Assist chairs with simple mechanism to flip up a seat at the front edge to assist sit-to-stand (STS) is well known as commercial products developed from around 1980. The speed of flipping up the seat on the assist chairs would play a key role how to adjust assisting STS in individual cases, but there is almost no discussion how the speed of flipping up the seat affects the benefit of assisting STS. The aim of study is to investigate the optimised speed of flipping up the seat of the assist chairs. As a first step, a motorised assist chair with controllable speed of flipping up the seat was developed and tested with four healthy participants in three seat height conditions; High:520mm, Middle:420mm, and Low:320mm without and with flipping up the seat at maximum speed 30degree/second. Significant main effect of assisting was found on peak vGRF at Low seat height (p<0.005) by Wilcoxon rank test. There was no significant effect at Middle (p=0.126) and High seat height (p=0.507). With assisting at the Low seat height, the reduction of peak vGRF means that the flipping up the seat successfully supports trunk movement and knee function to lift up the hip from the seat. All participants felt the assisting was effective to reduce the hardness of STS at the Low seat height. With further studies to optimise the speed of flipping the seat, the motorised assist chair in this study would provide a proper assist for individual STS movement, with leaving certain part of physical load for keeping muscle functioning.