As most stroke patients generally spend a lot of time sitting in chairs or lying on bed due to their disability 1,2) , many occasions require them to start walking rapidly and safely from the sitting position; for example, answering the telephone or going to the restroom. In cases of healthy normal subjects, the sit-to-walk (STW) task constitutes fl uid merging of a discrete task (rising from the chair) and a rhythmic task (walking) 3) . Stroke, however, impairs the fl uidity of body movements during the STW task 4) . Because fl uidly merging the sit-to-stand task with the initiation of walking requires a control system to exploit the inertial characteristics of both tasks 3) , most patients with stroke have difficulty executing the STW task [4][5][6][7][8] , even though they can start walking smoothly from the standing position. Therefore, the STW task carries a high risk of fall for patients with stroke 7) . Malouin et al. developed the fl uidity index (FI) as a further specifi c index of the STW movement for assessing fl uidity of the STW task in patients with stroke 4,8) . Because the FI is calculated using horizontal body velocity after seat-off, high FI values indicate successful merging of the sit-to-stand and gait-initiation tasks. As horizontal body velocity in STW is impaired to various degrees after stroke, and because this index enables quantitative evaluation of the fl uidity of STW, FI is the gold standard indicator for STW in not only patients with stroke, but also healthy elderly individuals [9][10][11][12] .