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A comparative study of lumbar paraspinal muscle reflexes during sudden upper limb loading in healthy control subjects and patients with sciatica.To assess reflex activation of paraspinal muscles during sudden upper limb loading.Sudden upper limb loading and upper limb voluntary movements cause reflex activation of trunk muscles. A short latency response of ∼50 msec of lumbar muscles has been observed before, but the reflexes have not been studied in patients with sciatica.The paraspinal muscle responses for upper limb loading during unexpected and expected conditions were measured by surface EMG from 20 patients selected for an operation as a result of disc herniation-related chronic low back pain and 15 back-healthy controls. Pain, disability, and depression scores were recorded.Short latency response of paraspinal muscles for unexpected upper limb loading was similar in healthy controls and patients with sciatica in supported standing. During normal standing anticipation shortened the lumbar reflex latency in healthy controls but not among the patients.The results provide evidence for impaired feed-forward control of lumbar muscles in patients with sciatica.