Comparison Method of Biomechanical Analysis of Trans-Tibial Amputee Gait with a Mechanical Test Machine Simulation

applied sciences

Article Comparison Method of Biomechanical Analysis of Trans-Tibial Amputee Gait with a Mechanical Test Machine Simulation

Christophe Lecomte 1,2,* , Anna Lára Ármannsdóttir 3,4, Felix Starker 1, Kristin Briem 3,4 and Sigurður Brynjólfsson 2

1 Össur ehf., Grjótháls 5, 110 Reykjavik, Iceland; [email protected] 2 Faculty of Industrial Engineering, Mechanical Engineering and Computer Science, School of Engineering and Natural Sciences, University of Iceland, 107 Reykjavik, Iceland; [email protected] 3 Department of Physical Therapy, School of Health Sciences, University of Iceland, 101 Reykjavik, Iceland; [email protected] (A.L.Á.); [email protected] (K.B.) 4 Research Centre of Movement Science, University of Iceland, 101 Reykjavik, Iceland * Correspondence: [email protected]

Featured Application: The novel mechanical testing method for prosthetic feet presented here may be used by researchers and engineers to evaluate sagittal plane stiffness characteristics. The method gives comparable results to state-of-the-art biomechanical testing and can be ben- eﬁcial for evaluating and optimizing a prosthetic foot before user testing.

Abstract: Energy-storing-and-returning prosthetic feet are frequently recommended for lower limb amputees. Functional performance and stiffness characteristics are evaluated by state-of-the-art biomechanical testing, while it is common practice for design engineers and researchers to use Citation: Lecomte, C.; test machines to measure stiffness. The correlation between user-specific biomechanical measures Ármannsdóttir, A.L.; Starker, F.; and machine evaluation has not been thoroughly investigated, and mechanical testing for ramps is Briem, K.; Brynjólfsson, S. limited. In this paper, we propose a novel test method to assess prosthetic foot stiffness properties Comparison Method of Biomechanical Analysis of in the sagittal plane. First, biomechanical data were collected on five trans-tibial users using a Trans-Tibial Amputee Gait with a variable stiffness prosthetic foot on a split-belt treadmill. Gait trials were performed on level ground ◦ Mechanical Test Machine Simulation. and on an incline and a decline of 7.5 . The same prosthetic foot was tested on a roll-over test Appl. Sci. 2021, 11, 5318. https:// machine for the three terrains. The sagittal ankle moment and angle were compared for the two test doi.org/10.3390/app11125318 methods. The dorsiflexion moment and angle were similar, while more variability was observed in the plantarflexion results. A good correlation was found for level-ground walking, while decline Academic Editor: Philip Fink walking showed the largest differences in the results of the maximum angles. The roll-over test machine is a useful tool to speed up design iterations with a set design goal prior to user testing. Received: 10 May 2021 Accepted: 31 May 2021 Keywords: prosthetic foot; variable stiffness; trans-tibial amputee; motion capture; mechanical Published: 8 June 2021 property; roll-over emulator

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- 1. Introduction iations. The human foot is a complex mechanical structure containing twenty-six bones, thirty- three joints, and more than a hundred muscles, tendons, and ligaments [1]. By comparison, while their aim is to replace lost function of the biological foot, prosthetic feet are simpler and less adaptive. Energy-storing-and-returning (ESR) prosthetic feet are regularly fitted Copyright: © 2021 by the authors. for medium to high active users. Since their first introduction in the 1980s, the design of ESR Licensee MDPI, Basel, Switzerland. This article is an open access article prosthetic feet has evolved and more complex devices have been engineered, fine-tuning distributed under the terms and their benefits [2,3]. During gait, the same basic principle can be applied to ESR prosthetic conditions of the Creative Commons feet, where the composite leaf springs in the device store energy during load application and Attribution (CC BY) license (https:// subsequently return the energy [4]. The stiffness of the elastic structures in the prosthetic creativecommons.org/licenses/by/ foot depends on the geometry and materials used, so each prosthetic foot design has its 4.0/). own stiffness characteristics dictated by its design. During our research, we counted more

Appl. Sci. 2021, 11, 5318. https://doi.org/10.3390/app11125318 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 5318 2 of 12

than a hundred different ESR prosthetic feet available for lower limb amputees [4,5]. The large number of devices makes it difficult for the certified prosthetist and orthotist (CPO) to select the most appropriate prosthetic foot for each user. Every prosthetic foot type has its own stiffness category selection chart, which complicates the decision-making process. The CPO needs to select the “right” stiffness category by considering the user’s weight, mobility level, and impact level. Next, the CPO’s work continues with the alignment process, which needs to be adapted for each user based on visual evaluation of their gait and feedback. Advanced measurement tools are available to quantify the changes in alignment [6,7]; however, the success of the alignment depends highly on the dialogue between the prosthetist and the user to find the optimal adjustment settings. Previous studies have measured the effect of ESR prosthetic foot stiffness on amputee gait [8,9]. However, the mechanical evaluation of prosthetic feet can be challenging as stiffness is design dependent [10]. Advanced test equipment for prosthetic foot testing could help during the design process and reduce the burden of user testing. Data collection on a machine representing values closer to biomechanical data during different gait tasks would help the communication between design engineers and biomechanical engineers. The ESR prosthetic feet have typically no evident center of rotation or articulation compared with the anatomical ankle joint, making it difficult to relate the investigation of stiffness characteristics to the mechanical deformations during the stance phase [11]. The functional joint center location provides a visual indication of the virtual ankle joint and depends on prosthetic foot design [12]. It is not clear how sagittal stiffness and functional joint center position are related. In this study, our objective was to assess two methods of collecting data on the same prosthetic foot. State-of-the-art biomechanical analysis of amputee gait for level-ground and ascending and descending ramp walking was compared to a mechanical test machine simulating each gait task. A quality machine assessment can be beneficial for researchers to compare different prosthetic foot designs. Additionally, the user testing could be reduced during the design phase or stiffness characterization and therefore provide an additional tool for prosthetic foot designers. Our specific aims were to (1) propose load and angle profile data for the machine roll-over test for level-ground and ramp walking, (2) collect sagittal ankle moment and ankle angle data on the test machine and on users and contrast the results with each test method, and (3) compare functional joint center positions for three different prosthetic foot stiffnesses between the two test methods for level-ground walking. We hypothesized that an advanced mechanical test method can be used to evaluate prosthetic foot stiffness to compare roll-over test data with state-of-the-art biomechanical analysis of amputee gait for all tasks. We further hypothesized that there is a linear relationship between the maximum angles collected on the machine and on the users with increasing stiffness of the prosthetic foot. For example, as the prosthetic foot stiffness increases, the maximum measured angles on the machine and on users decrease. The same prosthetic foot was used during the study to allow comparison across amputees and machine.

