Optimizing Ankle Exoskeleton Assistance for Walking Across the Life Span
Phase: NA
Conditions: Aging
Sponsor: Georgia Institute of Technology

Project Title: Optimizing Ankle Exoskeleton Assistance for Walking Across the Life 
Span 
 
 
 
 
 
 
 
 
 
 
 
NCT Number: NCT04033146 
 
 
 
 
 
 
 
 
 
 
 
Date: April 25, 2024 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 
Study Protocol 
 
We used commercially available bilateral ankle exoskeletons (Dephy Inc, Boxborough, 
MA) with custom controllers to provide plantarflexion assistance to each user. The 
exoskeletons have on-board encoders and inertial measurement units to enable state-
based control.  
 
Participants walked in six conditions: (1) without ankle exoskeletons (NoExo), (2) ankle 
exoskeletons without assistance (NoTorque, (3) exoskeletons with springlike assistance 
(Spring), and exoskeletons with three motorized assistance strategies with increasing 
peak torque: (4) 10 N-m (MotorLow), (5) 20 N-m (MotorMed), (6) 30 N-m (MotorHigh). 
The spring-like assistance used impedance control without damping to apply torque with 
a spring stiffness of 70 Nm/rad. The assistance began in the foot-flat position after heel 
strike and ended at toe off mimicking a spring and clutch mechanism than enabled 
storage and release of elastic energy in stance and free foot motion during swing. The 
torque profile for the motor assistance conditions was specified by a spline generated 
from four nodes: peak time, rise time, fall time, and peak torque. The spline settings 
were based on a previously published study that used human-in-the loop optimization to 
minimize metabolic cost for a group of younger adults walking at medium speed. For 
this experiment, we scaled the HiLO torque pattern to match our desired low, medium 
and high peaks values at preset values of 10, 20, and 30 N-m. 
 
Participants completed testing over three sessions walking at 1.25 m/s on an 
instrumented split-belt treadmill (CAREN, Motek, Netherlands). The sessions were 
conducted in a set order: 1) habituation, 2) gait mechanics, and 3) steady-state 
metabolic energy consumption. Waiting periods between sessions were minimized to 
allow for learning and retention of walking patterns with exoskeleton assistance (2-7 
days on average). 
 
Session 1 – Habituation.  
Previous work has shown the importance of habituation for the acceptance of 
assistance from exoskeletons. In the first session, each participant walked with spring-
like assistance for three, seven-minute intervals for a total of 21 minutes. They also 
walked with motorized assistance for three, seven-minute intervals while increasing 
peak torque (10, 20, 30 Nm) with each bout. 
 
Session 2 - Gait Mechanics. 
The primary goal for the second session was to obtain biomechanical measurements. 
Reflective markers were placed on participants’ lower limbs and torso to collect motion 
of the hip, knee and ankle joints and an instrumented treadmill was used to collect 
ground reaction forces form each limb. We instrumented participants with surface 
electromyography to record muscle activity on the right leg [tibialis anterior (TA), lateral 
gastrocnemius (LG), soleus (SOL), rectus femoris (RF), biceps femoris (BF), gluteus 
medius (GMd), and gluteus maximus (GMx)]. Participants were re-habituated to the 
exoskeleton assistance for 6-minutes prior to walking with exoskeletons without 

assistance, with spring-like assistance, and with motorized assistance. During the 
motorized assistance habituation period, we increased the peak torque every 2 minutes. 
After each condition’s habituation trial, participants walked for a 2-minute data 
acquisition period. The exoskeleton assistance order was randomized for each 
participant. 
 
Session 3 - Steady-State Metabolic Energy Consumption. 
Participants began the third session with a five-minute standing trial to record their 
metabolic resting baseline. Participants were fitted to a portable indirect calorimetry 
device (COSMED K5, Rome, Italy) to collect energy expenditure for each condition. We 
repeated the habituation process conducted during Session 2 - Gait Mechanics. The 
participants walked for six minutes in each of the six conditions in a randomized order with the 
respective habituation trial preceding a measurement trial.