906 IEEE TRANSACTIONS ON NEURAL SYSTEMS AND REHABILITATION ENGINEERING, VOL. 25, NO. 7, JULY 2017

WAKE-Up Exoskeleton to Assist Children WithCerebral Palsy: Design and Preliminary

Evaluation in Level WalkingFabrizio Patané, Stefano Rossi, Fausto Del Sette, Juri Taborri, and Paolo Cappa

Abstract— This paper presents the modular design andcontrol of a novel compliant lower limb multi-joint exoskele-ton for the rehabilitation of ankle knee mobility and locomo-tion of pediatric patients with neurological diseases, suchas Cerebral Palsy (CP). The device consists of an untetheredpowered knee–ankle–foot orthosis (KAFO), addressed asWAKE-up (Wearable Ankle Knee Exoskeleton), character-ized by a position control and capable of operating synchro-nously and synergistically with the human musculoskeletalsystem. The WAKE-up mechanical system, control architec-ture and feature extraction are described. Two test bencheswere used to mechanically characterize the device. The fullsystem showed a maximum value of hysteresis equal to8.8% and a maximum torque of 5.6 N m/rad. A pre-clinicaluse was performed, without body weight support, by fourtypically developing children and three children with CP.The aims were twofold: 1) to test the structure under weight-bearing conditions and 2) to ascertain its ability to provideappropriate assistance to the ankle and the knee duringoverground walking in a real environment. Results confirmthe effectivenessof the WAKE-up design in providing torqueassistance in accordance to the volitional movements espe-cially in the recovery of correct foot landing at the start ofthe gait cycle.

Index Terms— Assistive device, cerebral palsy,exoskeleton, gait feature, series elastic actuators.

I. INTRODUCTION

CEREBRAL Palsy (CP) can be referred as an umbrellaterm covering a group of non-progressive motor impair-

ment syndromes secondary to lesions or anomalies of the brainarising in the early stage of development [1]. This pathologicalstatus represents the commonest cause of physical disabilityin early childhood [2].

Children with CP have a reduced movement capacity, caus-ing walking asymmetries, which may considerably limit their

Manuscript received April 14, 2016; revised July 28, 2016 andDecember 12, 2016; accepted January 3, 2017. Date of publicationJanuary 11, 2017; date of current version August 6, 2017. J. Taborri is thecorresponding author.

F. Patané is with the School of Mechanical Engineer-ing, “Niccol Cusano” University, 00166 Rome, Italy (e-mail:fabrizio.patane@unicusano.it).

S. Rossi is with the Department of Economics and Management,Industrial Engineering, University of Tuscia, 01100 Viterbo, Italy (e-mail:stefano.rossi@unitus.it).

F. Del Sette and J. Taborri are with the Department of Mechanical andAerospace Engineering, “Sapienza” University of Rome, 00184 Rome,Italy (e-mail: fausto.delsette@uniroma1.it; juri.taborri@uniroma1.it).

P. Cappa, deceased, was with the Department of Mechanical andAerospace Engineering, “Sapienza” University of Rome, 00184 Rome,Italy.

Digital Object Identifier 10.1109/TNSRE.2017.2651404

ability in the main daily living activities and also influencetheir motor skill development [3]. Briefly, patients with neu-rological diseases can be affected by a mixture of severalfunctional deficits, each one with a specific severity range [4].Focusing on gait, children with CP are characterized by slowspeed and disturbed motor control that are caused by anincreased prevalence of pain, fatigue, musculoskeletal dys-function (i.e., spasticity and contractures), decreased balance,and deterioration in overall physical conditioning and musclestrength [5], [6]. The drop foot is the main, persistent, andlong-term disability in 20% of CP population [7]. Drop foot isthe inability to dorsiflex the ankle joint during the swing phaseand, consequently, the initial contact occurs with the toe andnot with the heel. Other common dysfunctions [4], [8], with aless extent, are: the slap foot, which represents the inability ofindividual to control the falling of her/his foot after heel strike,so that the foot slaps the ground; the toe walking, that is theinability to clear toe during swing; and, excessive knee flexionduring stance, which causes an elevated walking energy cost.

In the last decades it emerged that traditional rehabilitativetreatments, focused on the passive and manual movements ofthe hemiplegic limb, do not improve gait patterns [9]. Actually,manually assisted rehabilitation is: 1) a highly often labor-intensive task supporting and facilitating movements that areunsafe or difficult to perform, and 2) beyond the capabilities oftherapists in terms of speed, sensing, strength, and repeatabilityof mobilization exercises [10], [11]. Conversely, the usageof robotic interventions demonstrates the potentiality to sig-nificantly improve the outcome of rehabilitation treatments,to decrease the period of hospitalization, and to transfer therehabilitation in house [9], [12], [13].

