Due to its position on the convexity of the anterior/posterior (A/P) curve, the thoracis spinalis narrows the space between the ribs, flattens the ipsilateral kyphosis, & rotates the vertebral bodies contralaterally, toward the concavity of the lateral curve. The left spinalis thoracis activates to help stabilize a right nutation lesion.

Origin: Proximal Attachment: SPs T1-8 (variable) [1p138] [2p519]
Insertion: Distal Attachment: SPs T11-12-L1-2 (sometimes L3) [1p138, 2p519]
Functions:

• Flattens the thoracic curve
• Rotates the vertebral bodies contralaterally, toward the concavity of the lateral curve, in alignment with the upper lumbar spine and sacrum
• Narrows the space between the ribs

Note that the spinalis thoracis ends at the peak of the lumbar curve, L3 [3]p92-93. If it went one vertebra lower, it would create a nutation effect in the lumbar spine; this can be seen as an indication that the principle of nutation/counternutation may be the driving force in development of musculoskeletal function.

References:

1. Kendall, F., E. McCreary, and P. Provance, Muscles Testing and Function. 4th ed. 1993: Lippincott Williams & Wilkins.
2. Oatis, C.A., Kinesiology. The Mechanics and Pathomechanics of Human Movement. 2004: Lippincott Williams & Wilkins.
3. Kapandji, I.A., The Physiology of the Joints. Vol. 3. 1977: Churchill Livingstone.