  Mobility disabilities are prominent in society with wide-ranging detriments
to affected individuals. Addressing the specific deficits of individuals within
this heterogeneous population requires modular, partial-assist, lower-limb
exoskeletons. This paper introduces the Modular Backdrivable Lower-limb
Unloading Exoskeleton (M-BLUE), which implements high torque, low mechanical
impedance actuators on commercial orthoses with sheet metal modifications to
produce a variety of hip- and/or knee-assisting configurations. Benchtop system
identification verifies the desirable backdrive properties of the actuator, and
allows for torque prediction within 0.4 Nm. An able-bodied human subject
experiment demonstrates that three unilateral configurations of M-BLUE (hip
only, knee only, and hip-knee) with a simple gravity compensation controller
can reduce muscle EMG readings in a lifting and lowering task relative to the
bare condition. Reductions in mean muscular effort and peak muscle activation
were seen across the primary squat musculature (excluding biceps femoris),
demonstrating the potential to reduce fatigue leading to poor lifting posture.
These promising results motivate applications of M-BLUE to additional subject
populations such as hip/knee osteoarthritis and geriatric frailty, and the
expansion of M-BLUE to bilateral and ankle configurations.
