A histochemical and electromyographic study of the lumbar multifidus muscle in 17 patients with L4-L5 lumbar disk herniation was performed. Electromyography (EMG) was recorded preoperatively with a needle electrode according to Haig's method. Biopsy specimens were obtained intraoperatively from the L5 band of the multifidus muscle on the affected and unaffected sides. Patients with lumbar disk herniation showed atrophy of type 1 and type 2 fibers with structural changes in the multifidus muscle at the involved level. Furthermore, patients with abnormal EMG results had severe muscle atrophy compared with patients with normal EMG results.