From this perspective, novel robotic systems, to assistand complete traditional therapeutic treatments delivered tochildren with CP, received recently a great deal of inter-est [8], [14], [15]. Briefly, exoskeletons/active orthoses fitclosely and operate in parallel with the human legs andthey are intrinsically capable to: deliver massed practices,engage the children, carry out the desired actions, semi-autonomously practice their movement training for longerperiods, and, quantify via objective indices the therapeuticprocess [16], [17], [18]. The main critical issues, which couldnegate the benefit of the assisted movement, are related to themechanical structure (e.g., weight, inertia and friction) [19]and the physical and cognitive interactions between the patientand the exoskeleton/active orthoses [20], [21].

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PATANÉ et al.: WAKE-UP EXOSKELETON TO ASSIST CHILDREN WITH CEREBRAL PALSY 907

Whereas several assistive robotic systems for upper limbof children with CP are exhaustively reported in litera-ture [22]–[29], few systems were designed for the recovery oflower limb movements, such as: Lokomat, Gait Trainer, Atlasand Anklebot. The Lokomat and Gait Trainer operate with abody weight support [13], [15], [30]–[32], Atlas and Anklebot,instead, were designed for level walking trials without bodysupport [14], [33], [34]. The previously indicated exoskeletonscan be used only in research laboratories or clinics. Hence,to the best of our knowledge, no lower limb exoskeletonsdesigned for knee and ankle joints have been proposed torehabilitate CP population in daily activities outside clinicalenvironment. In this context, we here described the designand the development of the WAKE-up (Wearable Ankle KneeExoskeleton), a lower-limb assistive modular exoskeleton forCP patients. The WAKE-up is an assistive device addressed toa cohort of population able, although with limited movements,to walk. The exoskeleton is equipped with robotic jointslocated alongside the knee and ankle, and has been designed tobe portable, lightweight, comfortable and safe. The WAKE-upis a device designed to provide torque assistance to the jointswith Rotary Series Elastic Actuators (RSEAs) during givenevents associated to the gait pattern: heel strike, mid/forestrike and toe off. By extending the comparison between theWAKE-up and the exoskeletons for adults after spinal cordinjury [35], [36], it emerged three main distinctive novelties.Firstly, the WAKE-up mass is lower than the other devicesthat are designed for children and adults, i.e., 2.5 kg [37] andin the range of 10–25 kg [36], respectively; actually, the lowmass value is a mandatory design constraint for exoskeletonsaddressed to children. Secondly, the WAKE-up design includesthe RSEA to improve the user’s safety; that design specifica-tion was proposed only in the IHMC prototype [38]. Thirdly,the modularity of the WAKE-up, i.e., the feasibility to use thetwo modules together or alone, represents a novelty in the fieldof exoskeletons.

Starting from the mechanical design of the structure, theneeded sensors and actuator systems are discussed as wellas the pre-clinical study conducted on children with hemiple-gia (HC). The hypothesis was that, targeting the more affectedleg, the synergistically power application to the ankle and kneejoints would advantageously alter gait patterns.

In particular, the main objective of the WAKE-up is to pro-vide assistance for recovering physiological kinematic patternsduring level walking. In order to evaluate the WAKE-up effec-tiveness, we comparatively examine the joint kinematics in HCand in age-matched typically developing children (TDC).

II. WAKE-UP DESIGN

The WAKE-up, shown in Fig. 1, has been built withinthe framework of the ITINERE1 project and it is a lowerlimb exoskeleton for the recovery of knee and/or ankle joints.It represents the second version of the alpha prototype whichhas already been descripted in [37]. WAKE-up is designed

1Project “ITINERE - INTERACTIVE TECHNOLOGY: AN INSTRU-MENTED NOVEL EXOSKELETON FOR REHABILITATION” Project seedFondation IIT, 670kPrincipal Investigator: Paolo Cappa.

Fig. 1. WAKE-up system, Joint Module and shoe insole details.

for the rehabilitation of children aged from 5 to 13 yearsold with neurological diseases, such as CP. The device isconceived as modular, allowing the infrastructure to work withone or two modules per leg simultaneously. Thus, the chosenrehabilitative scenario is related to the number and type ofinvolved modules. In the current version, the WAKE-up isarticulated in two joint modules: the knee and the ankle.

A. Mechanical Design

One of the most difficult design constraint that still requiresattention is the development of lightweight exoskeletons ableto avoid encumbering the movement ability. We found in apreliminary experimental work that a wearable robotic devicewith a mass up to 2.5 kg does not noticeably modify the gaitpatterns in pediatric population [39]. Other relevant issuesare related to the ergonomics and comfort, allowing thesafe adaptation to patients. We decided to allow active andpassive movements only in the sagittal plane. The range ofmotion (ROM) of the actuated joints is mechanically limitedto 100◦ for the knee and 45◦ for the ankle, for safety reasons.The WAKE-up mechanical frame, Fig. 1, is composed of twobraces where the joint modules are mounted. The braces arerigidly attached to the thigh and the shank by adjustable Velcrostraps for customization to patient requirements. Thigh andshank frame lengths are adjustable allowing the use of thedevice by children with leg length in the range of 0.5–0.8 m.

