First published online December 16, 2008
Journal of Experimental Biology 212, 32-41 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.017277
Mechanics and energetics of incline walking with robotic ankle exoskeletons
Gregory S. Sawicki* and
Daniel P. Ferris
Human Neuromechanics Laboratory, University of Michigan-Ann Arbor, Ann
Arbor, MI 48109, USA
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Fig. 1. Experimental set-up. Subjects walked on a motorized treadmill at 1.25 m
s–1 for 7 min with exoskeletons unpowered, then rested for 3
min, then walked for 7 min with exoskeletons powered at surface inclines of
0%, 5%, 10% and 15% grade presented in randomized order. Boxes indicate
periods when data were collected (minutes 4–6) in both unpowered and
powered conditions. During powered walking, bilateral ankle–foot
orthoses (i.e. exoskeletons) were operated under proportional myoelectric
control. Users' soleus muscle activity generated a real-time control signal
commanding timing and amplitude of artificial pneumatic muscle forces. We used
reflective markers and motion capture to collect joint kinematics,
open-circuit respirometery to measure O2 consumption and
CO2 production, surface electromyography to record ankle muscle
activity and compression load transducers to record artificial muscle
forces.
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Fig. 2. Joint kinematics. Thick lines are the mean ankle (left column), knee
(middle column) and hip (right column) joint angles over the stride from heel
strike (0%) to heel strike (100%) of nine subjects. Data are averages of left
and right legs. Each row is walking data at 1.25 m s–1 on a
single surface gradient (0%-level at top to 15%-uphill at bottom). In each
subplot, curves are for unpowered (black circles), and powered (gray circles)
walking and thin lines are +1 s.d. Stance is  0–60% of the stride,
swing 60–100%. For all joints 0 deg. is upright standing posture. Ankle
joint plantarflexion, knee joint extension and hip joint extension are all
positive.
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Fig. 3. Ankle exoskeleton mechanics. The thick lines are mean ankle joint angular
velocity (left column), exoskeleton torque (middle column) and exoskeleton
mechanical power (right column) over the stride from heel strike (0%) to heel
strike (100%) of nine subjects. Data are average of left and right legs. Each
row is walking data at 1.25 m s–1 on a single surface
gradient (0%-level at top to 15%-uphill at bottom). In each subplot, bold
lines are for unpowered (black circles), and powered (gray circles) walking,
and thin lines are +1 s.d. Stance is 0–60% of the stride, swing
60–100%. Ankle joint angular velocity (deg. s–1) is
positive for ankle plantarflexion. Exoskeleton torque that acts to plantarflex
the ankle is positive. Torque is the product of artificial muscle load and
moment arm length and is normalized by subject mass (Nm
kg–1). Exoskeleton mechanical power is the product of
exoskeleton torque and ankle joint angular velocity and is normalized by
subject mass (W kg–1). Positive exoskeleton power indicates
transfer of energy from exoskeletons to the user's biological ankle
muscle–tendon system.
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Fig. 4. Exoskeleton performance. Bar graphs show the mean (A) change in net
metabolic power (powered–unpowered; W kg–1) as a result
of powered assistance from bilateral ankle exoskeletons, (B) exoskeleton
average positive (black), negative (white) and net (dark gray) mechanical
power (W kg–1) over a stride for powered walking and (C)
exoskeleton performance index (unitless) of the nine subjects. Performance
index indicates the fraction of ankle muscle–tendon positive work
performed by plantar flexor muscle shortening rather than Achilles' tendon
recoil (i.e. muscle work fraction). Numbers above bars are equivalent ankle
muscle–tendon `apparent efficiency' values (see Materials and methods
for details). For all panels, surface inclines increase from left (0%-level)
to right (15%-uphill). Error bars are ±1 s.e.m. Asterisks (in A)
indicate statistical significance for comparison of powered versus
unpowered net metabolic power (P<0.05).
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Fig. 5. Ankle muscle root mean square electromyography (EMG) for soleus (Sol.;
top), medial gastrocnemius (MG), lateral gastrocnemius (LG) and tibialis
anterior (TA; bottom). Values are mean stance phase root mean square (r.m.s.)
average muscle activation for nine subjects (error bars are ± 1
s.e.m.). Surface gradients increase from left (0%-level) to right (15%-uphill)
with unpowered walking (minutes 4–6) in white and powered walking
(minutes 4–6) in gray. All r.m.s. values (unitless) are normalized to
the unpowered 15% grade condition. Values above bars indicate percentage
difference in powered versus unpowered condition. Asterisks indicate
a statistically significant difference between powered and unpowered walking
(ANOVA, P<0.05).
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© The Company of Biologists Ltd 2009
