Background and Aim: A push-off event is a significant gait phase that contributes to initiating the swing leg and propelling the body forward. This study aims to explore a method for estimating push-off initiation, focusing on pelvic and saddle-seat motion peculiar to saddle-seat-type body-weight-supported (BWS) treadmill walking. Methods: The pelvic/saddle-seat rotation and ground reaction force during saddle-seat-type BWS treadmill walking were measured using an IMU sensor and a force plate in the treadmill, respectively. The hypothesis posits that the transverse rotational displacement of the pelvis and saddle seat aids in estimating push-off initiation. Results: The analysis results show that (1) the timing estimated from transverse pelvic rotation moderately agreed with push-off initiation timing (Spearman's rank correlation coefficient $\rho=0.59, \mathrm{p} < 0.01)$, and (2) the timing estimated from transverse saddle rotation strongly agreed with push-off initiation timing $(\rho=0.67,\mathrm{p} < 0.01)$. Conclusions: The utilization of a single IMU sensor requires a minimal setup for identifying push-off initiation, which contributes to reducing time, effort, and costs in clinical settings. This preliminary study using intact subjects should be a meaningful first step in the research for gait rehabilitation with saddle-seat-type BWS.