908 IEEE TRANSACTIONS ON NEURAL SYSTEMS AND REHABILITATION ENGINEERING, VOL. 25, NO. 7, JULY 2017

A fine alignment of each joint module with the anatomicalrotation axis can be obtained changing the inclination ofthe belt/pulley stage. The braces are designed from a 3Dscanner acquisition (Laser ScanArm V3, FARO, FL, USA) ofa mock leg and they are realized with a FDM rapid prototyp-ing machine (3D-Printer Inspire S250, Tiertime Technology,China). To avoid excessive pressure of the brace against theskin and prevent eventual damages, foam pads are used tocover the braces. An ad-hoc quick release system between thebrace and the joint module is designed, Fig. 1. It is composedof a sliding dovetail joint where the joint module can be easilyinserted after the patient worn the braces. A second quickrelease system is designed to permit the integration with asize-specific shoe, which a sole equipped with six footswitchesis placed inside (FSR 400, force sensing resistor, InterlinkElectronics, USA).

B. Joint Modules

The modules of the knee and the ankle are identical inthe design and specifications in terms of maximum rotation,speed, and torque. The choice is justified considering that themaximum torque exerted at the knee and ankle, by typicallydeveloping children (TDC) during walking, are quite similar,i.e., 1 N m/kg and 1.3 N m/kg, respectively [40], [41]. Suchphysiological property allowed us to simplify the WAKE-uphardware design. Each module is equipped with a RotarySeries Elastic Actuator (RSEA) and a belt/pulley reductionstage, Fig. 1.

The RSEA consists in a servomotor with integratedPID (Proportional Integral Derivative) position/velocity con-troller (Dynamixel EX-106+, Trossen Robotics, IL, USA), andan ad-hoc designed torsion spring. The spring, which has anominal stiffness of 2.0 N m/rad and nominal fatigue limitequal to ±100◦, can transmit a maximum torque of 3.5 N m.The nominal stiffness was chosen coherently with the maxi-mum actuator torque and stroke limit, considering a maximumspring rotation of 180◦. In addition, the spring is sensorizedwith an absolute 14-bit magnetic encoder (MA3, US Digital,WA, USA). The use of RSEA allows many benefits in naturaland repetitive tasks, such as the human gait; indeed, RSEAs,with respect to the traditional stiff actuators, provides highforce fidelity, lower inertia and more energy storage [42], [43].A belt/pulley stage with a reduction ratio of τ = 2/3 is usedto increase of 1/τ = 1.5 the value of the assisting torqueat the anatomical joint, up to 5.2 N m. The assisting torquecorresponds ∼15% of the maximum knee and ankle net-moments occurring during the walking of TDC. The maximumassistive torque was chosen considering that: 1) the waveformsof net moment at knee and ankle level [40], [41] are both 15%lower than the maximum at the instants when the WAKE-upprovides assistance (heel strike and toe off for the knee,mid/fore strike and toe off for the ankle); 2) the selectedvalue represents a trade-off between lifting the weight of thechild and helping the paretic leg during gait; and, 3) themaximum torque lies in the range of resistive plantarflexiontorque in children with CP, considering a maximum speedof 0.5 rad/s [44]. Thus, the nominal stiffness at the anatomicaljoint results equal to 4.5 N m/rad.

The control strategy is based on the fine regulation of theequilibrium point of the RSEA, which is locally controlled bythe integrated PID. Since the device consists essentially of aspring with known stiffness k, with a precisely controllableequilibrium position �θ , this method allows to transmit to thearticulation a torque m = k�θ . Controlling the position �θof RSEA, therefore, is an indirect method of controlling thetorque m. The position accuracy and the speed fidelity withwhich the device is able to reproduce a given position profilecorrespond to the accuracy of the torque profile in staticcondition and in dynamic one, respectively.

The choice of a position control is justified by the presenceof a reduction of 100:1 in the servomotor, which impedes adynamic impedance control based on the maximum speed oracceleration at the motor shaft. However, due to the limiteddynamics in the interaction between exoskeleton and thehere targeted patients, the position control implemented inthe WAKE-up can be considered acceptable also in a realenvironment.

C. Control Architecture

Taking into account the modularity of the device, theWAKE-up control architecture was designed in accordance toan addressable master/slave model. There is only one master,called WU-S, for the coordination and configuration of theslave nodes, each one addressed as WU-N.

The computation and control capability is distributed in theperiphery, thus reducing to a minimum the dependencies andthe communications between master and slaves. Dependingon the operative scenario, the WU-S configures appropriatelythe connected WU-N, and starts coordinating transfers ofinformation. Local control of joint torque, for example, ismanaged by only the corresponding WU-N, without involvingthe WU-S. In parallel to the control of the lower level, eachslave, upon request, can receive, compute and send data to themaster, which can forward the information to another slave, ifrequired by the current scenario. The WU-S and the WU-Ns,within the general architecture, assume, therefore, the role ofSupervisor and Local Controller, respectively. In addition tothe task of overall supervision, the WU-S manages the highlevel GUI, which the user can access via WiFi connection, withan internet browser to configure/activate scenarios, to monitorthe status of the system, and to download logfiles.

