IEEE Robotics & Automation Magazine - March 2020 - 98

back and forth on the staircase at a self-selected cadence for
10 consecutive repetitions. Then, a certified prosthetist fitted the subjects with the powered polycentric ankle. The
prosthetist was instructed to follow the same procedure normally used with commercially available prostheses, which
consisted of adjusting the build height of the experimental
prosthesis based on the subject's anthropometry and aligning the prosthesis joints to achieve comfortable and stable
standing posture. Approximately 30 min of training with the
powered polycentric ankle was provided to subjects prior to
the data collection.
After the training was completed, the treadmill test was
repeated [Figure 11(a)]. The test for stairs ambulation
occurred on a staircase that allows for two consecutive strides
on the prosthesis side [Figure 11(b)]. The subjects were asked
to ambulate back and forth on the staircase at a self-selected
cadence for 10 consecutive repetitions. Both tests consisted of
the subject wearing the powered polycentric prosthesis. For
the goal of this experiment, transitions between different
ambulation modes were manually triggered by the experimenter using the GUI.
During the tests, we recorded signals from all the embedded prosthesis sensors at a sampling frequency of 500 Hz.
During postprocessing, we divided the acquired data into
separate strides using the outputs from the GRF sensors and
the finite-state machine. Each recorded stride was normalized
to the stride duration and resampled to 1,000 samples. Additionally, we normalized both the ICR and ankle joint torque
to body weight. Finally, we computed the mean and standard
deviations of the normalized ankle angle, ankle torque (measured using the instrumented pyramid), and ICR torque (estimated from motor current as previously done for ankle
torque in [41]) profiles of all recorded strides and combined
them for each subject.

Biomechanical Results
For both subjects, the powered ankle prosthesis provided
kinematics and kinetics that qualitatively resemble the nominal walking profiles of able-bodied individuals. The average
ankle kinematics and kinetics for each subject during the
treadmill walking test are depicted in Figure 12. Positive
angles and torques represent plantarflexion, whereas negative
angles and torque represent dorsiflexion. Nominal kinematics
and kinetics extracted from able-bodied individuals are
shown using dashed red lines in all the panels. As seen in
Figure 12(a) and (b), for both subjects, the powered ankle
kinematics (solid blue lines) closely approximated the physiological ankle kinematics, including the peak plantarflexion
angles in early stance and swing and the dorsiflexion angle in
midstance; however, for subject 1 the latter was roughly 40%
higher than that of the former.
Compared to the prescribed passive prostheses, the powered polycentric ankle produced a marked increase in ROM,
especially in late stance and swing. Analysis of the ankle
kinetics during walking [Figure 12(c) and (d)] shows a similar
trend for the powered ankle prosthesis torque (blue lines) and
the nominal able-bodied torque (red dashed lines). However,
the peak torque of the powered prosthesis was roughly 20
and 28% lower than that of the nominal able-bodied trajectory for subjects 1 and 2, respectively. In agreement with the
dynamic simulations (Figure 3), higher dorsiflexion torque
and lower plantarflexion torque are seen at the ICR (solid
orange line) compared to the ankle joint (solid blue line).
Thus, the experimental data show that the proposed polycentric mechanism has the ability reduce the physiological plantarflexion torque bias, as predicted by our simulations.
The ability to inject net positive energy into the gait cycle is
one of the key advantages of a powered prosthesis over a conventional passive prosthesis. While walking on the treadmill
at 1 m/s, subject 1 had a self-selected
cadence of 92 steps/min. During the
walking tests, the powered polycentric
ankle injected 10.1 ± 1.2 J/stride of
mechanical energy, consuming 20.9 ±
1.7 J/stride of electrical energy. For subject 2, the self-selected cadence was ~96
steps/min. The powered ankle injected
8.6 ± 0.4 J/stride of mechanical energy,
consuming 25.6 ± 1.2 J/stride of electrical energy. Interestingly, the observed
mechanical energy injection matches
that of previous experiments [52],
where a powered ankle prosthesis was
shown to reduce the metabolic cost
of walking in seven individuals with
below-knee amputations. However, the
powered polycentric ankle is approximately 900 g lighter than the powered
(a)
(b)
ankle used in [52], which may lead to
further metabolic cost reductions [53].
Figure 11. Still photos of subject 2 as he (a) walks on the treadmill and (b) ambulates
on the staircase with the powered polycentric ankle.
With Wi-Fi and data recording on, the

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