Preferred and fast speed sit-to-stand and stand-to-sit (STS) tests are prevalent in literature, but biomechanical changes between the different speeds of STS have never been studied. Understanding differences between these STS techniques will better inform experimental design for research assessing functional ability in clinical populations. The purpose of this study was to investigate the effect of different speeds of STS transfers on lower body and trunk kinematics and kinetics in healthy adults. Nineteen healthy middle-aged and older adults participated in this study. Two different speeds of STS were tested: self-selected speed and fast speed (as quickly as possible). Ten Vicon cameras and two AMTI force platforms were used to collect three-dimensional kinematic and kinetic data. During sit-to-stand transfer, peak knee extension velocity and knee extension moment were significantly increased for the fast speed STS as compared to the preferred speed STS. During stand-to-sit transfer, peak knee extension moment and lower back moment were significantly increased while STS time was decreased for the fast speed STS as compared to the preferred speed STS. Our results indicate that the fast speed STS could be more challenging for participants compared to the preferred speed STS evidenced by greater knee and lower back joint movements. Therefore, fast STS tests should be reconsidered when testing middle-aged and older adults with chronic low back pain and knee joint problems.