Comparison Method of Biomechanical Analysis of Trans-Tibial Amputee Gait with a Mechanical Test Machine Simulation
Comparison Method of Biomechanical Analysis of Trans-Tibial Amputee Gait with a Mechanical Test...
Lecomte, Christophe;Ármannsdóttir, Anna Lára;Starker, Felix;Briem, Kristin;Brynjólfsson, Sigurður
2021-06-08 00:00:00
applied sciences Article Comparison Method of Biomechanical Analysis of Trans-Tibial Amputee Gait with a Mechanical Test Machine Simulation 1 , 2 , 3 , 4 1 3 , 4 Christophe Lecomte * , Anna Lára Ármannsdóttir , Felix Starker , Kristin Briem and Sigurður Brynjólfsson Össur ehf., Grjótháls 5, 110 Reykjavik, Iceland; fstarker@ossur.com Faculty of Industrial Engineering, Mechanical Engineering and Computer Science, School of Engineering and Natural Sciences, University of Iceland, 107 Reykjavik, Iceland; sb@hi.is Department of Physical Therapy, School of Health Sciences, University of Iceland, 101 Reykjavik, Iceland; ala20@hi.is (A.L.Á.); kbriem@hi.is (K.B.) Research Centre of Movement Science, University of Iceland, 101 Reykjavik, Iceland * Correspondence: clecomte@ossur.com 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 in the sagittal plane. First, biomechanical data were collected on five trans-tibial users using a Biomechanical Analysis of variable stiffness prosthetic foot on a split-belt treadmill. Gait trials were performed on level ground Trans-Tibial Amputee Gait with a and on an incline and a decline of 7.5 . The same prosthetic foot was tested on a roll-over test Mechanical Test Machine Simulation. machine for the three terrains. The sagittal ankle moment and angle were compared for the two test Appl. Sci. 2021, 11, 5318. https:// 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. prosthetic feet has evolved and more complex devices have been engineered, fine-tuning This article is an open access article their benefits [2,3]. During gait, the same basic principle can be applied to ESR prosthetic distributed under the terms and feet, where the composite leaf springs in the device store energy during load application and conditions of the Creative Commons subsequently return the energy [4]. The stiffness of the elastic structures in the prosthetic Attribution (CC BY) license (https:// foot depends on the geometry and materials used, so each prosthetic foot design has its creativecommons.org/licenses/by/ own stiffness characteristics dictated by its design. During our research, we counted more 4.0/). 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 (Figure 1). 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 Appl. Sci. 2021, 11, 5318 3 of 12 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) Figure 1. VSA prosthetic foot drawing: (a) sagittal view and (b) isometric view. Figure 1. VSA prosthetic foot drawing: (a) sagittal view and (b) isometric view. (a) (b) The three stiffness conditions tested during this study are schematized in Figure 2. (a) Condition 1 (softest stiffness setting) (b) Condition 2 (mid stiffness setting) (c) Condition 3 (stiffest setting) 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. Figure 2. Schematic of leaf spring support for three stiffness settings for the VSA prosthetic foot: (a) condition 1—softest; 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. 2.2. Mechanical Testing (b) condition 2—mid; and (c) condition 3—stiffest. 2.2.1. Level Ground 2.2. Mechanical Testing 2.2. Mechanical Testing The level-ground roll-over evaluation was performed using a prosthetic test machine 2.2.1. Level Ground 2.2.1. Level Ground (Shore Western, Monrovia, CA, USA) with a mechanical setup according to International The level-ground roll-over evaluation was performed using a prosthetic test machine The level-ground roll-over evaluation was performed using a prosthetic test machine Organization for Standardization technical specifications ISO/TS 16955 [14]. 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 this test method is to apply a more realistic progression and magnitude of loading com- Organization for Standardization technical specifications ISO/TS 16955 [14]. The aim of Organization for Standardization technical specifications ISO/TS 16955 [14]. The aim of this pared to static tests derived from ISO 10328:2016 [15]. Using this new test procedure, this test method is to apply a more realistic progression and magnitude of loading com- test method is to apply a more realistic progression and magnitude of loading compared to quantification of physical parameters of the prosthetic foot can be calculated. pared to static tests derived from ISO 