At the T9-T12 level, which trunk muscles are functional?

• A. Abdominals and paraspinals
• B. Abdominals only
• C. Paraspinals only
• D. No trunk muscles

Answer: (A)

Trunk control relies on two main muscle groups: the abdominal wall and the paraspinal (erector spinae) muscles, each with its own nerve supply. The abdominal muscles are innervated by thoracoabdominal nerves from roughly T7 to T11 and the subcostal nerve (T12), with some contribution from lower nerves. Because these nerves originate above the T9–T12 level, the abdominal muscles retain function even when a lesion is at T9–T12. The paraspinal muscles are innervated by the dorsal rami of the thoracic nerves, which also provide segmental input that remains functional around this level, supporting trunk extension and posture. Put together, both the abdominal wall and the paraspinal muscles can be functional at T9–T12, giving trunk stability and control.