  We present a personal mobility device for lower-body impaired users through a
light-weighted exoskeleton on wheels. On its core, a novel passive exoskeleton
provides postural transition leveraging natural body postures with support to
the trunk on sit-to-stand and stand-to-sit (STS) transitions by a single gas
spring as an energy storage unit. We propose a direction-dependent coupling of
knees and hip joints through a double-pulley wire system, transferring energy
from the torso motion towards balancing the moment load at the knee joint
actuator. Herewith, the exoskeleton maximizes energy transfer and the
naturalness of the user's movement. We introduce an embodied user interface for
hands-free navigation through a torso pressure sensing with minimal trunk
rotations, resulting on average $19^{\circ} \pm 13^{\circ}$ on six unimpaired
users. We evaluated the design for STS assistance on 11 unimpaired users
observing motions and muscle activity during the transitions. Results comparing
assisted and unassisted STS transitions validated a significant reduction (up
to $68\%$ $p<0.01$) at the involved muscle groups. Moreover, we showed it
feasible through natural torso leaning movements of $+12^{\circ}\pm
6.5^{\circ}$ and $- 13.7^{\circ} \pm 6.1^{\circ}$ for standing and sitting,
respectively. Passive postural transition assistance warrants further work on
increasing its applicability and broadening the user population.
