At the T6-T9 level, which muscles are primarily functional?

• A. Upper trunk (abdominals)
• B. Lower trunk (abdominals)
• C. Paraspinals
• D. Hip flexors

Answer: (A)

At mid-thoracic levels, trunk control is mainly supported by the upper abdominal muscles. The nerves that exit around T6 to T9 innervate the upper portion of the rectus abdominis and the upper obliques, so these upper abdominal muscles stay functional and can contribute to trunk flexion and stability when the lesion is around T6–T9. Lower trunk abdominal muscles rely on nerves lower in the thoracic or upper lumbar region (T10–T12 and beyond), so they’re not the primary group preserved at this level. The back (paraspinal) muscles are primarily innervated by dorsal rami across multiple levels, and hip flexors depend on lumbar nerves (L1–L3), which are not the ones preserved at a mid-thoracic level. Therefore, the upper trunk (abdominal) muscles are the ones most likely to remain functional at T6–T9.