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Fig. S1. Placebo test results (n=4). Four participants completed the same protocol as the main
experimental group, but were given an exotendon with stiffness less than 5% that of a normal
exotendon. These participants showed no change in running economy, relative to natural
running, with the placebo exotendon (two-tailed one-sample t-tests). Error bars represent one
standard deviation.
net m
etab
olic
pow
er(%
cha
nge
from
nat
ural
)day 1 day 2
trial 1 trial 2 trial 3 trial 4
mor
e ef
ficie
ntle
ss e
ffici
ent
p = 0.16
p = 0.67 p = 0.88p = 0.52
-15
-10
-5
0
5
Journal of Experimental Biology: doi:10.1242/jeb.202895: Supplementary information
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Fig. S2. Joint-level kinematics and kinetics (n=4). Traces show average joint angles, moments,
and powers across the gait cycle for natural running (dark red) and exotendon running. Kinetics
from exotendon running are separated into exotendon contributions (blue), biological tissue
(muscles, tendons, etc.) contributions (light red), and the total joint kinetics (black) for the four
participants from Experiment 3. Thin traces show stride-averaged trajectories for individual
participants (n=4) while the thick traces show trajectories averaged across participants. Vertical
dashed lines indicate across-participant average toe-off time for exotendon running (light red)
and natural running (dark red).
Journal of Experimental Biology: doi:10.1242/jeb.202895: Supplementary information
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Fig. S3. Muscle activity (n=4). Average muscle activity for each participant (thin traces) and
across participants (solid lines) for natural running (dark red) and running with the exotendon
(light red) as a function of gait cycle. The vertical dashed lines indicate the average time at which
the toe lifts off the ground during exotendon running (light red) and natural running (dark red).
Journal of Experimental Biology: doi:10.1242/jeb.202895: Supplementary information
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Fig. S4. Average muscle activity (n=4). Comparisons of average, normalized muscle activity,
computed from EMG recordings, across stance and swing phases of gait. Statistical comparison
(paired t-test with Holm-Šidák corrections, α = 0.05) revealed no significant changes in muscle
activity as a result of running with the exotendon.
0
0.02
0.04
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stance swing
aver
age
mus
cle
activ
atio
niliopsoas sartorius adductor group
rectus femoris vastus lateralis vastus medialis
gluteus maximus gluteus medius tibialis anterior
biceps femoris semitendinosus lateral gastrocnemius
medial gastrocnemius soleus peroneus
stance swing stance swing
p=5.2x10-1 p=8.5x10-1 p=2.4x10-1 p=5.2x10-2 p=6.2x10-1 p=8.8x10-1
p=3.7x10-3 p=2.1x10-1 p=6.4x10-3 p=5.8x10-1 p=1.3x10-1 p=5.3x10-1
p=7.4x10-1 p=3.3x10-1 p=6.8x10-1 p=2.2x10-2 p=4.1x10-2 p=2.2x10-2
p=1.5x10-1 p=1.9x10-3 p=2.8x10-2 p=3.7x10-1 p=5.2x10-2 p=5.4x10-1
p=9.1x10-1 p=5.9x10-1 p=3.3x10-1 p=8.6x10-1 p=2.2x10-2 p=3.3x10-1
aver
age
mus
cle
activ
atio
nav
erag
e m
uscl
e ac
tivat
ion
aver
age
mus
cle
activ
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nav
erag
e m
uscl
e ac
tivat
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Journal of Experimental Biology: doi:10.1242/jeb.202895: Supplementary information
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Fig. S5. Exotendon hypothesized mechanism of savings including ‘force rate’ costs. The
total energetic cost (bold, dark red line) comprises costs that increase with stride frequency (thin,
dark red line), and costs that decrease with stride frequency (thin, black line). The increasing
costs can be broken into subcomponents, here force rate costs (long dashed, dark red line)
and swing costs (short dashed, dark red line), although others could be added as well. When the
exotendon assists leg swing, the swing costs subcomponent line shifts (short dashed, light red
line). This in turn shifts the net increasing costs line (thin, light red line), resulting in a new total
energy curve (bold, light red line), and a new the optimal stride frequency (vertical, dashed light
red line). The operating points on the curves of both the increasing costs and decreasing
costs (Point B) as well as on the total energy expenditure curve (Point A) shift down and to the
right with the exotendon, as does Point D (the operating point on the curve of the swing
costs subcomponent of the increasing costs). The net savings in energy expenditure occurs
despite Point C (operating point on the curve of the force rate costs) shifting up and to the right.
Thus, even though some costs might increase with adaptation to the exotendon, decreases in
other costs can result in net savings.
theo
retic
alen
ergy
expe
dntiu
re
stride frequency
total energy,natural
total energy,exotendon
decreasing costs
increasing costs, natural
increasing costs, exotendonforce rate costsswing costs, natural
swing costs, exotendon
A
B
C
D
Journal of Experimental Biology: doi:10.1242/jeb.202895: Supplementary information
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Movie 1. Running with the exotendon.
Journal of Experimental Biology: doi:10.1242/jeb.202895: Supplementary information
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