© Springer International Publishing Switzerland 2015. Lumbar erector spinae muscle has been little explored for knee exoskeleton control through sEMG signals, which could improves the onset and offset of motion. In this study, a simultaneous sEMG record was obtained in three movements routine (knee extension-flexion, stand-up from seated position and seat-down from up position) from five trunk levels, and five muscles of the leg. sEMG signals were smoothed to obtain the envelope through the captured sample entropy method, which can be used to detect the onset/offset through an adaptive threshold value. The onset/offset related to knee motion was obtained on trunk muscles, during knee extension-flexion, stand-up and sitdown activities, which is suitable for knee exoskeleton control.