Design and Simulation of a Pneumatic,
Stored-energy, Hybrid Orthosis
for Gait Restoration
William K. Durfee
Adam Rivard
Department of Mechanical Engineering, University of
Minnesota, Minneapolis, MN 55439
The reciprocating gait orthosis 共RGO兲 links opposite joints so that
extension of the hip on one side leads to flexion on the contralateral side 关30兴. Gharooni et al. proposed that stored spring energy
and limb-segment potential energy could be used to replace stimulation of the hip flexors or withdrawal reflex 关31兴. In their spring
brake orthosis 共SBO兲, excess quadriceps energy is stored in a
mechanical spring that resists knee extension. Spring release
causes knee flexion, which due to inertial properties of the leg
forces the hip to flex. Our system goes further to provide decoupled hip extension and flexion that increases gait-assist performance.
Energy Storing Orthosis
Loss of mobility due to lower limb paralysis is a common result of
thoracic level spinal cord injury. Functional electrical stimulation
(FES) can restore primitive gait in the vicinity of a wheelchair by
using electrical stimulation to generate muscle contractions. A
new concept for FES-assisted gait is presented that combines
electrical stimulation with an orthosis that contains a fluid power
system to store and transfer energy during the gait cycle. The
energy storage orthosis (ESO) can be driven through a complete
gait cycle using only stimulation of the quadriceps muscles. The
conceptual design of the ESO was completed and implemented in
a dynamic simulation model and in a benchtop prototype for engineering measurements. No studies were conducted with human
subjects. The results demonstrate the potential of the ESO concept
for a feasible gait-assist system and the validity of the simulation
model as a means for designing the system.
关DOI: 10.1115/1.2050652兴
Introduction
The inability to walk because of lower limb paralysis is a common result of a thoracic-level spinal cord injury 共SCI兲. Functional
electrical stimulation 共FES兲, which uses electrical stimulation of
motor nerves to trigger muscle contractions, is one means of restoring rudimentary standing and gait for limited mobility in the
vicinity of a wheelchair to some individuals with SCI 关1–7兴. The
user must have good trunk control and a strong upper body because considerable effort is required from the arms engaging parallel bars, a walker, or crutches for support. Despite these restrictions, successful FES users are able to ambulate for hundreds of
meters with many years of use from their system 关8,9兴.
Two limitations of FES-aided gait systems are rapid muscle
fatigue and the inability to precisely control joint torques, which
leads to erratic stepping trajectories 关10兴. Hybrid systems that
combine electrical stimulation with a lower limb orthotic brace
have been developed to address these problems 关11–26兴. Our lab
developed the controlled brake orthosis 共CBO兲, a hybrid FESaided standing and gait system that contains computer-regulated
friction brakes at the knee and hip to lock the joints during stance
phase and control motion during swing phase 关27,28兴. The goal of
our present research is to develop an FES-aided gait system that
requires only one channel of surface muscle stimulation. This can
be achieved by combining FES, a mechanical orthosis, and energy
storage. Excess energy generated by electrical stimulation of one
muscle is harvested, stored, and transferred to other joints that
cannot be conveniently driven by direct muscle stimulation.
The concept of using orthotics to store energy is not new. For
example, Van den Bogert used elastic exotendons for gait assist
关29兴. Greene and Granat utilized a cam-slider mechanism to transfer energy from the knee to the ankle to assist dorsiflexion 关24兴.
Contributed by the Bioengineering Division for publication in the JOURNAL OF
BIOMECHANICAL ENGINEERING. Manuscript received by the Bioengineering Division
April 7, 2005; revision received July 14, 2005. Associate Editor: Mary Frecker.