2. Materials and Methods 2.1. Prosthetic Foot Assessed The device used in this study is the second version of the variable stiffness ankle (VSA) prosthetic foot (Figure1). The VSA prosthetic foot allows for controlled adjustments of the sagittal prosthetic foot stiffness for each participant. The previously described ﬁrst generation [13] of the VSA prosthetic foot is a combination of an ESR base prosthetic foot and an adjustable-stiffness unit with a ﬁxed center of rotation around the pivot connection. Enhancements on the prototype device have been implemented to improve usability with a wireless communication to the control unit. The actuator used in the ankle unit is self- locking. This feature allows the control unit and the battery to be disconnected from the Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 12

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Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 12 disconnected from the prosthetic foot during the walking trial, lowering the mass by 250 g and maintaining the prosthetic foot mass with foot cover at 1.1 kg. prosthetic foot during the walking trial, lowering the mass by 250 g and maintaining the The three stiffness conditions tested during this study are schematized in Figure 2. prosthetic foot mass with foot cover at 1.1 kg. disconnected from the prosthetic foot during the walking trial, lowering the mass by 250 g and maintaining the prosthetic foot mass with foot cover at 1.1 kg. The three stiffness conditions tested during this study are schematized in Figure 2.

(a) (b)

FigureFigure 1. VSA 1. VSA prosthetic prosthetic foot drawing:foot drawing: (a) sagittal (a) sagittal view view and (andb) isometric (b) isometric view. view. (a) (b) (a) Condition 1 (softest stiffness setting)The three (b stiffness) Condition conditions 2 (mid stiffness tested setting) during this (c study) Condition are schematized 3 (stiffest setting) in Figure 2. Figure 1. VSA prosthetic foot drawing: (a) sagittal view and (b) isometric view.

(a) Condition 1 (softest stiffness setting) (b) Condition 2 (mid stiffness setting) (c) Condition 3 (stiffest setting)

Figure 2. Schematic of leaf spring support for three stiffness settings for the VSA prosthetic foot: (a) condition 1—softest; (b) condition 2—mid; and (c) condition 3—stiffest. FigureFigure 2. Schematic 2. Schematic of leaf of springleaf spring support support for threefor three stiffness stiffness settings settings for for the the VSA VSA prosthetic prosthetic foot: foot: (a ()a) condition condition 1—softest; 1—softest; (b) condition 2—mid; and (c) condition 3—stiffest. (b) condition 2—mid; and (c) condition2.2. Mechanical 3—stiffest. Testing 2.2.2.2.1. Mechanical Level Ground Testing 2.2. Mechanical Testing 2.2.1. LevelThe level-ground Ground roll-over evaluation was performed using a prosthetic test machine 2.2.1.(Shore Level Western, Ground Monrovia, CA, USA) with a mechanical setup according to International The level-ground roll-over evaluation was performed using a prosthetic test machine OrganizationThe level-ground for Standardization roll-over evaluation technical was performed specifications using ISO/TS a prosthetic 16955 test[14]. machine The aim of (Shore Western, Monrovia, CA, USA) with a mechanical setup according to International (Shore Western, Monrovia, CA, USA) with a mechanical setup according to International Organizationthis test method for Standardizationis to apply a more technical realistic specifications progression ISO/TS and magnitude 16955 [14]. of The loading aim of com- Organization for Standardization technical specifications ISO/TS 16955 [14]. The aim of this thispared test to method static istests to applyderived a more from realistic ISO 10328:2016 progression [15]. and Using magnitude this new of loading test procedure, com- test method is to apply a more realistic progression and magnitude of loading compared to paredquantification to static testsof physical derived parameters from ISO of10328:2016 the prosthetic [15]. Usingfoot can this be new calculated. test procedure, staticquantification testsSagittal derived footof from physical motion ISO 10328:2016parameters was recorded [of15 the]. with Using prosthetic a video this new footcameratest can procedure,(Rx0be calculated. II, Sony, quantification New York City, of physicalNY, SagittalUSA). parameters Thefoot testmotion of is theintended was prosthetic recorded to simulate foot with can a videoa be heel-to-toe calculated. camera (Rx0roll-over II, Sony, walking New cycle.York City, The test NY,sampleSagittal USA). is foot subjectedThe motiontest is to intended wasboth recordedan toM-shaped simulate with forca aheel-to-toe videoe and the camera roll-over motion (Rx0 of walking II,a rotating Sony, cycle. New plate The Yorksynchro- test City,samplenized NY, USA).with is subjected the The vertical test to both is force intended an M-shapedprofile. to The simulate forc platee and angle a the heel-to-toe motion starts at of roll-over− a20° rotating for heel walking plate strike synchro- cycle.and ends Thenized testat +40° sample with for the push-off, is vertical subjected andforce to the bothprof maximumile. an The M-shaped plate force angle force applied starts and theatis −105020° motion for N. heelThe of a strike rotatingmachine and plateendsset-up is ◦ synchronizedatdescribed +40° for withinpush-off, Figure the vertical and3. the force maximum profile. force The applied plate angle is 1050 starts N. atThe−20 machinefor heel set-up strike is ◦ anddescribed ends at +40 in Figurefor push-off, 3. and the maximum force applied is 1050 N. The machine set-up is described in Figure3. Markers were placed on the cosmetic foot shell and the pyramid adapter of the foot, as shown in Figure 5, and a roll-over test was performed without shoes. Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 12

