Final answer:
The least variable part of the spinalis muscle is the spinalis thoracis, which is a member of the erector spinae group in the back muscles.
Explanation:
The least variable of the different parts of the spinalis muscle is the spinalis thoracis. The spinalis muscle group, including spinalis capitis, spinalis cervicis, spinalis thoracis, is a part of the erector spinae, which plays a crucial role in extending and laterally flexing the vertebral column. Among the components of the spinalis group, the spinalis thoracis shows the least variability.
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