1014 / Vol. 127, NOVEMBER 2005
The energy storing orthosis 共ESO兲 hybrid FES gait system uses
stimulated muscle power to not only move the limb but also to
push on the orthosis, storing energy in the process. The stored
energy is piped to another joint and released to drive joint motion
without having to stimulate additional muscles. The following
sections of the paper describe the engineering design for an ESO
concept that utilizes a fluid power system to capture excess energy
from the quadriceps that is then released to drive hip and knee
joints. A mathematical model was developed, and a bench top
prototype was fabricated and mounted onto a simulated leg for
conducting baseline engineering tests and for validating the simulation model. No experiments or device evaluations were done on
humans.
Design Requirements
Joint Range of Motion. The system must allow flexion and
extension at the hip and knee. Only sagittal plane motions are
considered, and the ankle is assumed to be locked with an ankle
foot orthosis. Prior work with the CBO 关27兴 showed that the
orthosis must accommodate −10– 25 deg of hip flexion and
0 – 60 deg of knee flexion.
Muscle Stimulation. The system must stimulate only the quadriceps muscle. Quadriceps stimulation is attractive for several reasons. First, the quadriceps 共technically four muscles兲 are large and
relatively large amounts of power can be generated through stimulation. Second, the quadriceps drive knee extension, which undergoes a large excursion during normal gait, simplifying the task of
designing an energy capture system. Third, the quadriceps are
simple to activate with surface electrodes; users can apply and
remove quadriceps electrodes relatively easily. Although completely implanted stimulation systems are the ultimate goal, easyto-use surface systems have a role as a bridge to implanted systems and for those users who do not want surgery.
Available Energy. All energy to drive the leg comes from the
stimulated quadriceps. Additional energy for forward progression
can come from the upper body because the system will be used
with a walker. The available energy from the quadriceps for one
extension cycle moving was estimated from previous work by
calculating the area under a typical torque-angle curve from
60 deg of flexion to full extension, resulting in 31.4 J 关10兴. To be
conservative and to maximize the total number of steps before
fatigue, the system was designed to extract 14 J from the quadriceps per step cycle.
Efficiency. Stimulated muscles behave as nonlinear, timevarying actuators with significant power and energy limitations
关27,32兴. Thus, the mechanical system must minimize energy loss
during operation. Of primary concern is the process of storing,
channeling, and discharging the energy from the quadriceps. Any
energy dissipation during this process will increase the amount of
energy required from the quadriceps, therefore, increasing their
rate of fatigue and limiting the total number of steps that can be
taken.
Copyright © 2005 by ASME
Transactions of the ASME
Fig. 1 Stored energy orthosis concept: „a… biased equilibrium
position, „b… stimulation of quadriceps and storage of excess
energy, and „c… discharge of stored energy
Size, Weight, Don/Doff Time. Minimum size, weight, and time
to don and doff the orthosis is essential for the success of a future
assistive technology product. The purpose of this study was to
demonstrate technical feasibility of the concept; therefore, size
and weight were not considered.
System Description and Physical Prototype
The ESO concept is shown in Fig. 1. Elastic energy-storage
elements on the orthosis hip and knee joints hold the leg in a
flexed equilibrium position 共Fig. 1共a兲兲. Stimulation of the quadriceps extends the knee, placing excess energy in both the equilibrium spring and an energy transfer element 共Fig. 1共b兲兲. The stored
energy is transferred to the hip where it is discharged and used to
extend the hip against its equilibrium spring, and to aid in forward
progression 共Fig. 1共c兲兲. A new step is initiated by releasing the hip
and knee joints from the straight leg position to the flexed position
shown in Fig. 1共a兲.
The realization of the ESO concept is shown in Fig. 2. Gas
springs crossing the hip and knee joints are the flexed equilibrium
energy-storage elements. Their energy is used locally and not
transferred to other joints. A pneumatic fluid power system implements the energy-storage and transfer system. An air cylinder at
the knee acts as a compressor while a matching cylinder at the hip
acts as a joint actuator. An accumulator stores compressed air
energy, and three-way-controlled valves selectively connect the
piston ports to the accumulator and to the atmosphere.