Appl.Appl. Sci. Sci. 20212021,, 1111,, x 5318 FOR PEER REVIEW 4 4of of 12 12

(a) (b)

Figure 3. Schematic illustration of foot roll-over testing according to ISO/TS 16955: (a) machine setup with a VSA prosthetic foot sample (yellow arrows showing force and rotation); (b) vertical force on ball joint and rotation of the tilt-table, as functions of stance phase time.( a) (b) FigureFigure 3. 3. SchematicSchematic illustration illustration of of foot foot roll-over roll-over testing testing according to ISO/TS 16955: 16955: ( (aa)) machine machine setup setup with with a a VSA VSA prosthetic prosthetic footfoot sample sample (yellow (yellow arrows arrows showing showingMarkers force force andwere and rotation); rotation); placed on( (bb)) thevertical vertical cosmetic force force onfoot on ball ball shell joint joint and and and the rotation rotation pyramid of of the the adapter tilt-table, tilt-table, of theas as foot, functionsfunctions of of stance stance phase phase time. time.as shown in Figure 5, and a roll-over test was performed without shoes.

2.2.2.2.2.2.Markers RampRamp Input Inputwere placed on the cosmetic foot shell and the pyramid adapter of the foot, as shown in Figure 5, and a roll-over test was performed without shoes. RampRamp testingtesting isis notnot partpart ofof thethe ISOISO 16955 16955 nor nor ISO ISO 22675 22675 standards standards [ 16[16].]. However,However, machinemachine inputs inputs can can be be adjusted adjusted to to simulate simulate different different types types of of walking walking patterns patterns and and allow allow 2.2.2. Ramp Input evaluationevaluation ofof otherother types of of devices devices than than prosthetic prosthetic feet feet [17,18]. [17,18 ].Vertical Vertical force force input input on onthe themachine machineRamp for testing for the the simulatedis simulated not part ramp of ramp the tasks tasksISO was16955 was calculated calculated nor ISO 22675using using standardspreviously [16]. collectedcollected However, over-over- machinegroundground ramp inputsramp data data can of beof trans-tibial trans-tibialadjusted to amputees. amputees.simulate different TheseThese vertical vertical types force offorce walking data data were werepatterns resampled resampled and allow and and evaluationadjustedadjusted to toof keep keepother smooth smooth types of loading loading devices and and than unloading. unloading. prosthetic The Thefeet stance[17,18].stance timetime Vertical waswas force modiﬁedmodified input to to on allow allow the machinethethe test test machine machinefor the tosimulated to reach reach the the ramp correct correct tasks loads loads was and and calculated plate plate angle angle using synchronization. synchronization. previously collected Machine Machine over- input input groundproﬁlesprofiles ramp for for the the data test test of are are trans-tibial described described amputees. in in Figure Figure4 These. 4. vertical force data were resampled and adjusted to keep smooth loading and unloading. The stance time was modified to allow

theMachine test Forces machine to reach the correct loads and plate angleMachine synchronization. Angles Machine input 1400 50 profiles for the test areLevel described in Figure 4. Level Incline 40 Incline 1200 Decline Decline

Machine Forces 30 Machine Angles 14001000 50 Level Level 20 Incline 40 Incline 1200800 Decline Decline 10 30 1000600 0 20 800400 -10 10 600 200 -20 0

400 0 -30 0 100 200 300 400 500 600 700 800 900 -10 0 200 400 600 800 1000 1200 Time [s] Time [s] 200 -20 (a) (b) 0 -30 FigureFigure 4. 4.0Machine Machine 100 200 inputs inputs 300 for for 400 vertical vertical 500 force 600force 700and and tilt-table 800tilt-table 900 rotation: rotation: ( a(a)) level-ground, 0level-ground, 200 incline, 400incline, and600 and decline decline 800 vertical vertical 1000 force force 1200 input; input; Time [s] Time [s] (b(b)) level-ground, level-ground, incline, incline, and and decline decline tilt tilt table table rotation rotation input. input. (a) (b) Figure 4. Machine inputs for2.3. vertical User force Testing and tilt-table rotation: (a) level-ground, incline, and decline vertical force input; (b) level-ground, incline, and decline tilt table rotation input. A randomized, participant-blinded crossover study design was used. The order of stiffness settings during data collection was randomly chosen, and both the user and the investigator were blinded to the settings throughout. The user testing protocol was Appl. Sci. 2021, 11, 5318 5 of 12

approved by the Icelandic National Bioethics Committee, and all participants gave written informed consent to participate in the study.