10328:2016 [15]. Using this new test procedure, static tests derived from ISO 10328:2016 [15]. Using this new test procedure, quantification Sagittal foot motion was recorded with a video camera (Rx0 II, Sony, New York City, quantification of physical parameters of the prosthetic foot can be calculated. of physical parameters of the prosthetic foot can be calculated. NY, USA). The test is intended to simulate a heel-to-toe roll-over walking cycle. The test Sagittal foot motion was recorded with a video camera (Rx0 II, Sony, New York City, Sagittal foot motion was recorded with a video camera (Rx0 II, Sony, New York sample is subjected to both an M-shaped force and the motion of a rotating plate synchro- NY, USA). The test is intended to simulate a heel-to-toe roll-over walking cycle. The test City, NY, USA). The test is intended to simulate a heel-to-toe roll-over walking cycle. nized with the vertical force profile. The plate angle starts at −20° for heel strike and ends sample is subjected to both an M-shaped force and the motion of a rotating plate synchro- The test sample is subjected to both an M-shaped force and the motion of a rotating plate at +40° for push-off, and the maximum force applied is 1050 N. The machine set-up is nized with the vertical force profile. The plate angle starts at −20° for heel strike and ends synchronized with the vertical force profile. The plate angle starts at 20 for heel strike described in Figure 3. at +40° for push-off, and the maximum force applied is 1050 N. The machine set-up is and ends at +40 for push-off, and the maximum force applied is 1050 N. The machine described in Figure 3. set-up is described in Figure 3. 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. Sci. 2021, 11, 5318 4 of 12 Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 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 (a) (b) functions of stance phase time. Figure 3. Figure 3. SSchematic chematic illustrat illustration ion of of foot foot roll- roll-over over te testing sting a accor ccording to ding to ISO/T ISO/TS S 16955: ( 16955: ( aa )) machine setup machine setup with with aa VSA prosthet VSA prosthetic ic Markers were placed on the cosmetic foot shell and the pyramid adapter of the foot, foot sample (yellow arrows showing force and rotation); (b) vertical force on ball joint and rotation of the tilt-table, as foot sample (yellow arrows showing force and rotation); (b) vertical force on ball joint and rotation of the tilt-table, as as shown in Figure 5, and a roll-over test was performed without shoes. functions of stance phase time. functions of stance phase time. 2.2.2. Ramp Input Markers were placed on the cosmetic foot shell and the pyramid adapter of the foot, 2.2.2. Ramp Input as shown in Figure 5, and a roll-over test was performed without shoes. Ramp testing is not part of the ISO 16955 nor ISO 22675 standards [16]. However, Ramp testing is not part of the ISO 16955 nor ISO 22675 standards [16]. However, machine inputs can be adjusted to simulate different types of walking patterns and allow machine inputs can be adjusted to simulate different types of walking patterns and allow 2.2.2. Ramp Input evaluation of other types of devices than prosthetic feet [17,18]. Vertical force input on the evaluation of other types of devices than prosthetic feet [17,18]. Vertical force input on machine Ramp testi for thne s g iismu not pa latedrt of ramp t the IS askO 1695 s was c 5a nor IS lculate O d 22 usin 675g sta prev nda ious rds [1 ly co 6]llect . However, ed over - the machine for the simulated ramp tasks was calculated using previously collected over- ma ground ra chine inputs ca mp data of tra n be adjusted to si ns-tibial am mula putees. te dif These ferent types of vertical force wa d lking pa ata wetterns re resam and a pled llo and w ground ramp data of trans-tibial amputees. These vertical force data were resampled and eval adju uat stied t on of o k ot eep smoot her types of h l devices oading and than uprost nload hing etic fee . The st t [17, anc 18e ]. tVert ime ica was l fo modi rce inp fied u t t on t o allow he adjusted to keep smooth loading and unloading. The stance time was modified to allow machine the test m for ach th in e s e tio mu relat ach ed th r e correct amp task los ad was s and p calclul atat e e an dgle usin synchron g previous izatly ion co . Ma llect chi ed over ne inp- ut the test machine to reach the correct loads and plate angle synchronization. Machine input profiles for the test are described in Figure 4. ground ramp data of trans-tibial amputees. These