When TrA contracts, the force generated by TrA is transferred to the linea alba , and the outward contraction force increases the tension of the linea alba. The high tension of the linea alba and dorsal sheath push the rectus abdominis closer to the midline, shortening the transverse diameter of the abdomen and is better for the transfer of force [20,32-33].
Pathological features of linea alba in patients with DRA
Among women without DRA, the semi-curl up resulted in progressively higher stiffness but no distortion in the linea albat. This is likely because the intact linea alba is taut due to the tensile forces generated by the muscles of the lateral abdominal wall. In contrast, among women with DRA the linea alba distorted along its length and did not become stiffer during the semi-curl-up [34], but rather experienced narrowing of the inter rectus distance [20]. These results suggest that there are both morphological and mechanical alterations in the linea alba among women with DRA. In patients with mild DRA, those generated high stiffness in their linea alba during the semi-curl-up task had values similar to those seen in women without DRA.These data suggest that we may be able to predict functional impairments based on a combination of inter rectus distance, linea alba stiffness and/or linea alba distortion [34]. The Contraction of the TrA increased inter-rectus distanc, and no obvious distortion of the linea alba is observed. It can be seen that Contraction of the TrA improved the tension the linea alba [39]. These results suggest that TrA is unable to produce effective force and transfer, resulting in a widened linea alba, reduced tension, and obvious distortion among women with DRA. These are the pathological features of DRA. In rehabilitation exercise, reducing the distortion of the linea alba might be the goal. Since the tension of the linea alba is maintained by the TrA, the main exercise goal for DRA is strengthen the TrA [40].