<qa>  What is the purpose of the study?Rocuronium block neuromuscular transmission at the neuromuscular junction, causing paralysis of the affected skeletal muscles. There have been several studies reported positive relationship between muscle mass and amount of neuromuscular junction. Until now, dose of rocuronium has been judged by patients weight and height. When the rocuronium was developed, 50 effective dose and 95 effective dose of rocuronium was calculated by patient corrected body weight which used weight and height. However, height and weight do not represent exact muscle mass. The purpose of this study is to find 50 effective dose and 95 effective dose of rocuronium according to muscle mass. Measurement of muscle mass will be obtained using bioelectrical impedence analysis. 