Effect of the Robot-assisted Gait Training Frequency on Balance and Gait in Stroke Patients in Convalescent Rehabilitation Hospitals: A Preliminary Study by Moo-Hong Yoon, Bong-Sik Woo, Yong-Hwa Park, Dae-Hwan Lee, Eung-Sung Kim, Jin-Ook Choi, Jong-Hyeon Lim, Dae-Seong Han, Tae-lim Yoon