Objective
To investigate the response of back muscle, the flexion moment and the kinematic change of lumbar spine during sudden upper limb loading between expected and unexpected conditions. Method: 23 healthy young subjects (13 male and 10 female, mean age 26.6±3.3) were recruited. We measured the latency of paraspinal muscle contraction (erector spinae and lumbar multifidus), flexion moment of lumbar spine, and kinematic change of lumbar spine during sudden upper limb loading by using surface EMG system, motion analysis system, and force platform. 6 trials with 3 eye opened and 3 eye closed were performed randomly. Results: The latency of paraspinal muscle contractions was significantly slower during eyes closed condition than during eyes opened condition after sudden upper limb loading (p＜0.05). The flexion moment and the flexion change of lumbar spine increased significantly during eyes closed condition compared with eyes opened condition (p＜0.05). Conclusion: The response of paraspinal muscle was significantly slower and the flexion moment and the flexion change of lumbar spine was higher during unexpected condition than during expected condition after sudden upper limb loading. Therefore, the spinal stability is more decreased during unexpected condition than expected condition. (J Korean Acad Rehab Med 2007; 31: 718-724)