vertical force data were resampled and profiles for the test are described in Figure 4. adjusted to keep smooth loading and unloading. The stance time was modified to allow the test machine to reach the correct loads and plate angle synchronization. Machine input Machine Forces Machine Angles 1400 50 profiles for the test are described in Figure 4. Level Level Incline Incline Decline Decline Machine Forces Machine Angles 1400 50 Level Level Incline Incline 800 Decline Decline -10 -20 0 -30 -10 0 100 200 300 400 500 600 700 800 900 0 200 400 600 800 1000 1200 Time [s] Time [s] -20 (a) (b) 0 -30 0 100 200 300 400 500 600 700 800 900 0 200 400 600 800 1000 1200 Figure 4. Machine inputs for vertical force and tilt-table rotation: (a) level-ground, incline, and decline vertical force input; Figure 4. Machine inputs for vertical force and tilt-table rotation: (a) level-ground, incline, and decline vertical force input; Time [s] Time [s] (b) level-ground, incline, and decline tilt table rotation input. (b) level-ground, incline, and decline tilt table rotation input. (a) (b) Figure 4. Machine inputs for vertical force and tilt-table rotation: (a) level-ground, incline, and decline vertical force input; 2.3. User Testing (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 (Table 1). 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/m ) 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. Sci. 2021, 11, 5318 6 of 12 Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 12 2.3.3. Functional Joint Center Calculation 2.3.3. Functional Joint Center Calculation The functional joint center (FJC) visually indicates the prosthetic foot’s center of ro- The functional joint center (FJC) visually indicates the prosthetic foot’s center of tation [11,12,22] and provides additional information to help understand the ESR defor- rotation [11,12,22] and provides additional information to help understand the ESR de- mation characteristics during the stance phase. The ankle moment and ankle angle data formation characteristics during the stance phase. The ankle moment and ankle angle commonly collected during user testing describe the prosthetic foot’s stiffness character- data commonly collected during user testing describe the prosthetic foot’s stiffness char- istics. In contrast, the FJC’s position provides an average intersection location of the foot acteristics. In contrast, the FJC’s position provides an average intersection location of the foot and ti and bial tibial segments rota segments r ti otation on duriduring ng the the stanstance ce phaphase. se. The FJ The C is FJC an is esti an m estimation ation of the vi of the r- tual joint center and therefore enhances the interpretation of the ESR flexible leaf spring virtual joint center and therefore enhances the interpretation of the ESR flexible leaf spring deformation deformation durin during gr ro oll-over ll-over. . The The funct functional ional joint joint cent center er was was c calculated alculated for bot for both h ma machine chine and b and biomechanical iomechanical t tests. ests. Marker Marker po positions sitions were were pr proc ocessed essedusing usingthe the video video trac tracking king softw softwar are e Tema (Image Tema (Image Sy Systems, stems, Linköping, Sweden) for machine tests. The marker positions for the biomechanical tests Linköping, Sweden) for machine tests. The marker positions for the biomechanical tests were exported as an average for the five users. The points coordinates were projected were exported as an average for the five users. The points coordinates were projected to to the sagittal plane (Figure 5). The rigid-body assumption between markers was used the sagittal plane (Figure 5). The rigid-body assumption between markers was used for for the calculation. The center of zero velocity was calculated during the stance phase the calculation. The center of zero velocity was calculated during the stance phase using using a MATLAB custom script (MathWorks, Natick MA, USA) between M1–M2 and a MATLAB custom script (MathWorks, Natick MA, USA) between M1–M2 and M3–M4 M3–M4 segments for the test machine and between segments M -M and M -M toe segments for the test machine and between segments Mknee-MAnkle knee and Ankle Mheel-Mtoe segments heel segments for the biomechanical test. The position of the instantaneous center of rotation for the biomechanical test. The position of the instantaneous center of rotation was then was then averaged for the stance phase over five steps. The X and Y coordinates of the FJC averaged for the stance phase over five steps. The X and Y coordinates of the FJC were were compared between the two test methods. compared between the two test methods. Figure 5. Schematic of marker positions used for the functional joint center: (left) user test and (right) Figure 5. Schematic of marker positions used for the functional joint center: (left) user test and (right) machine test. machine test. 2.4. 