The infrastructure software is therefore designed accordingto four levels:

1. SA-L, Sensor and Actuator Layer: management ofmovement sensors, local PID controller for the RSEAs;

2. NC-L, Node Controller Layer: implementation of thescenario at the local level (5 ms time cycle);

3. MS-L, Main Supervisor Layer: coordination and config-uration of the slaves (10 ms time cycle);

4. UL-L, User Level layer: user interaction, graphical userinterface (asynchronous).

The control architecture was shown in Fig. 2.1) Supervisor: WU-S: The WU-S consists in a MyRio

controller (National Instruments, TX, USA) equipped withan ARM Cortex-A9. The development environment is Lab-VIEW RT (National Instruments, TX, USA) based on a Linux

PATANÉ et al.: WAKE-UP EXOSKELETON TO ASSIST CHILDREN WITH CEREBRAL PALSY 909

Fig. 2. Control architecture.

RT kernel. The WU-S manages the communication betweenthe WU-Ns through the software layer MS-L. Because thecontrol is distributed more on the periphery, the routines imple-mented in the WU-S for the configuration of the slaves areasynchronous and with a low priority. The WU-S implementsalso a GUI (UL-L), which the user can access via an internetbrowser through WiFi connection. The GUI permits: 1) tochoose the scenario; 2) to configure the associated parameters;3) to send low-level commands to the slaves; 4) to monitorthe correct functioning of the system; and 5) to activate thedata logging.

2) Local Controller: WU-N: The key element of the sys-tem is the WU-N, which is articulated in the followingcomponents: 1) 32 bit microcontroller (PIC32MX440F512H,Microchip, USA); 2) 9-axis inertial measurement unit (IMU),with an accelerometer, a gyro, and a magnetometer(MPU-9350, InvenSense, USA); 3) 14-bit magnetic absoluteencoder for the angular deflection of the torsional spring;4) foot insole with six footswitches; and 5) high torque digitalservomotor.

Each module is able to gather the following data from thesensor systems: the rotation of the motor, the angular defor-mation of the torsion spring of the RSEA, the IMU outputs,and the state of the connected footswitches. Depending on thepreviously mentioned quantities and on the selected scenario,the WU-N calculates the target rotation of the actuator andsends it to the local PID controller connected via RS-485 bus.

The software levels implemented in the WU-N are SA-Land NC-L. The latter layer is associated with the state machineshown in Fig. 3.

The module, after a first initialization phase (General Init),in which it self-assigns an address on the bus, passes toinitialize the remaining hardware (Init) and, if no error occurs,it waits (Idle) a start command to trigger the scenario control.

Fig. 3. State machine associated to the WU-N.

The start may be provided via software by the GUI availableat the WU-S, or by a physical button present on each module.

After the start, the module moves to the state of fullfunctionality (Update Scenario), where the position commandis sent to the motor every 5 ms. In parallel to this branch ofthe state machine, the module processes continuously the statesMeasure, data Pre-processing, Feature Recognition, and Sce-nario Processing, to compute the target rotation of the motor.In addition, a state (Command Execution), corresponding tothe interaction with the master, is present, where commandsand information received from the WU-S are processed.

D. Feature Recognition

The capability to discriminate the features of human move-ment in real time represents a critical issue in the control ofpowered orthoses or exoskeletons. The proper scenario, i.e.,the subject-machine interaction model, can provide therapeuticassistance only if the volitional intention of the patient isproperly estimated. Actually, the given assistance must beprovided by the device, both in timing and level, on the baseof the recognition of these features. The discrimination of gaitphases can be performed by several methodologies based on aplethora of sensor systems. The synergistic activation is herebased on a set of footswitches, considered as a more reliablesolution in comparison with IMU data for the present pilotstudy [45]. The feature recognition is handled by the anklemodule, which collects the footswitch data. As soon as theankle module controller is capable to classify a gait phase, theinformation is sent to the knee module in real time.

E. Scenario

In the present paper, only one scenario has been tested.The scenario consisted, for each module, of providing a givenassistive torque in flexion or extension direction, according tothe estimated gait phases. The implemented scenario requiredto classify three gait phases: heel strike, mid/forefoot strike,and swing.

The generation of the assistive torque, during the mentionedphases, was accomplished by tuning in realtime the equilib-rium point of the RSEA modules such as to produce themaximum available torque in a given direction. Specifically,each local controller sets autonomously the target rotation ofthe connected RSEA according to the gait phases in Table I.

910 IEEE TRANSACTIONS ON NEURAL SYSTEMS AND REHABILITATION ENGINEERING, VOL. 25, NO. 7, JULY 2017

TABLE IGAIT FEATURES AS A FUNCTION OF THE COMBINATION OF ACTIVATED FOOTSWITCHES AND THE

RELATED EQUILIBRIUM ANGLES SET AT THE ANKLE AND KNEE MODULES

The equilibrium point variations reported in the Table I pro-duces: 1) for the knee module, the maximum torque in flexiondirection during the swing phase, and in extension directionduring the remaining two phases; 2) for the ankle module, themaximum torque in flexion direction during swing and heelstrike, and in extension direction during mid/forefoot strike.

