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Experimental study of muscle changes after lumbar spinal injury.To investigate effects of intervertebral disc and nerve root lesions on cross-sectional area, histology and chemistry of porcine lumbar multifidus.The multifidus cross-sectional area is reduced in acute and chronic low back pain. Although chronic changes are widespread, acute changes at 1 segment are identified within days of injury. It is uncertain whether changes precede or follow injury, or what is the mechanism.The multifidus cross-sectional area was measured in 21 pigs from L1 to S1 with ultrasound before and 3 or 6 days after lesions: incision into L3–L4 disc, medial branch transection of the L3 dorsal ramus, and a sham procedure. Samples from L3 to L5 were studied histologically and chemically.The multifidus cross-sectional area was reduced at L4 ipsilateral to disc lesion but at L4–L6 after nerve lesion. There was no change after sham or on the opposite side. Water and lactate were reduced bilaterally after disc lesion and ipsilateral to nerve lesion. Histology revealed enlargement of adipocytes and clustering of myofibers at multiple levels after disc and nerve lesions.These data resolve the controversy that the multifidus cross-sectional area reduces rapidly after lumbar injury. Changes after disc lesion affect 1 level with a different distribution to denervation. Such changes may be due to disuse following reflex inhibitory mechanisms.