2.3.1. Users The users were recruited at the Össur domestic workshop. The inclusion criteria were (1) 18 years of age or older, (2) unilateral trans-tibial amputation for more than 1 year, (3) prosthetic user for more than 1 year, (4) having no issue with the current socket. The exclusion criteria were (1) user body mass less than 50 kg or higher than 120 kg and (2) patient prosthetic build height lower than 175 mm, to allow correct ﬁtting of the VSA prosthetic foot.

2.3.2. User Data Collection and Processing Five male users (age: 57 ± 11 years, mass: 97.5 ± 8.7 kg) participated in and com- pleted the study (Table1). The users walked at 0.8 m/s during level-ground and ramp ascent/descent data collection. All gait trials were performed on an instrumented dual-belt treadmill (Bertec, Columbus, OH, USA), and an eight-camera-based 3D motion capture system (Qualisys AB, Gothenburg, Sweden) was used to track markers for defined body segments (400 Hz). A six DoF model was constructed, and data were processed and analyzed in Visual 3D (C-motion, Germantown, MD, USA). A fourth-order low-pass But- terworth filter was applied for the kinematic and kinetic data, with a cut-off at 6 Hz and 10 Hz, respectively [19]. All users wore their daily prosthetic socket. The prototype foot tested was the only alteration to each user’s prosthetic set-up. The prosthetic foot was aligned for condition 1 (mid-stiffness) by a certified prosthetist with more than ten years of experience. The order of the stiffness conditions tested was randomized for each user who was blinded of the stiffness condition. The same VSA ankle unit was used for all trials. The change in the sole blade size and side was the only change to the ESR base foot. This modification was done to maintain the same foot length as the participant’s prescribed foot. The treadmill ramp angle was set to 7.5◦ in accordance with previous ramp angle protocols [3,20,21]. The test time was set to allow the collection of 15 consecutive steps on each side at constant speed. The study was performed in two visits for each user. During the first session, each participant was fitted with the VSA prosthetic foot, which was aligned by the CPO. During the second visit, biomechanical data were collected. All users used the same type of shoe (Viking, Norway) during both test sessions.

Table 1. Characteristics of the users enrolled in the study.

User A B C D E Gender Male Male Male Male Male Age (years) 53 60 42 59 72 Height (cm) 179 187 177 180 182 Mass (kg) 103 82 100 100 102 Foot size (cm) 27 27 26 27 27 BMI (kg/m2) 32.1 24 31.9 30.9 30.8 Amputated side Right Right Right Left Left Time since amputation (years) 16 43 9 4 8 Vari-Flex with Prescribed prosthetic foot Pro-Flex Pivot Vari-Flex XC Pro-Flex XC Pro-Flex XC Quick Align Prosthetic foot stiffness category Cat. 5 Cat. 5 Cat. 6 Cat. 6 Cat. 6 Cause of amputation Trauma Trauma Trauma Trauma Trauma Appl.Appl. Sci. Sci.2021 2021, ,11 11,, 5318 x FOR PEER REVIEW 6 ofof 1212

2.3.3.2.3.3. FunctionalFunctional JointJoint CenterCenter CalculationCalculation TheThe functionalfunctional joint center center (FJC) (FJC) visually visually indicates indicates the the prosthetic prosthetic foot’s foot’s center center of ro- of rotationtation [11,12,22] [11,12,22 and] and provides provides additional additional in informationformation to tohelp help understand understand the the ESR ESR defor- de- formationmation characteristics characteristics during during the the stance stance phase. phase. The Theankle ankle moment moment and andankle ankle angle angle data datacommonly commonly collected collected during during user usertesting testing describe describe the prosthetic the prosthetic foot’s foot’s stiffness stiffness character- char- acteristics.istics. In contrast, In contrast, the FJC’s the FJC’s position position provides provides an average an average intersection intersection location location of the of foot the footand andtibial tibial segments segments rotation rotation during during the thestance stance phase. phase. The TheFJC FJCis an is estimation an estimation of the of thevir- virtualtual joint joint center center and and therefore therefore enhances enhances the the in interpretationterpretation of of the ESR flexibleflexible leafleaf springspring deformationdeformation duringduring roll-over.roll-over. TheThe functionalfunctional jointjoint centercenter waswas calculatedcalculated forfor bothboth machinemachine andand biomechanicalbiomechanical tests.tests. MarkerMarker positionspositions werewere processedprocessed usingusing thethe videovideo trackingtracking softwaresoftware TemaTema (Image(Image Systems,Systems, Linköping,Linköping, Sweden)Sweden) forfor machinemachine tests.tests. TheThe markermarker positionspositions forfor thethebiomechanical biomechanical teststests werewere exportedexported asas an average for the the five five user users.s. The The points points coordinates coordinates were were projected projected to tothe the sagittal sagittal plane plane (Figure (Figure 5).5 ).The The rigid-body rigid-body assumption assumption between between markers markers was was used used for forthe thecalculation. calculation. The Thecenter center of zero of zerovelocity velocity was wascalculated calculated during during the stance the stance phase phase using usinga MATLAB a MATLAB custom custom script script(MathWorks, (MathWorks, Natick NatickMA, USA) MA, between USA) between M1–M2 M1–M2 and M3–M4 and M3–M4segments segments for the test for themachine test machine and between and between segments segments Mknee-M MAnkleknee -MandAnkle Mheeland-Mtoe M segmentsheel-Mtoe segmentsfor the biomechanical for the biomechanical test. The position test. The of position the instantaneous of the instantaneous center of centerrotation of was rotation then wasaveraged then averaged for the stance for the phase stance over phase five over step fives. The steps. X Theand XY andcoordinates Y coordinates of the of FJC the were FJC werecompared compared between between the two the test two methods. test methods.