The mid/fore strike event was on average detected at 14.3%,10.7%, and 14.5% of the gait cycle for patient #1, #2, and #3,respectively; while, the start of the swing was on averageestimated at 62.4%, 68.7%, and 70.0% of the gait cycle forpatient #1, #2, and #3.

F. Joint Module Performance Evaluation

A test bench composed of a torque meter with 10 N mfull scale (4520A10 Kistler, CH) and a Joint Module wasassembled. Two configurations of the test bench were used:the former allowed the computation of the hysteresis andthe stiffness of RSEA, while the latter the estimation of thehysteresis, maximum torque, and PID accuracy of the module.

In the first configuration, the RSEA was directly connectedto the torque meter, without the belt transmission (Fig. 4 a),The spring could rotate only at the motor side, whereas theshaft of the torque meter was fixed to the L-shape plateshowed in Fig. 4. During the test, a set of rotations from−100◦ to +100◦, with steps of 20◦, was imposed to theactuator, and the load curve was obtained. Tests were repeatedthree times. The maximum value of hysteresis resulted equalto 0.1%. The stiffness calculated as the best fit of theangle/torque curve, resulted equal to 2.2 N m/rad, comparablewith the nominal stiffness of the tested spring 2.0 Nm/rad.

In the second configuration, the RSEA was connected tothe torque meter via the belt/pulley stage (Fig. 4b). As inthe previous case, the joint rotation was locked by the torquemeter shaft. Motor rotations from −100◦ to +100◦, with stepsof 20◦, were imposed for estimating hysteresis, torque andrise PID accuracy; the test was repeated three times. Theresults of the tests corresponding to the second configurationare reported in Fig. 5.

The maximum value of the acquired torque at the joint sidewas 5.6 Nm, comparable to the assisting torque required forthe anatomical joint, and the maximum value of hysteresiswas 8.8%. The stiffness resulted 4.7 N m/rad, similar to thenominal value. The PID position-tracking accuracy was quiteindependent from the imposed rotation amplitude; the torqueslopes, in fact, were nearly the same for any angle step, withoutbeing affected by the increase of spring elastic torsion. The

Fig. 4. Test benches for the device characterization. Joint moduleconnected to the torque meter (a) without the belt transmission stage,and (b) with the belt transmission stage.

maximum velocity during the rotations was always equal tothe maximum value of about 160◦ /s, at the joint side.

III. METHODS

A. Experimental Protocol

Four typically developing children (TDC, 10.2 ± 2.6 years)and three children with CP (HC, 11.0 ± 2.6 years), affected byhemiplegia on the right lower limb, were recruited. TDC wereenrolled in order to obtain reference waveforms of lower limb

PATANÉ et al.: WAKE-UP EXOSKELETON TO ASSIST CHILDREN WITH CEREBRAL PALSY 911

Fig. 5. Second test bench configuration outputs: (A) hysteresis and(B) accuracy in rise PID as a function of the equilibrium points, the steadystate position equilibrium tracked by the PID control corresponds to thesteady state torque.

TABLE IIDETAILS OF CHILDREN INVOLVED IN THE EXPERIMENTAL PROTOCOL

joint angles. Only patients with no evident reduction in cog-nitive functions and comprehension of the verbal commandswere included. Written consent was obtained from the parentsof all children enrolled in this study and the procedure wasapproved by the Ethics and Medical Board of the “BambinoGesù” Children’s Hospital, Rome. More details of childrenwere shown in Table II.

For TDC group, the experimental protocol consisted in fiverepetitions of level walking without exoskeleton in order toestimate the gait pattern of typically developing children. ForHC group, the experimental protocol consisted in a walkingtask that included three conditions: Normal Walking (NW)

without the exoskeleton, Motor OFF (MOFF), walking withexoskeleton with motor switched off, and Motor ON (MON)walking with exoskeleton with motor switched on.

Each condition was repeated five times. During each repe-tition, patients of both groups were asked to walk at the self-selected speed. Participants were tested individually. In orderto reduce the risk of falls and to minimize interference fromexternal support, a trainer stayed close alongside or behind theparticipant. Before trials with exoskeleton, subjects were askedto wear it and to walk in the laboratory in order to adapt theirgait to the presence of the exoskeleton. Subjects that experi-enced fatigue at the end of each walking trial were allowedto rest on a chair until they felt ready to perform the nextrepetition. All subjects were able to complete all the requiredtrials. The session per participant lasted approximately 45 min.

In order to compute kinematic variables, 16 retro-reflectivemarkers were placed on the subject’s skin surface. An expertoperator provided the placement of markers according to thePlugInGait marker set based on the Davis’ protocol [46].

When the subject worn the WAKE-up, markers on the thighwere moved externally during walking with exoskeleton; itis worthy to note that the re-attachment procedure did notaffect the estimation of gait kinematics due to the new subject-specific calibration. In Fig. 1 marker placement on right lowerlimb was shown.

