Jtam-A4.dvi JOURNAL OF THEORETICAL AND APPLIED MECHANICS 55, 1, pp. 369-376, Warsaw 2017 DOI: 10.15632/jtam-pl.55.1.369 ANATOMICAL PROTOCOL FOR GAIT ANALYSIS: JOINT KINEMATICS MEASUREMENT AND ITS REPEATABILITY Magdalena Żuk, Marta Trzeciak Wroclaw University of Technology, Department of Biomedical Engineering, Mechatronics and Theory of Mechanisms, Wrocław, Poland; e-mail: magdalena.zuk@pwr.edu.pl International Society of Biomechanics has proposed a general reporting standard for joint kinematics based on anatomical reference frames. Nevertheless, the gait analysis protocols based on this standard are still poorly reported. The purpose of the current study is to pro- pose and preliminarily assess the potential of an anatomically based ISB 6-DOF protocol, which combines the ISB reporting standard together with a marker cluster technique. The proposed technicalmarker set enables full descriptionof the lower limbkinematics (including three-dimensional ankle-foot complex rotations) according to the current biomechanical re- commendation. Themarker set provides a clinically acceptable inter-trial repeatability and minimal equipment requirements. Keywords: joint kinematics, gait analysis protocol, repeatability, anatomical protocol, marker-set, motion capture system 1. Introduction -Three-dimensional kinematic measures of the human gait provide useful data for clinical prac- tice and biomechanical research (Baker, 2006; Syczewska et al., 2012). Increasingly, quantitative description of the humanmovement is used as input data in dynamic simulation of themusculo- skeletal system (Delp et al., 2007), including joint moment identification using inverse dynamic methods as well as muscle force estimation using optimization based methods (Erdemir et al., 2007; Żuk andPezowicz, 2016). Furthermore, such datamay be helpful in the design of walking machines, exoskeletons (Oliński et al., 2015), limbprosthesis or active orthoses (Dollar andHerr, 2008). Contemporary quantitative analysis of gait incorporates advanced, still expensive motion capture systems for tracking marker location. The marker set together with the related biome- chanical model for mathematical description of lower limb kinematics is called the gait analysis protocol. The widely used protocol in clinical gait analysis is Conventional Gait Model (Davis et al., 1991; Kadaba et al., 1990) which is better standardised and validated than othermodels (Baker, 2006); therefore, it seems to be themost appropriatemodel in clinical practice at themoment. In this protocol, markers are placed both above bony landmarks andwand, therefore, this protocol is not fully anatomical. Simultaneously, the Conventional Gait Model is inconsistent with the ISB reporting standard. Protocols based on the current ISB recommendation (Wu et al., 2002) are still poorly repor- ted. A recent study evaluated the performance of anatomically based protocols (Manca et al., 2010; Leardini et al., 2007; Ferrati et al., 2008), including those usingmarker clusters (Collins et al., 2009). However, both those protocols are not fully consistent with the ISB recommendation. Gait kinematics measured using an anatomically based protocol, which also enables tracking of each segment independently, could increase the accuracy of musculoskeletal modelling and also seems to bemore appropriate for consideration of orthoses and exoskeletons design. 370 M. Żuk,M. Trzeciak The purpose of the current study is to propose and assess the protocol, which fulfils ISB standard, as well as to present reference data for normal subjects obtained using the proposed protocol. The proposed anatomically based protocol combines the general reporting standard recommended by the International Society of Biomechanics (ISB) together with a marker clu- ster technique. In the previous paper (Żuk and Pezowicz, 2015), the proposedmethodology was presented and comparative analysis with a conventional protocol was conducted on the limi- ted group as a preliminary verification of applied methods. In the current study, the reference data for normal subjects have been collected and inter-trial reproducibility has been validated. Furthermore, the applied methodology have been described in greater detail. 