FigureFigure 5.5. SchematicSchematic ofof markermarker positionspositions usedused forfor thethe functionalfunctional jointjoint center:center: (left)(left) useruser testtest andand (right)(right) machinemachine test.test.

2.4.2.4. StatisticsStatistics AA PearsonPearson correlation was was selected selected to to cont contrastrast maximum maximum sagittal sagittal prosthetic prosthetic foot foot an- anglesgles between between the the machine machine test test and and biomecha biomechanicalnical data. Analysis waswas performedperformed usingusing MATLAB.MATLAB. StatisticalStatistical analysisanalysis waswas notnot performedperformed onon thethe mechanicalmechanical machinemachine data. data.

3.3. ResultsResults 3.1. Force Displacement–Static 3.1. Force Displacement–Static The prototype VSA prosthetic foot was ﬁrst evaluated in a state-of-the-art stiffness The prototype VSA prosthetic foot was first evaluated in a state-of-the-art stiffness test, based on AOPA guidelines [20]. Stiffness results are shown in Figure6. The heel test, based on AOPA guidelines [20]. Stiffness results are shown in Figure 6. The heel dis- displacement changes between stiffness settings ranged from 17.8 to 18.5 mm, while the placement changes between stiffness settings ranged from 17.8 to 18.5 mm, while the keel keel displacement ranged from 39.5 to 43.6 mm. This test was performed to provide a baselinedisplacement stiffness ranged measure from of 39.5 the to prosthetic 43.6 mm. foot’s This test stiffness was performed for the three to conditions.provide a baseline stiffness measure of the prosthetic foot’s stiffness for the three conditions. Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 12

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Load [N] Load [N]

Figure 6. VSA prosthetic foot load versus displacement heel and keel stiffness test results for condition 1 (softest), condition 2 (mid), and condition 3 (stiffest). Figure 6. VSA prosthetic foot load versus displacement heel and keel stiffness test results for Figure 6. VSA prosthetic foot load versus displacement heel and keel stiffness test results for condi- condition 1 (softest), condition 2 (mid), and condition 3 (stiffest). tion3.2. 1 (softest), Ankle Sagittalcondition Moment-Test 2 (mid), and Machinecondition & 3 (stiffest).Biomechanical Results 3.2.3.2.1. Ankle Level Sagittal Ground Moment-Test Machine & Biomechanical Results 3.2. Ankle Sagittal Moment-Test Machine & Biomechanical Results 3.2.1.The Level results Ground for the ankle function for the machine and biomechanical tests are con- 3.2.1. Level Ground trastedThe in results Figurefor 7. The the ankle ankle moment function and for angl the machinees were averaged and biomechanical for the five users tests arefor con-each trastedstiffnessThe results in condition. Figure for7 the. The Theankle ankle machine function moment test for and allows the angles machine the were presentation and averaged biomechanical of for data the ﬁvefor tests prosthetic users are for con- each foot trastedstiffnessankle in jointFigure condition. angles 7. The and ankle The moments machine moment in test and a comparable allows angles thewere presentationlayout averaged to the for ofusers’ the data five biomechanical for users prosthetic for each data. foot stiffnessankleThe load condition. joint cell angles moment The and machine momentswas moved test in allows ato comparable the thepivot presentation point layout location to theof data users’of the for biomechanicalVSA prosthetic ankle tofoot allow data. ankleTheanalysis joint load angles with cell momentthe and user moments wasresults moved in on a allcomparable to tasks. the pivot layout point to location the users’ of thebiomechanical VSA ankle todata. allow Theanalysis load cell with moment the user was results moved on to all the tasks. pivot point location of the VSA ankle to allow analysis with the user results on all tasks. 1.2 1.2

1 1 1.2 1.2

0.8 0.8 1 1

0.6 0.6 0.8 0.8

0.4 0.4 0.6 0.6

0.2 0.2 0.4 0.4

0 0 0.2 0.2 COND 1 COND 1 -0.2 COND 2 -0.2 COND 2 0 0 COND 3 COND 3 -0.4 COND 1 -0.4 COND 1 -0.2 -10 -5 0 5 10COND 15 2 20 -0.2 -10-50 5 101520COND 2 Ankle Angle [°] COND 3 Ankle Angle [°] COND 3 -0.4 -0.4 -10 -5 0 5(a) 10 15 20 -10-50 5(b) 101520 Ankle Angle [°] Ankle Angle [°] FigureFigure 7.7.Ankle Ankle momentmoment versus(versusa) ankleankle angleangle duringduring level-groundlevel-ground walkingwalking forfor threethreestiffness stiffness(b) settings-condition settings-condition 11 (softest),(softest), conditioncondition 22 (mid),(mid), andand conditioncondition 33 (stiffest):(stiffest): ((aa)) testtest machinemachine resultsresults andand ((b)) biomechanicalbiomechanical test results–average of ﬁvefive users. Figure 7. Ankle moment versus ankle angle during level-ground walking for three stiffness settings-condition 1 (softest), condition 2 (mid), and condition 3 (stiffest): (a) test machine results and (b) biomechanical test results–average of five users. 3.2.2.3.2.2. RampRamp The incline-walking prosthetic foot ankle moment and angle for the machine and 3.2.2. RampThe incline-walking prosthetic foot ankle moment and angle for the machine and biomechanicalbiomechanical tests are depicted in in Figure Figure 88.. The decline-walking prosthetic foot ankle The incline-walking prosthetic foot ankle moment and angle for the machine and moment and angle results for the machine and biomechanical tests are shown in Figure9. biomechanical tests are depicted in Figure 8. Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 12

Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 12 Appl. Sci. 20211.2, 11, 5318 1.2 8 of 12

1 1

0.8 0.8 1.2 1.2

0.6 0.6 1 1

0.4 0.4 0.8 0.8

0.2 0.2 0.6 0.6

0 COND 1 0 COND 1 COND 2 COND 2 0.4 0.4 COND 3 COND 3 -0.2 -0.2 0.2-5 0 5 10 15 20 0.2-5 0 5 10 15 20 Ankle Angle [°] Ankle Angle [°] 0 COND 1 0 COND 1 (a) COND 2 (b) COND 2 COND 3 COND 3 Figure -0.28. Ankle moment versus ankle angle during 7.5° incline walkin-0.2g for three stiffness settings-condition 1 (softest), con- -5 0 5 10 15 20 -5 0 5 10 15 20 dition 2 (mid), and conditionAnkle 3 (stiffest): Angle [°] (a) test machine results and (b) biomechanical testAnkle results–average Angle [°] of five users. (a) (b) The decline-walking prosthetic foot ankle moment and angle results for the machine Figure 8. 8. AnkleAnkle moment moment versus versusand anklebiomechanical ankle angle angle during during tests 7.5° 7.5 areincline◦ incline shown walkin walking ing Figure for forthree 9. three stiffness stiffness settings- settings-conditioncondition 1 (softest), 1 (softest), conditioncondition 2 (mid), 2 (mid), and andcondition condition 3 (stiffest): 3 (stiffest): (a) test (a) machine test machine results results and and(b) biomechanical (b) biomechanical test results–average test results–average of five of users. ﬁve users.

1.2 The decline-walking prosthetic foot ankle moment and angle results for the machine 1 and biomechanical tests are shown in Figure 9.

0.8

0.6 1.2 0.4 1 0.2 0.8 0 0.6 -0.2 Ankle Moment [N.m/kg] Moment Ankle 0.4 -0.4 0.2 COND 1 -0.6 COND 2 0 COND 3 -0.8 -0.2-15 -10 -5 0 5 10 15 20 Ankle Moment [N.m/kg] Moment Ankle Ankle Angle [°] -0.4 COND 1 -0.6 (a) COND 2 (b) COND 3 ◦ FigureFigure -0.89. 9. AnkleAnkle moment moment versus versus ankle ankle angle angle during during 7.5° 7.5 declinedecline walk walkinging for three for three stiffness stiffness setting settings-conditions-condition 1 (softest), 1 (softest), con- -15 -10 -5 0 5 10 15 20 ditioncondition 2 (mid), 2 (mid), and andcondition conditionAnkle 3 (stiffest): Angle 3 (stiffest): [°] (a) test (a) machine test machine results results and ( andb) biomechanical (b) biomechanical test results–average test results–average of five of users. five users. (a) (b) TheThe maximum ankleankle anglesangles for for plantarflexion plantarflexion and and dorsiflexion dorsiflexion were were recorded recorded for each for Figure 9. Ankle moment versuseachstiffness ankle stiffness angle condition, condition,during task, 7.5° task,decline and testand walk method.testing me forthod. three The resultsstiffnessThe results are setting presented ares-condition presented in Table1 (sinoftest), Table2. con- 2. dition 2 (mid), and condition 3 (stiffest): (a) test machine results and (b) biomechanical test results–average of five users. TableTable 2. 2. MaximumMaximum ankle ankle angles angles for for plantarflexion plantarflexion and dorsifle dorsiflexionxion in the test machine and biomechanical tests. The maximum ankle angles for plantarflexion and dorsiflexion were recorded for Maximum ankle Plantarflexion (°)◦ Maximum Ankle Dorsiflexion◦ (°) Maximumeach Ankle stiffness Plantarflexion condition, ( ) task, and test method. Maximum The results Ankle are Dorsiflexion presented ( in) Table 2. Condition Correlation Correlation Condition Machine Test BiomechanicalBiomechanical Test Correlation p Machine Test BiomechanicalBiomechanical Test Correlation p Machine Test p Machine Test p Test CoefficientCoefficient Test CoefficientCoefficient COND 1:Table Level 2. Maximum−9.6 ankle angles− 5.9for plantarflexion and dorsiflexion in the17.2 test machine and16.5 biomechanical tests. COND 1: Level −9.6 −5.9 17.2 16.5 COND 2: Level −9.4 −5.7 0.961 0.179 16.1 15.9 0.998 0.043 COND 2: Level −9.4Maximum ankle− Plantarflexion5.7 (°)0.961 0.179 16.1 Maximum Ankle 15.9 Dorsiflexion (°)0.998 0.043 CONDCondition 3: Level −−8.88.8 −−5.55.5 Correlation 14.8 15.015.0 Correlation Machine Test Biomechanical Test p Machine Test Biomechanical Test p COND 1: Incline −4.6 −0.2 Coefficient 17.4 18,4 Coefficient COND 1: Incline −4.6 −0.2 17.4 18,4 COND 2: Incline −4.2 −0.5 −0.564 0.619 16.6 17.7 0.995 0.065 CONDCOND 2:1: InclineLevel −−9.64.2 −−5.90.5 −0.564 0.619 16.617.2 17.716.5 0.995 0.065 CONDCOND 3: 2: Incline Level −−3.69.43.6 −−0.45.70.4 0.961 0.179 15.816.1 16,716,715.9 0.998 0.043 COND 1: Decline −14.8 −8.0 13.5 15.7 CONDCOND 1: 3: Decline Level −8.814.8 −−5.58.0 13.514.8 15.715.0 COND 2: Decline −14.5 −7.8 0.963 0.173 12.6 15.1 0.986 0.106 COND 2:1: DeclineIncline −4.614.5 −−0.27.8 0.963 0.173 12.617.4 15.1 18,4 0.986 0.106 CONDCOND 3:2: DeclineIncline −−14.14.214.1 −−7.70.57.7 −0.564 0.619 12.216.6 14.614.617.7 0.995 0.065 COND 3: Incline −3.6 −0.4 15.8 16,7 COND 1: Decline −14.8 −8.0 13.5 15.7 COND 2: Decline −14.5 −7.8 0.963 0.173 12.6 15.1 0.986 0.106 COND 3: Decline −14.1 −7.7 12.2 14.6 Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 12