The optoelectronic system and the WAKE-up sensor systemwere synchronized in each MON trial taking as a reference thefirst heel-strike event detected by the RF footswitch.

B. Data Analysis

Kinematic variables consisted of sagittal angles of hip, kneeand ankle joints of the affected lower limb. Angles werenormalized for each stride. TDC data were averaged andassumed as reference waveforms.

For each patient, four kinematic indices were computed toquantify the effectiveness of the WAKE-up assistive action inNW and MON conditions. In particular, the maximum of theknee angle during the swing phase, that is αMON and αMON,and the ankle angle at the heel contact with the ground, thatare θNW and θMON. Means and Standard Deviations (SDs)for the five repetitions per participant were calculated. Dataaveraging among subjects has not been computed due to thelow inter-subject reproducibility, and to the small number ofparticipant included in the present pilot study.

C. Statistical Analysis

In order to quantify the differences among the examinedwalking conditions, we performed a 1D paired t-test based onStatistical Parametric Mapping (SPM) theory [47] to analyzestatistical differences among curves. A significance level equalto 0.05 was set, and the test was performed individually foreach patient considering the comparison: NW-MON.

IV. RESULTS AND DISCUSSIONS

Sagittal hip, knee, and ankle angles of HCs and TDCs areshown in Fig. 6. The kinematic indices related to the knee and

912 IEEE TRANSACTIONS ON NEURAL SYSTEMS AND REHABILITATION ENGINEERING, VOL. 25, NO. 7, JULY 2017

Fig. 6. Hip, knee and ankle trajectories in the sagittal plane for the three children with hemiplegia (HC) and the reference waveform obtained byaveraging the data gathered for the four typically developing children (TDC). Solid lines represent mean values and dotted lines the SDs. Trajectoriesare represented as a percentage of the gait cycle, from heel strike to next heel strike.

TABLE IIIMEANS (SDS) OF KINEMATIC INDICES FOR THE EVALUATION

OF KNEE MODULE PERFORMANCE

the ankle modules are reported in Tables III and IV, respec-tively; the outcomes of SPM are shown in Fig. 7. A generaltrend of the joint kinematics cannot be identified; therefore, asmentioned in Section III-C, we examined separately the jointangles for each patient.

A. Hip Joint

As regards HC1 (Fig. 6a), the following differences emergedfor NW in respect to the healthy reference: 1) a reductionof the maximum flexion at the heel strike; 2) a reducedcapability to extend the hip joint during the stance phase;3) a higher flexion during the swing phase; and, 4) a reducedvalue of the range of motion during the entire gait cycle.

TABLE IVMEANS (SDS) OF KINEMATIC INDICES FOR THE EVALUATION

OF ANKLE MODULE PERFORMANCE

Considering the statistical outcomes (Fig. 7a), a statisticaldifference (p<0.001) was observed between 63% and 87% ofthe gait cycle. Therefore, while the WAKE-up did not provideeffects during the stance phase, it permitted a partial recoveryof the physiological value of the hip flexion during the swingphase.

Focusing on HC2 (Fig. 6b), the kinematics related tothe NW condition, with respect to the reference waveform,showed: 1) a decrease of the flexion at the heel strike;2) a reduction of the extension before the swing phase;3) a reduction of the flexion at the end of the stride; and,4) a reduction of the range of motion during the entire gaitcycle. In NW and MON trials, statistical differences were

PATANÉ et al.: WAKE-UP EXOSKELETON TO ASSIST CHILDREN WITH CEREBRAL PALSY 913

Fig. 7. Maps of the t-value for the comparison Normal Walking (NW) and Motor On (MON) trials related to hip, knee and ankle kinematics in thesagittal plane for the three children with hemiplegia (HC). t* represents the t limit, while gray areas represent the found differences with relativep-value.

found in the following time intervals of the stride: 31%–34%(p<0.001) and 49%–52% (p=0.007), see Fig. 7b. Thus, forHC2, the WAKE-up allowed a partial recovery of the hipextension in the terminal phase of the stance; while no effectsemerged during the swing phase.

By examining HC3 (Fig. 6c), NW trials resulted the closestto the reference. From an examination of Fig. 6c, it emergeda reduction of both the range of motion and the maximumextension before the swing phase. Statistical differences werefound in the whole stride (p<0.001), with the exception of theinterval 64%–90% (Fig. 7c). Thus, it emerged that wearing theWAKE-up allowed a more physiological joint kinematics dur-ing the extension phase; conversely, it determined a worseningof the flexion at the heel strike.

Finally, for the hip joint, the delivery of an assistive torque atthe knee and ankle joints, allowed, for the examined patients,a partial recovery of the extension before the toe-off.