A bench model physical prototype of the ESO is shown in Fig.
3. The ESO components were mounted on wooden links weighted
with steel blocks to model the weight, center of mass, and inertia
of a 180 cm, 76 Kg male 关33兴. The gas springs 共McMaster Carr
6465K11, 6465K12兲 had 90 lb reducible force with stroke lengths
of 2.36 in. 共hip兲 and 3.94 in. 共knee兲. The springs attached to the
links with miniature nylon ball sockets. The pneumatic cylinders
共Bimba D-90366-A-2.5兲 were single-acting, nonspring return,
rear-pivot mount cylinders, with a 7 / 8 in. bore and 2.5 in. stroke
length. Single-acting cylinders are required because they only allow air flow to and from the accumulator in a single port and
minimize overall length of the unit. The pistons were pivot
mounted to prevent side loads on the shaft. Two-position, threeway, 12 V dc solenoid-operated valves 共McMaster Carr 61245K2兲
with a maximum operating pressure of 120 psi were used for control. The accumulator was made from 1 / 16 in. i.d., 1 / 8 in. o.d.
Teflon® tubing with a rated burst strength of 440 psi. Thickwalled Teflon tubing is stiff, and the accumulator did not visibly
expand under pressure.
Gas springs were chosen over mechanical springs for the bias
elements because of their excellent force-to-weight and storedenergy-to-weight ratio and because they produce an almost constant force throughout their entire stroke. A pneumatic fluid power
system was chosen for energy storage and transfer because large
amounts of energy can be stored as highly compressed air in a
Journal of Biomechanical Engineering
Fig. 2 Realization of ESO. Gas springs at the hip and knee
bias joints in flexion. Pneumatic cylinders at the hip and knee
convert mechanical to fluid power. Compressed air stored in
accumulator tubing that couples hip and knee cylinders.
small volume and with almost no weight. The amount of stored
potential energy in compressed air is E = PV ln共P / Po兲 where E
= energy, P = pressure of the compressed air, V = volume of compressed air, and Po = atmospheric pressure. For example, pressurized air at 250 psi contains 79 J / in3, and air at 1500 psi contains
777 J / in3. Pressurized air can be stored in lightweight plastic or
metal tubing, allowing for a high energy-to-weight ratio in comparison to gas springs or traditional coil springs. Energy transfer
and conversion is simple because hoses can transfer compressed
air to remote locations and across rotary joints, while air cylinders
can convert between mechanical translation and fluid power domains.
Three considerations were involved in air-cylinder size and
placement. First, the cylinders must be small bore and located
close to the body to avoid unnecessary weight and bulk. Smaller
bore, higher pressure cylinders would be optimal, but would result
in an accumulator whose volume was so small that parasitic effects would dominate. Second, the cylinders must be aligned so
that they are parallel to the thigh when fully retracted to produce
the highest mechanical advantage when the maximum cylinder
force is exerted. This minimizes losses in the transmission during
operation. Third, because the two cylinders are equal bore and
have the same volume, they must have equal stroke length over
the motion of the respective joint. The placement distance from
the central axis of the leg can then be estimated based on a relationship between cylinder length, stroke length, and angular disNOVEMBER 2005, Vol. 127 / 1015
Fig. 4 ADAMS dynamic simulation model. Head-arm-trunk
loads were simulated with a controlled, moving “floor” that applied equal and opposite loads to the bottom of the leg linkage.
Fig. 3 Bench model physical prototype. The gas springs are
on the back side.
placement of the joint. To prevent energy loss from the system due
to release of pressurized air to atmosphere, the cylinders must be
at zero pressure at full extension so that all energy goes into the
system and no pressurized air is exhausted into the atmosphere.
The pneumatic components and operating conditions, including
piston diameter, maximum pressure, and accumulator volume,
were selected using an iterative design process. The resulting
maximum system pressure was calculated to be 120 psi with an
accumulator volume of 0.15 cu. in.