Appl. Sci. 2021, 11, 5318 9 of 12

3.2.3. Functional Joint Center 3.2.3.Functional Functional joint Joint center Center positions for biomechanical and machine data are depicted in Figure 10.Functional Since the joint machine center test positions was performe for biomechanicald without shoes, and machine the pivot data joint are of depictedthe VSA in unitFigure was used10. Since to align the machineresults between test was the performed two test methods. without shoes, the pivot joint of the VSA unit was used to align results between the two test methods.

FigureFigure 10. 10.FunctionalFunctional joint joint center center location location for forthree three stiffness stiffness settings, settings, condition condition 1 (softest), 1 (softest), condition condition 2 (mid),2 (mid), and and condition condition 3 (stiffest), 3 (stiffest), calculated calculated for formachine machine test test and and biomechanical biomechanical tests tests during during level- level- groundground walking. walking.

4. Discussion4. Discussion ProstheticProsthetic foot foot stiffness stiffness category category significan significantlytly impacts impacts biomechanical biomechanical variables; variables; how- how- everever after after some some accommodation accommodation time, time, the user’s the user’s perception perception of stiffness of stiffness becomes becomes inconsistent inconsis- [21].tent While [21]. a Whilefew studies a few have studies evaluated have evaluated prosthetic prosthetic foot functions foot using functions quasi-static using quasi-static machine testingmachine methods, testing limited methods, work limited has been work done has on been dynamic done testing on dynamic [22]. In testing addition, [22 ].studies In addition, studies focusing on mechanical testing have been limited to heel and keel loading focusing on mechanical testing have been limited to heel and keel loading in separate mo- in separate motions [23,24]. Hansen et al. showed changes in roll-over characteristics tions [23,24]. Hansen et al. showed changes in roll-over characteristics between different between different prosthetic feet for level-ground ambulation using a mechanical testing prosthetic feet for level-ground ambulation using a mechanical testing apparatus [25]. Sim- apparatus [25]. Similar studies have investigated the impact of prosthetic foot sagittal ilar studies have investigated the impact of prosthetic foot sagittal stiffness on trans-tibial stiffness on trans-tibial amputees by measuring oxygen consumption and ankle push-off amputees by measuring oxygen consumption and ankle push-off power [26,27]. power [26,27]. This study presents a novel approach to evaluating prosthetic feet for ramp ascent This study presents a novel approach to evaluating prosthetic feet for ramp ascent and descent using a mechanical roll-over test. and descent using a mechanical roll-over test. In the comparison of biomechanical test results and test machine simulation for level- In the comparison of biomechanical test results and test machine simulation for ground walking, we found a strong correlation between the ankle angles during dorsi- level-ground walking, we found a strong correlation between the ankle angles during flexion.dorsiflexion. However, However, the plantarflexion the plantarflexion angle was angle larger was in larger the machine in the machine test than test in thanthe bio- in the mechanicalbiomechanical test, which test, which may maybe because be because the machine the machine test testwas was performed performed without without shoes, shoes, whilewhile the the biomechanical biomechanical test test was was performed withwith shoes. shoes. The The cushioning cushioning provided provided by shoesby shoesmight might have have affected affected the the correlation correlation between between the twothe two test test methods. methods. ForFor ramp ramp ascent, ascent, the the maximum maximum dorsiflexion dorsiflexion prosthetic prosthetic foot foot angles angles obtained obtained during during machinemachine and and biomechanical biomechanical testing testing were were comparable comparable between thethe threethree stiffness stiffness conditions. conditions.However, However, the the plantarflexion plantarflexion angles angles were were lower lower in in biomechanical biomechanical test test data data versus versus the themachine machine test test data. data. This This result result could could be be explained explained by by the the participants’ participants’ gait gait strategy strategy for for ramp rampascent ascent to ensureto ensure flexion flexion angles angles and and moments moments up up the the movement movement chain chain of theof the lower lower limb limbduring during weight weight acceptance acceptance [28 ].[28]. This This may may be achievedbe achieved by, forby, example,for example, initiating initiating a stance a stancewith with the footthe foot flat toflat ease to ease the momentthe moment on the on knee.the knee. Appl. Sci. 2021, 11, 5318 10 of 12