B. Knee Joint

As regards the HC1 (Fig. 6d), the joint kinematics obtainedduring the NW trials was characterized by a greater flexionduring the stance phase, in comparison with the referencewaveform. During the swing phase, instead, HC1 showedgait patterns comparable to TDC in NW condition. Statistical

differences were found between NW and MON at heel strike(p=0.003), and in the following time intervals of the stride:12%–26% (p<0.001), 77%–87% (p<0.001), and 93%–99%(p<0.001), Fig. 7d. These statistical differences correspondto a flexion increase during the stance phase and a flexiondecrease during the swing phase, with respect to the trials con-ducted without wearing the exoskeleton. The flexion decrease,confirmed by kinematic indices in Table III, implies that theknee module of WAKE-up was not working as an assistivedevice. This worsening of the kinematics suggests that WAKE-up inertia was not negligible for the patient HC1, especiallyduring the swing phase.

Focusing on the HC2 (Fig. 6e), the joint patterns in NWcondition were substantially different from the healthy ref-erence. Statistical differences were found in the followingintervals of the gait cycle: 1%–10% (p<0.001), 15%–48%(p<0.001), 69%–85% (p<;0.001), Fig. 7e. Therefore, theWAKE-up permitted to reach a lower extension value duringthe stance, and a noticeable flexion increase during the swingphase. The statistical difference found at the start of thestance corresponds to a flexion recovery; while the differencesobserved during the swing phase indicate a flexion incre-ment, confirmed also by the kinematic indices reported inTable III. The WAKE-up knee module, therefore, was able to

914 IEEE TRANSACTIONS ON NEURAL SYSTEMS AND REHABILITATION ENGINEERING, VOL. 25, NO. 7, JULY 2017

synergistically assist the anatomical joint during walking eventhough a full recovery was not reached in HC2.

Taking into account the kinematics in NW for HC3 (Fig. 6f),it emerged an increase of the flexion during the stance phase,a lack of extension at the toe-off, a reduced value of theflexion during the first part of the swing, and a higher valueof the flexion at the end of the swing. Comparing MON andNW, statistical differences were found in the following timeintervals of the gait cycle: 4%–8% (p<0.001) and 91%–99%(p<0.001), see Fig. 7f. The difference occurred at the startof the stride indicates a recovery of the flexion; while thedifference at the end of the swing corresponds to a worseningof the extension before the heel strike.

Thus, considering the three patients, the WAKE-up nega-tively affected the knee gait patterns especially in the swingphase, i.e., when the CP joint kinematics recorded in NWtrials was already close to reference waveforms. On thecontrary, when the WAKE-up was used for CP patient witha higher severity (HC2), it allowed a partial recovery of theknee flexion in stance and swing phases. Nevertheless, themaximum value of the flexion during the swing phase suggeststhat the maximum torque provided by the device did not permita full recovery of physiological gait pattern. To overcome thislimitation, we suppose that an additional motor at thigh levelcan help to increase the provided torque at the knee jointduring level walking. By increasing the maximum torque, theWAKE-up should be able to correctly assist the knee jointby guaranteeing a knee flexion angle closer to the healthyreference, especially during the swing phase.

C. Ankle Joint

The ankle joint kinematics for HC1 (Fig. 6g) in NWwas characterized, with respect to the healthy reference, by:1) a lower dorsiflexion at the heel strike; 2) a greater maximumplantarflexion at the end of the stance phase; and, 3) a higherplantarflexion at the end of the swing phase. As reportedin Fig. 7g, statistical differences were found in the follow-ing intervals of the stride: 43%–46% (p=0.003), 55%–59%(p<0.001), and 76%–100% (p<0.001). The observed statisti-cal differences before the toe-off correspond to a plantarflexionreduction, allowing a correct toe-clearance before the swingphase; the difference, instead, emerged in the terminal swingallowed a correct placement of the foot before the heel contact,as also confirmed by the kinematic indices in Table IV. Theimprovements were due to the assistive torque provided by theWAKE-up during the swing phase and mid/fore strike.

As concerns HC2 (Fig. 6h), the joint kinematics relatedto the NW showed a different pattern from the healthyreference. It is worth highlighting that the patient in NWcondition was characterized by the absence of the heel strike,since the toe touched the ground before the heel. We foundthat the correct sequence of gait phases was restored duringMON trials, as shown in the Table IV. Comparing NW andMON (Fig. 7h), we found statistical differences at the heelstrike (p=0.006) and in the following intervals of the stride:7%–12% (p<0.001), at 51% (p=0.003), and 75%–100%(p<0.001). The differences found at the heel strike and at

the terminal swing confirmed that the WAKE-up was able torecover the correct placement of the foot before the contactwith the ground. The difference observed immediately afterthe heel strike reflected an improvement in the plantarflexion.The recovery of the overall trend was less evident in HC2 thanin HC1, and it can be ascribed to the spasticity level of theHC2, see Table II.

As regards HC3 (Fig. 6i), the time history of ankle anglein NW was the closest to the reference one. However, thefollowing discrepancies were observed: 1) a reduction ofdorsiflexion at the heel strike; 2) a delay in the occurrence ofthe maximum plantarflexion; and, 3) a greater plantarflexionat the end of the swing. No differences between NW andMON via SPM analysis were found (Fig. 7i); so, the effectof WAKE-up on the ankle joint of HC3 can be consid-ered negligible even though a weak increment of the dor-siflexion at the heel strike was found by kinematic indices,see Table IV.

