The sit-to-stand (STS) motion is an activity of daily living which requires significant torque generation and coordinated movement at multiple joints. It is therefore important to consider the whole-body biomechanics when designing an assistive device for STS. In this study, a passive elastic orthotic was developed which provides bilateral knee extension assistance. Initial human experiments were conducted with two subjects under two foot-placement conditions. The human and device kinematics and dynamics were modelled, allowing for the assessment of the biomechanical effects of the device. The assistance resulted in a decrease in the human knee torque as well as changes in whole-body biomechanics, notably an increase in the linear momentum of the upper body and a decrease in the anterior excursion of the center of mass. These results indicate that single-joint assistance at the knee has the potential to both facilitate successful STS and positively alter whole-body biomechanics.