2.4. Sta Statistics tistics A Pearson correlation was selected to contrast maximum sagittal prosthetic foot A Pearson correlation was selected to contrast maximum sagittal prosthetic foot an- angles between the machine test and biomechanical data. Analysis was performed using gles between the machine test and biomechanical data. Analysis was performed using MATLAB. MATLAB. Statistical analysis was not performed on the mechanical machine data. Statistical analysis was not performed on the mechanical machine data. 3. Results 3. Results 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 Figure 6. 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 displacement ranged from 39.5 to 43.6 mm. This test was performed to provide a baseline baseline stiffness measure of the prosthetic foot’s stiffness for the three conditions. stiffness measure of the prosthetic foot’s stiffness for the three conditions. Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 12 Appl. Sci. 2021, 11, 5318 7 of 12 Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 12 Figure 6. VSA prosthetic foot load versus displacement heel and keel stiffness test results for condi- tion 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). tion 1 (softest), condition 2 (mid), and condition 3 (stiffest). 3.2. Ankle Sagittal Moment-Test Machine & Biomechanical Results 3.2. Ankle Sagittal Moment-Test Machine & Biomechanical Results 3.2.1. Level Ground 3.2. Ankle Sagittal Moment-Test Machine & Biomechanical Results 3.2.1. Level Ground The results for the ankle function for the machine and biomechanical tests are con- 3.2.1. Level Ground The results for the ankle function for the machine and biomechanical tests are con- trasted in Figure 7. The ankle moment and angles were averaged for the five users for each The results for the ankle function for the machine and biomechanical tests are con- trasted in Figure 7. The ankle moment and angles were averaged for the five users for each stiffness condition. The machine test allows the presentation of data for prosthetic foot trasted in Figure 7. The ankle moment and angles were averaged for the five users for each stiffness condition. The machine test allows the presentation of data for prosthetic foot ankle joint angles and moments in a comparable layout to the users’ biomechanical data. stiffness condition. The machine test allows the presentation of data for prosthetic foot ankle joint angles and moments in a comparable layout to the users’ biomechanical data. The load cell moment was moved to the pivot point location of the VSA ankle to allow ankle joint angles and moments in a comparable layout to the users’ biomechanical data. The load cell moment was moved to the pivot point location of the VSA ankle to allow analysis with the user results on all tasks. The l analysis oad cell with moment wa the user rs esults moved to the pi on all tasks.vot 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 -0.2 COND 2 COND 2 0 0 COND 3 COND 3 -0.4 COND 1 -0.4 COND 1 -0.2 -10 -5 0 5 10 15 20 -0.2 -10 -5 0 5 101520 COND 2 COND 2 Ankle Angle [°] Ankle Angle [°] COND 3 COND 3 -0.4 -0.4 -10 -5 0 5 10 15 20 -10 -5 0 5 101520 (a) (b) Ankle Angle [°] Ankle Angle [°] Figure Figure 7. 7. Ankle Ankle mom moment ent ve versus rsusankle ankle a angle ngle during duringlevel-gr level-gr ound oundwalking walking for three stiffness setti for three stiffness settings-condi ngs-condition 1 (softes tion 1 (softest), t), (a) (b) condition 2 (mid), and condition 3 (stiffest): (a) test machine results and (b) biomechanical test results–average of five users. condition 2 (mid), and condition 3 (stiffest): (a) test machine results and (b) biomechanical test results–average of five 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. Ramp 3.2.2. Ramp The incline-walking prosthetic foot ankle moment and angle for the machine and The incline-walking prosthetic foot ankle moment and angle for the machine and 3.2.2. Ramp biomechanical tests are depicted in Figure 8. biomechanical tests are depicted in Figure 8. 