By an overall exam of the results collected with the threepatients, it emerged that the ankle module effectively works inthe prevention of an incorrect foot landing at the first phase ofthe gait cycle. The ankle module also provided benefits in theplantarflexion before the toe-off and in the correct foot place-ment during the swing phase in HC1 and HC2. In addition, inthe on-going research project we are planning to move froman assistive scenario to a rehabilitative one. Considering thealready significant results obtained by the ankle module, webelieve that the WAKE-up, upgraded to a rehabilitative device,could be useful to provide therapeutic effects if it is used foran extended period of time by stimulating the motor cortexplasticity.

V. CONCLUSION

This paper describes the exoskeleton WAKE-up designedto assist gait in children with neurological diseases, such asCerebral Palsy. Experimental outcomes obtained from differentwalking trials showed that WAKE-up assisted the here exam-ined patients for recovering the physiological gait patterns,especially at the ankle joint. Two patients, HC1 and HC2,presented a correct foot placement at the heel strike andbefore the toe-off. Conversely, the results showed that the kneemodule was not capable to fully correct the joint patterns,in particular in the recovery of the maximum value of kneeflexion during the swing phase. Only a weak improvement ofthe flexion during the swing phase was observed in HC2, whois the patient with the greatest pathology severity. In addition,the WAKE-up allowed in the three examined patients topartially recover the extension of the hip joint before thetoe-off.

On the basis of the obtained findings, we can approach therefinements of the on-going WAKE-up design by addressingthe following critical issues. Firstly, the power provided by themotor at the knee joint should be increased, always consideringthe overall device mass limitation, for example using anadditional motor at the thigh level. Secondly, to improve therobustness of the sensor system and to extend its service time,we plan to drive the motors via the IMU (Inertial MeasurementUnit) already embedded in each joint module. Actually, we

PATANÉ et al.: WAKE-UP EXOSKELETON TO ASSIST CHILDREN WITH CEREBRAL PALSY 915

demonstrated that a Hidden Markov Model algorithm, fed withangular velocity and linear acceleration of thigh and shank,has the capability to correctly and in real-time discriminatethe gait phases [48], without training the algorithm with thefootswitch outputs [49]. Thirdly, we will modify the WAKE-upcontrol system in order to allow a rehabilitative scenario.In addition, since the recovery of kinematic patterns may resultin a dynamic distortion, a quantification of the WAKE-upeffects on joint moments will be performed.

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Fabrizio Patanè received the M.S. degree inmechanical engineering from the “Sapienza” Uni-versity of Rome, Italy, in 2000, and the Ph.D.degree in mechanic measurement for engineer-ing from the University of Padua, Padua, Italy,in 2004.

Until 2014, he worked as Research Associateat the “Sapienza” University of Rome in the fieldof the mechanical and thermal measurements.He is currently Associate Professor at the Uni-versity Niccolò Cusano, Rome, Italy, where he

is the Head of the Mechanical Measurements and Microelectronics(M3-Lab) Laboratory. His research interests include sensor and mea-surement technique design and validation, and development and testingof biomedical devices.

Stefano Rossi received the 5-year Laureadegree (cum laude) in mechanical engineeringfrom the Sapienza University of Rome, Italy,in 2004, and the Ph.D. degree in mechanicaland thermal measurements with specializationin measurements in biomechanics from theUniversity of Padua, Italy, in 2008.

He currently is Assistant Professorat the Department of Economics andManagement–Industrial Engineering, Universityof Tuscia, Viterbo, Italy. His research interests

include biomechanics, design of robotic devices for rehabilitation robotsand performance analysis of measurement systems for the analysis ofhuman movements.

Fausto Del Sette received the M.S. degreein mechanical engineering from the Sapienza,University of Rome, Italy, in 2011, and thePh.D. degree in industrial production engineer-ing, in 2016.

His research interests include mechanical andaerospace measurements, 3D prototyping anddesign of robotic device for rehabilitation.

Juri Taborri received the master’s degree(cum laude) in biomedical engineering from theSapienza, University of Rome, Italy, in 2014. Hecurrently is a third year Ph.D. degree student inindustrial and management engineering.

His research interests include biomechanics,muscle synergies, machine learning algorithmsfor gait partitioning and control of robotic devicefor rehabilitation.

Paolo Cappa received the 5-year Laurea degree(cum laude) in mechanical engineering from theSapienza University, Rome, Italy, in 1980.

He was full Professor at the Departmentof Mechanical and Aerospace Engineering,Sapienza University of Rome, Rome, Italy. Healso was adjunct Professor at the Departmentof Mechanical and Aerospace Engineering, NewYork University, NY, USA, and as researcher atthe Movement Analysis and Robotics Laboratory,“Bambino Gesù” Children’s Hospital, Rome, Italy.

He passed away on August 26, 2016.