During ramp descent, a variability was seen in plantarflexion between the two test methods. The shoe heel cushion likely affected this result. Additionally, the input on the tilt-table angle may have a significant impact on the increased sagittal moment on this terrain. A lower tilt-table angle at heel strike would generate a decreased sagittal reaction moment, corresponding more to the biomechanical test data. The functional joint center is an additional parameter to help interpret the prosthetic foot response during roll-over. Our calculation method of the functional joint center can describe its change of position, depending on the prosthetic foot stiffness. However, the FJC position calculated using the machine was on average, 4.5 cm more posterior and 1 cm more proximal than the FJC position calculated using level-ground biomechanical gait measurements. The FJC position moved proximally and posteriorly with decreasing stiffness of the VSA. This change in position can be explained by the decreasing stiffness constant of the VSA ankle unit from stiffest to softest, as a result of which, the FJC moves closer to the VSA pivot point. Our calculation of the FJC position is sensitive to the marker position collected and is limited to the sagittal plane. The novelty of this study is the use of a machine-based roll-over test for a prosthetic foot and evaluation for three types of terrains, level, ramp ascent, and ramp descent, for three different prosthetic foot stiffnesses. To the best of our knowledge, this is the first study providing a machine test input for common functional tasks such as incline/decline walking. The methods described to compare biomechanical and machine data in the sagittal plane for ankle moment and angles are promising, as demonstrated by the correlation seen for dorsiflexion angle data between the two test methods. The output measurements from the test machine used in conjunction with the camera provide a good surrogate for state-of-the-art biomechanical data generally obtained from gait analysis. The roll-over machine test allows researchers and prosthetic foot designers to evaluate device characteristics in a controlled and reproducible manner. Researchers could use this tool to compare different types of devices or adjust device properties before starting trials on users, ultimately reducing the burden of user testing. The limitations of the study presented are the small sample size for the user data collected and the use of a 2D camera to track the prosthetic foot angle on the test machine. Best-practices recommendations from Michelini were adopted to improve the video record reliability [29]. Future work should focus on the machine input parameters for heel strike during ramp-walking simulation to achieve an improved plantarflexion correlation. The VSA prototype foot was well accepted, and all users engaged in the study com- pleted the treadmill trial. The VSA unit allows an assessment of different stiffness settings while the user is wearing the device. This reduces the time and effort to change to the prosthetic foot stiffness category or alignment. In addition, an adaptable prosthetic foot could be beneficial for different terrains and tasks, as previously reported by prosthetic users [30]. Our proposed method is not a fully comprehensive analysis that describes all the prosthetic foot characteristics. However, it allows for calculation and post-processing for different gait tasks, with multiple data sets, such as moments and forces, combined with the deformations of the ESR foot flexible elements. Machine testing combined with a variable stiffness unit can be beneficial for researchers and prosthetic foot designers to better understand prosthetic foot properties and possibly the specific user preferences. Shepherd and Rouse showed the interaction between the CPO and the user when comparing ankle–foot stiffness preference [31]. We foresee the potential of these types of machine testing results to be optimized for user-specific input profiles. A user-specific gait could be used as input for the machine, and prosthetic foot stiffness can be changed while testing on the equipment to achieve a desired ankle range of motion for the user. Zelik et al.’s ankle elasticity model could be used in conjunction with a variable stiffness prosthetic foot to improve the economy of ankle- or hip-powered walking [32]. Following these two paths in future research, a more individualized prosthetic foot prescription depending on user gait could be developed. The high gait variability Appl. Sci. 2021, 11, 5318 11 of 12

between users, as described by Zelik et al., during incline walking, may call for user-specific prosthetic foot adjustability [33]. Safaeepour et al. described the change in ankle quasi- stiffness, depending on the gait phase and speed of healthy subjects [34]. The ability for users to adjust their prostheses for increased dorsiflexion, e.g., during faster walking, may be considered a significant advantage when using a variable stiffness prosthetic foot.

5. Conclusions We successfully contrasted two different methods of measuring prosthetic foot sagittal movement for level ground, ramp ascent, and ramp descent and proposed input parameters for a machine-based testing for ramps. Our hypothesis regarding the use of machine testing for different terrains was conﬁrmed. The proposed test method and input parameters to evaluate prosthetic feet under ramp ascent and ramp descent conditions is feasible and provides meaningful data that could allow comparison with prosthetic user gait for the speciﬁc terrains. This method can be valuable for researchers and designers to evaluate devices prior to user testing and lower the burden on users. Additionally, this method can help in distinguishing prosthetic foot design stiffness characteristics in the sagittal plane.

Author Contributions: Conceptualization, K.B. and S.B.; methodology, C.L. and F.S and A.L.Á.; formal analysis, C.L.; test machine investigation, F.S.; biomechanical investigation, A.L.Á.; data curation, C.L.; writing—original draft preparation, C.L.; writing—review and editing, F.S. and A.L.Á.; supervision, K.B. and S.B.; funding acquisition, S.B. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Technology Development Fund at the Icelandic Center for Research (grant no. 163805-0612). Institutional Review Board Statement: The study was conducted according to the guidelines of the Declaration of Helsinki and approved by the Institutional Review Board (or Ethics Commit- tee) of Lyfjastofnun–Icelandic Medicines Agency (protocol code CII20190907102, date of approval 04.12.2019). Informed Consent Statement: Informed consent was obtained from all the subjects involved in the study. Acknowledgments: Test machine for measurements were provided by Össur ehf. Conﬂicts of Interest: Christophe Lecomte and Felix Starker are employees of Össur ehf. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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