Simulation Model
An analytic simulation model of the ESO system was created
and realized using the ADAMS 共MSC Software, Santa Ana, CA兲
dynamic systems simulation software to produce a fully coupled
model of the leg operating in a gravity field 共Fig. 4兲. Although the
model greatly simplified that actual system, it included sufficient
detail for its primary purpose of helping select component properties and placements for the physical prototype. The human leg
was modeled as a two-link, frictionless joint pendulum with geometric and inertial properties matching that of an average male.
Quadriceps stimulation was approximated as a constant torque
source acting on the knee joint with a magnitude set sufficiently
high to move the leg and still be reasonable for the torque applied
by a real muscle. For simplicity, the action of the rectus femoris
about the hip was neglected. Joint locks were modeled as constraints. The gas springs and cylinders were modeled as prismatic
1016 / Vol. 127, NOVEMBER 2005
joints with controlled force profiles. Gas spring force was modeled as a constant pressure force in parallel with a damping element so that the output force changed with velocity, as it does in
a real gas spring. The cylinder was modeled as a frictionless air
chamber with a force that accounted for a varying accumulator
pressure, the extension and retraction of the cylinder, and the on/
off timing of the three-way valves. Airflow dynamics and characteristics and energy losses of the air through the system were not
incorporated into the force model.
Beginning with heel strike and throughout forward progression,
additional loads are placed on the foot due to the weight and
accelerations of the head, arms, and trunk 共HAT兲 during gait. To
avoid modeling the balance stability of upright humans, the thigh
was pinned to ground at the hip in the ADAMS model; therefore,
it was not possible to directly simulate the HAT loads. Instead,
HAT loads were implemented through an artificial moving floor
that applied loads to the bottom of the linkage equivalent to those
applied by the HAT, resulting in equivalent effects on the hip and
knee. The floor was modeled as a square block coupled to ground
with two frictionless translation joints and a link to generate twodimensional floor movement in the sagittal plane. A positive vertical force was applied to the floor to simulate the equal and opposite force that the HAT normally applies to the hip joint 关33兴.
Measurements were performed on components of the physical
prototype to verify whether the physical model matched the
model. The static force-displacement curve for the gas springs and
for the piston compressing air into the accumulator were measured by pressing the shaft against a calibrated scale and recording
force and position at several points. Friction was estimated by
measuring the force of an uncharged gas spring and an unpressurized cylinder against the scale while compressing at ⬃2 in./ s.
Trajectories of the prototype leg as it moved from full extension to
Transactions of the ASME
Fig. 5 Simulation model: hip and knee trajectories for one gait cycle
the flexed equilibrium position when released from a straight configuration were measured through frame-by-frame analysis of a
video recording.
Results
A simulation was run of the ADAMS model to show how the
leg responds. Figure 5 shows the change in hip and knee joint
angles throughout a simulation cycle. The simulation cycle is one
step in a gait cycle, but with artificial time intervals added to the
simulation to allow for adjustment of model parameters, such as
air-line solenoids opening and closing, and for simpler visualization of the cycle, resulting in a 15 s simulation time. Simulation
begins with the leg straight and vertical. In the first 0.9 s, the gas
springs force the hip and knee to their equilibrium positions of 25
and 60 deg of flexion. The knee then fully extends over the time
interval from 5.5 to 6.6 s, resisting against the pneumatic system.
Over the time interval from 10 to 11.25 s, the pneumatic system
discharges, causing the hip to extend to −10 deg. The simulation
ends after the leg is brought to equilibrium from 11.25 to 11.8 s.
Figure 6 shows the force and displacement for the hip and knee air
cylinders. Not shown are the additional simulation outputs of cylinder pressure and gas spring force and displacement.
