KIN 322 Kinesiology · Unit 1: Foundations and the axial skeleton · on Exam 1
1.6 Trunk muscles, posture and spinal cord injury
Study notes for Trunk muscles, posture and spinal cord injury in KIN 322 Kinesiology at Montana State University: the big ideas, a worked example, common mistakes and an exam tip. The same topic has endless practice questions and flashcards in Mathub.
Big ideas
- Abdominals: rectus abdominis (trunk flexion, posterior pelvic tilt), external oblique (rotation to the opposite side), internal oblique (rotation to the same side) and transversus abdominis (compresses the abdomen and stiffens the spine without moving it).
- Back and neck: erector spinae (iliocostalis, longissimus, spinalis: extension and lateral flexion), transversospinalis (multifidus for segmental stability, rotatores, semispinalis), quadratus lumborum (lateral flexion, hip hiking), sternocleidomastoid (both sides: neck flexion; one side: lateral flexion toward and rotation away), scalenes and splenius muscles.
- Deep “core” muscles (transversus abdominis, multifidus, diaphragm, pelvic floor) stiffen the spine; superficial muscles move it. Posture deviations: excess kyphosis, excess lordosis (often with anterior pelvic tilt), flat back, forward head and scoliosis (a lateral curve with vertebral rotation).
- The spinal cord ends near L1–L2 (conus medullaris); below it is the cauda equina. There are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. C1–C7 exit above their vertebra, C8 below C7, and the rest below theirs.
- Spinal cord injury: the level and completeness (ASIA A–E) set what function remains. Injuries at C3–C5 threaten breathing (“C3, 4, 5 keeps the diaphragm alive”). Cervical injuries cause tetraplegia; thoracic or lower, paraplegia. Key muscles: C5 elbow flexors, C6 wrist extensors, C7 elbow extensors, C8 finger flexors, T1 little-finger abductors; L2 hip flexors, L3 knee extensors, L4 ankle dorsiflexors, L5 long toe extensors, S1 ankle plantarflexors.
Worked example
After a complete spinal cord injury with C6 as the lowest normal level, can the person flex the elbows, extend the wrists and extend the elbows?
Show the solution
C5 (elbow flexors) and C6 (wrist extensors) work, so yes to elbow flexion and wrist extension. C7 (triceps, elbow extension) is below the injury, so elbow extension is lost. People with this level often use a tenodesis grip: extending the wrist pulls the fingers closed passively.
Common mistakes
- Getting sternocleidomastoid rotation backwards: the right SCM turns the face to the left.
- Crediting transversus abdominis with trunk flexion. It compresses and stabilizes.
- Assuming the spinal cord runs the whole length of the canal. It ends near L1–L2.
Exam tipLearn the key muscles as a ladder from C5 to S1. Many spinal cord injury questions are “what can this person still do?”
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