Uweda [17] described the morphology of the median septum of the soleus muscle in detail. In this study, the median septum was classified into two types: straight and curved. Our new classification is simpler and could be clinically applied using ultrasonography in the future.

In general, veins that accompany arteries are paired veins. Intramuscular veins are usually paired veins. There are more than ten veins in the soleus muscle, and the number and diameter of these veins vary significantly among individuals [4, 6]. In the present study, the presence and diameter of veins were observed at the level of the mid-length of the soleus muscle.

In previous ultrasonographic reports on the diameter of the central soleal vein and its branches, the diameter was defined as the maximum transverse diameter after at least 15 min of rest in the sitting position with the leg, which was 4.5 ± 1.5 mm for the medial branch, 6.3 ± 2.0 mm for the central branch, and 4.6 ± 1.5 mm for the lateral branch [18]. The diameter in the present study differed from Ohgi’s study as we used formalin-fixed cadavers. However, the central branch was the largest in diameter among the three branches of the central soleal vein in both the past ultrasonography results and the present study [18]. In our study, the diameters were 1.91 ± 0.62 mm for the medial branch, 2.71 ± 1.10 mm for the central branch, and 2.00 ± 0.56 mm for the lateral branch.

The central soleal vein was influenced by the morphology of the median septum, with the straight type having significantly fewer branches than the curved type (Table 1). The thickness of the median septum did not differ significantly between the two types. Still, there was a significant difference in the location of the tendon communicating branches. In the curved type, they were widespread, ranging from the proximal to distal areas to the soleus region, including the tendon portion of the distal median septum. In the straight type, communicating branches were not observed in the tendon.

During lower limb exercise, contraction of the lower limb muscles occurs, accompanied by increased intramuscular pressure. In the gastrocnemius muscle, the resting intramuscular pressure of 11 mmHg increases to 23 mmHg during contraction; in the rectus femoris muscle, it increases from 0 mmHg to 15 mmHg, and in the soleus muscle, it increases from 13 mmHg to 87 mmHg [19]. Wickiewicz et al. [20] examined and reported the muscle architecture of the lower limb in an anatomical study. The reported muscle fiber lengths were 4.3 cm for the gastrocnemius, 6.6 cm for the rectus femoris, and 1.9 cm for the soleus muscle. This difference in the change in intramuscular pressure of each muscle is influenced by the length of each muscle fiber, which we believe is inversely proportional to the length of each muscle fiber. Tabira [10] reported how the short myofibers of the soleus muscle are arranged, with the median septum, an intramuscular tendon, observed from the anterior surface of the soleus muscle, and myofibers present to distal locations that cannot be observed on the posterior surface, with the area of only the calcaneal tendon, a joint stop tendon with the gastrocnemius muscle, short relative to the total muscle length. In the present study, the length of tendon tissue only of the calcaneal tendon, which does not contain muscle fibers, was 2.8 ± 1.3 cm. This arrangement of short fibers in the soleus is a structure that can increase intramuscular pressure during muscle contraction and enhance active reflux by the leg muscle pump.

On the other hand, the most common causes of lower limb DVT are impaired venous return (e.g., immobile patients), endothelial injury or dysfunction (e.g., after lower limb fractures), or hypercoagulable states. The soleus is a muscle that acts on the ankle and is therefore susceptible to immobility, such as prolonged bed rest, which can lead to impaired venous return [21,22,23]. In addition, the presence of venous valves was also observed, putting it at high risk for thrombosis. We hypothesized that the venous branches of the median septum in contact with the tendon or aponeurosis were at greater risk of impaired venous return due to muscle immobility than the venous branches that pass between muscle fibers. This is because the muscle-tendon junction or tendon area is stiffer than the muscle area and more susceptible to blood flow effects. Sugama et al. [24] analyzed changes in intramuscular collagen in the soleus muscle of immobilized rats and demonstrated that immobilization leads to increased collagen content, causing stiffness in the muscle fibers. This muscle stiffness, particularly at the muscle-tendon junction, inhibits venous return. These findings indirectly support our hypothesis, suggesting that venous branches in contact with tendons or aponeurosis are at greater risk of impaired venous return due to immobility. However, further studies are needed to directly investigate the impact of muscle immobility on venous branches near the tendon or aponeurosis.

Although future studies should be performed in vivo with blood flow dynamics, the results of the present study highlight the importance of investigating how the morphology of the median septum and surrounding structures influence venous return, particularly in immobilized conditions. Further in vivo research is needed to directly assess the effect of muscle immobility on venous branches near the tendon or aponeurosis, which may have significant implications for understanding the development of DVT.

Illustration highlighting differences in vein structures between straight and curved types of the soleus muscle. These figures highlight the differences between the veins of the two types, as observed in our study. A Soleal veins in the straight type of median septum. B Soleal veins in the curved type of median septum. cBr, central branch; lBr, lateral branch; mBr, medial branch