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 Figure 9. biomechanical tests are depicted in Figure 8. Load [N] Load [N] Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 12 Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 12 1.2 1.2 Appl. Sci. 2021, 11, 5318 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 COND 1 COND 1 0 0 COND 2 COND 2 0.4 0.4 COND 3 COND 3 -0.2 -0.2 -5 0 5 10 15 20 -5 0 5 10 15 20 0.2 0.2 Ankle Angle [°] Ankle Angle [°] COND 1 COND 1 0 0 (a) (b) COND 2 COND 2 COND 3 COND 3 Figure 8. -0.2 Ankle moment versus ankle angle during 7.5° incline walk-0.2 ing for three stiffness settings-condition 1 (softest), con- -5 0 5 10 15 20 -5 0 5 10 15 20 dition 2 (mid), and condition 3 (stiffest): (a) test machine results and (b) biomechanical test results–average of five users. Ankle Angle [°] Ankle Angle [°] (a) (b) The decline-walking prosthetic foot ankle moment and angle results for the machine Figure 8. Ankle moment versus ankle angle during 7.5° incline walking for three stiffness settings-condition 1 (softest), con- Figure 8. Ankle moment versus and biomech ankle angle anic during al tests 7.5 are shown incline walking in Figure for 9. three stiffness settings-condition 1 (softest), dition 2 (mid), and condition 3 (stiffest): (a) test machine results and (b) biomechanical test results–average of five users. condition 2 (mid), and condition 3 (stiffest): (a) test machine results and (b) biomechanical test results–average of 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 0.2 0.8 0.6 -0.2 0.4 -0.4 0.2 COND 1 COND 2 -0.6 0 COND 3 -0.8 -0.2 -15 -10 -5 0 5 10 15 20 Ankle Angle [°] -0.4 COND 1 (a) (b) COND 2 -0.6 COND 3 Figure 9. -0.8 Ankle moment versus ankle angle during 7.5° decline walking for three stiffness settings-condition 1 (softest), con- Figure 9. Ankle moment versus ankle angle during 7.5 decline walking for three stiffness settings-condition 1 (softest), -15 -10 -5 0 5 10 15 20 dition 2 (mid), and condition 3 (stiffest): (a) test machine results and (b) biomechanical test results–average of five users. condition 2 (mid), and condition Ankle Angle [ 3 (stif °] fest): (a) test machine results and (b) biomechanical test results–average of five users. (a) (b) The maximum ankle angles for plantarflexion and dorsiflexion were recorded for each The maximum ankle angles for plantarflexion and dorsiflexion were recorded for Figure 9. Ankle moment versus ankle angle during 7.5° decline walking for three stiffness settings-condition 1 (softest), con- each st stiffness iffnes condition, s condittask, ion, tand ask, and test method. test method. The re The resultssults are pre are presented sented in T in Table 2 a.ble 2. dition 2 (mid), and condition 3 (stiffest): (a) test machine results and (b) biomechanical test results–average of five users. Table 2. Table 2. Maximum an Maximum ankle kle angles angles for plantarflexion and for plantarflexion and d dorsiflexion orsiflexion in the in the test m test machine achine a and nd biom biomechanical echanical t tests. ests. The maximum ankle angles for plantarflexion and dorsiflexion were recorded for Maximum ankle Plantarflexion (°) Maximum Ankle Dorsiflexion (°) Maximum Ankle Plantarflexion ( ) Maximum Ankle Dorsiflexion ( ) each stiffness condition, task, and test method. The results are presented in Table 2. Condition Correlation Correlation Condition Machine Test Biomecha Biomechanical nical Test Correlation p Machine Test Biomecha Biomechanical nical Test Correlation p Machine Test Coefficient p Machine Test Coefficient p Test Coefficient Test Coefficient Table 2. Maximum ankle angles for plantarflexion and dorsiflexion in the test machine and biomechanical tests. COND 1: Level −9.6 −5.9 17.2 16.5 COND 1: Level 9.6 5.9 17.2 16.5 COND 2: Level −9.4 −5.7 16. 0.961 0.179 1 15.9 0.998 0.043 Maximum ankle Plantarflexion (°) Maximum Ankle Dorsiflexion (°) COND 2: Level 9.4 5.7 16.1 15.9 0.961 0.179 0.998 0.043 COND 3: Level −8.8 −5.5 14.8 15.0 COND Cond3: ition Level 8.8 5.5 Correlation 14.8 15.0 Correlation Machine Test Biomechanical Test p Machine Test Biomechanical Test p COND 1: Incline −4.6 −0.2 17.4 18,4 Coefficient Coefficient COND 1: Incline 4.6 0.2 17.4 18,4 COND 2: Incline −4.2 −0.5 16. −0.564 0.619 6 17.7 0.995 0.065 COND 1: Level −9.6 −5.9 17.2 16.5 COND 2: Incline 4.2 0.5 16.6 17.7 0.564 0.619 0.995 0.065 COND 3: Incline −3.6 −0.4 15.8 16,7 COND 2: Level −9.4 −5.7 16. 0.961 0.179 1 15.9 0.998 0.043 COND 3: Incline 3.6 0.4 15.8 16,7 COND 1: Decline −14.8 −8.0 13.5 15.7 COND 3: Level −8.8 −5.5 14.8 15.0 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 1: Incline −4.6 −0.2 17.4 18,4 COND 2: Decline 14.5 7.8 12.6 15.1 0.963 0.173 0.986 0.106 COND 3: Decline −14.1 −7.7 12.2 14.6 COND COND 2: I 3: Decline ncline −