Methods
Soft exosuit design and operation
A lower extremity soft exosuit (Fig. 1a-c) and the associated
actuation system (Fig. 1d) were fabricated. The exosuit and
the actuation system were discussed in detail in [22]. Briefly,
the exosuit consisted of a structured textile extending bilaterally
from the waist to the feet and was composed of three
principle components: a waist belt, bilateral thigh pieces and
bilateral calf straps (Fig. 1e). Two actuation units (Fig. 1a-b)
were mounted on a backpack and connected to the exosuit
by Bowden cables. To operate, the actuation units retracted
the inner cable of the Bowden cable assembly, delivering a
controlled force to the wearer. The level of force transmitted
to the wearer was monitored by load cells using
force-based position control, as described in Additional
file 1: Text S1 and in Additional file 1: Figure S1.
Briefly, force-based position control imposes a predefined
position trajectory as a function of the gait cycle to the
motor acting on the Bowden cables to achieve a specific
force profile. The cable position trajectory generates
a force in the exosuit because the textile, the Bowden
cables, and the human tissue underneath the exosuit
are compliant. The combination of these factors, defined
as the “suit-human series stiffness” (Fig. 2), is a
mapping between the cable position and induced
force. While the position control method generated consistent
force profiles for fixed kinematics and suit
conditions, an algorithm that monitors key force profile
features iteratively adjusted assistive position profiles on a
step-by-step basis to account for small variations in gait
and drift of textile components. Similar iterative learning
techniques to achieve desired exoskeleton torque patterns
have been explored [26].
The load-transferring elements in the exosuit followed
two distinct paths in each leg as described in Fig. 1c.
One multiarticular load path extended from the waist,
over the front of the thigh