  Back injuries are the most prevalent work-related musculoskeletal disorders
and represent a major cause of disability. Although innovations in wearable
robots aim to alleviate this hazard, the majority of existing exoskeletons are
obtrusive because the rigid linkage design limits natural movement, thus
causing ergonomic risk. Moreover, these existing systems are typically only
suitable for one type of movement assistance, not ubiquitous for a wide variety
of activities. To fill in this gap, this paper presents a new wearable robot
design approach continuum soft exoskeleton. This spine-inspired wearable robot
is unobtrusive and assists both squat and stoops while not impeding walking
motion. To tackle the challenge of the unique anatomy of spine that is
inappropriate to be simplified as a single degree of freedom joint, our robot
is conformal to human anatomy and it can reduce multiple types of forces along
the human spine such as the spinae muscle force, shear, and compression force
of the lumbar vertebrae. We derived kinematics and kinetics models of this
mechanism and established an analytical biomechanics model of human-robot
interaction. Quantitative analysis of disc compression force, disc shear force
and muscle force was performed in simulation. We further developed a virtual
impedance control strategy to deliver force control and compensate hysteresis
of Bowden cable transmission. The feasibility of the prototype was
experimentally tested on three healthy subjects. The root mean square error of
force tracking is 6.63 N (3.3 % of the 200N peak force) and it demonstrated
that it can actively control the stiffness to the desired value. This continuum
soft exoskeleton represents a feasible solution with the potential to reduce
back pain for multiple activities and multiple forces along the human spine.