The electrically stimulated quadriceps must provide enough energy to fuel the operation of the system. The simulation model
was used to calculate how the 14 J extracted from the quadriceps
during each step was allocated during a cycle which, in turn, was
used to select ESO components. Figure 7 shows the start and
finish positions of the leg during knee extension and the corre-
sponding torque necessary for the movement. The required energy
from the quadriceps was estimated at 8.9 J by calculating the area
under the curve corresponding to the movement. Subtracting 8.9 J
from the initial 14 J leaves 5.1 J that can be stored in the pneumatic accumulator for driving hip extension later in the gait cycle.
Driving the hip from 25 deg of flexion to 10 deg of extension
stores energy in the hip gas spring. Gravity provides an assist in
the first part of the motion but resists the motion once the hip
passes zero degrees. Analysis of the hip motion showed that 1.4 J
of the 5.1 J available was needed to move the leg against gravity
and the gas spring, leaving 3.7 J to be used for actively driving
hip extension to assist forward progression and increase gait
speed.
Measurements of the physical gas springs and air cylinders
showed that ⬃0.25 J will be lost to friction during one gait cycle,
2% of the available 14 J. In the real system, there will be additional losses from the orthosis joints and fluid drag.
Figure 8 shows the measured force-displacement curves for the
hip and knee air cylinders. Superimposed are two curves from the
simulation model. The dashed line accounts for the extra
0.052 cu. in. of dead volume that is between the cylinder and the
valves at each end of the accumulator in the prototype. The dead
volume lowers the force in the system and is a loss of pressurized
air and, therefore, wasted energy that is never stored in the accumulator nor transferred to the hip.
Figure 9 shows the time course of the hip and knee joint angles
as the limbs of the physical prototype moved into their equilibrium position. The equivalent output from the simulation model is
Fig. 6 Simulation model: force „top… and displacement „bottom… for hip „left… and knee „right…
air cylinders for one gait cycle. Gas springs force leg to equilibrium during t = 0 – 0.9 s; knee
extends during t = 5.5– 6.6 s; pneumatic system forces hip extension during t = 10.0– 11.25 s; leg
returns to equilibrium during t = 11.25– 11.8 s.
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NOVEMBER 2005, Vol. 127 / 1017
Fig. 7 Simulation model of knee extending at constant angular velocity: „a… t = 2.0 s; 60 deg knee flexion, „b…
t = 4.0 s; knee fully extended, „c… knee torque versus time. Required energy is 8.9 J.
superimposed. The difference between the two curves is because
the model assumes the force in the gas springs is independent of
stroke length, whereas the force in the real spring varies by 10%
over its stroke.
Discussion and Conclusions
The ESO appears to be a viable concept for a FES-aided gait
system. Storing energy with gas springs and storing and transferring energy with a pneumatic fluid power system was shown to
work in the dynamic system model and the physical prototype.
The simulation model was essential for system design and was
exercised extensively to determine system component specifica-
tions and mounting positions. The evaluation experiments demonstrated that the simulation model could predict the performance of
the physical system with sufficient accuracy.
Significant technical issues must be addressed for the next design iteration. A rigorous energy loss analysis using a refined
model must be conducted that includes more accurate musculoskeletal dynamics and head losses in the fluid power system. The
system must then be optimized to minimize these losses because
they directly draw on the total available muscle energy. The dead
volume in the fluid power system must be eliminated along with
all other losses of compressed air due to valve switching. Finally,
the next design iteration must address total system size and weight
Fig. 8 Force-displacement properties of hip and knee air cylinders. Measured „markers… and two simulation
results „lines…. The dashed line accounts for dead volume between the cylinder and the valve.
Fig. 9 Prototype „markers… and simulation „line…: hip „a… and knee „b… knee angle trajectories as joints move
from full extension to equilibrium
1018 / Vol. 127, NOVEMBER 2005
Transactions of the ASME
as well as body attachment points for the orthosis. Once these
challenges are met, a study can be conducted to evaluate the efficacy of the ESO concept using human subjects.
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