2. Methods Lower limbmotionwas tracked using amotion capture system (OptotrakCertus,NDI, Canada) with one position sensor, equipped with three embedded infrared cameras (Fig. 1). The system tracked position and orientation of clusters of active markers. Fig. 1. (a) Marker set including technical markers and virtual markers, (b) marker placement, (c) motion capture system Four clusters of active markers were located on pelvis and right lower limb segments: thigh, shank, and foot. The clusters were placed laterally on the distal part of each segment. Each cluster, consisting of three active markers (infrared LEDs) attached on a rigid base (Optotrak SmartMarker Rigid Body, NDI, Canada), wasmountedwith an adhesive tape and a band.The pelvis cluster was mounted using only adhesive tape. Locations of marker clusters and virtual markers are shown in Fig. 1. Furthermore, two additional virtual markers (on the heel and the metatarsal head) were included for foot visualization and gait phase identification. Davis’s regression equation was applied to determine the hip joint centre (Davis et al., 1991) Anatomical landmarks were defined as virtual markers whose positions with respect to the technical markers (cluster) were measured using a tracked pointer during a static trial. The virtualmarker setwasdesignedon thebasis of the current ISBrecommendation (Wu et al., 2002) for anatomical reference frames.Anatomical coordinate systemsof each anatomical segmentwere defined in pursuance of the paper byWu et al. (2002) (Fig. 2). Anatomical protocol for gait analysis: joint kinematics... 371 Fig. 2. Anatomical coordinate systems definition according to ISB recommendation (Wu et al., 2002) based on following virual markeres: ASIS – anterior superior iliac spine, midPSIS –midpoint between posterior superior iliac spines, HJC – hip joint centre, FE – femur epicondyle, midFEs –midpoint between femur epicondyles, LC – the most lateral point on the boarder of the lateral tibial condyle, MC – the most medial point on the border of the medial tibial condyle, IC – the inter-condylar point located modway between theMC and LC, LM – tip of medial malleolus,MM – tip of the medial malleolus Cardan’s angular convention was used to describe relative orientation of adjacent segments (Tupling andPierrynowski, 1987; Kadaba et al., 1990). In this convention, the joint rotationsR are described as compound rotations R=    r11 r12 r13 r21 r22 r23 r31 r32 r33    =RZγRXαRYβ =    cosγ −sinγ 0 sinγ cosγ 0 0 0 1       1 0 0 0 cosα −sinα 0 sinα cosα       cosβ 0 sinβ 0 1 0 −sinβ 0 cosβ    =    cosγ cosβ− sinγ sinαsinβ −sinγcosα cosγ sinβ+sinγ sinαcosβ sinγ cosβ+cosγ sinαsinβ cosγcosα sinγ sinβ− cosγ sinαcosβ −cosαsinβ sinα cosαcosβ    (2.1) whereRZγ,RXα,RYβ are rotationmatrices corresponding to rotations aroundanatomical axes, respectively: rotation by an angle γ around the frontal axisZ, rotation by an angleα around the sagittal axisX and rotation by an angle β around the longituidal axis Y ; rij are rotationmatrix elements. Graphical interpretation of the adopted rotation sequence is presented in Fig. 3. According to the adopted joint angle definition, ifTALCS1→ALCS2 is the matrix of transfor- mation from the proximal segment coordinate system to the distal segment coordinate system, which can be like this TALCS1→ALCS2 =      r11 r12 r13 TX r21 r22 r23 TY r31 r32 r33 TZ 0 0 0 1      (2.2) where TX, TY , TZ refers to translations, then the anatomical joint angles can be calculated as follows α=arcsinr32 β=arcsin −r31 cosα γ =arcsin −r12 cosα (2.3) 372 M. Żuk,M. Trzeciak Fig. 3. Graphical representation of Cardan angle convention where α is abduction/adduction joint angle, β is external/internal rotation angle and γ is fle- xion/extension joint angle. Data acquisition and joint angle calculation were performed using custom-made software. Dataprocessing, includinggait cycle normalisation and smoothing,was performedusingMatlab. Ten able-bodied subjects without walking disability (five females and five males) were ana- lysed (aged 22±2 years, weight 66±11kg, height 1.75±0.11m). In the case of experimental methods or repeatability analysis, it was used to combine females and males while preserving the age range, which was shown in the paper byMcGinley et al. (2009). All participants providedwritten informed consent before participation. The subjectswalked barefoot at a preferred pace and three gait cycles were selected. Themean value and the standard deviation of 12 rotations were calculated over three trials for each sample of the gait cycle in ten subjects. Angle curves were plotted for a single re- presentative subject (mean of three cycles) and for ten subjects (averaged across mean curves of each subject). Inter-trial variability was calculated according to the recommended method (Schwartz et al., 2004; McGinley et al., 2009) and plotted. Average inter-trial variability (AIT) was compared to the corresponding values from recent papers (Manca et al., 2010; Schwartz et al., 2004). Averaged intra-protocol variability was defined as amean standard deviation over all subjects averaged across the gait cycle. 3. Results Calculated joint rotations (Fig. 4) are related to corresponding data derived from similar bio- mechanical models (Benedetti et al., 1998; Leardini et al., 2007; Collins et al., 2009). The lowest consistency of the range ofmotion (ROM) is observed for the ankle angle, forwhich the anatomi- cal framedefinition and themarker set differ considerably fromothermodels.Average inter-trial variability is low (Table 1) and similar to the corresponding data from other studies (Manca et al., 2010; Schwartz et al., 2004). The most repeatable rotation within the subject is pelvis obliquity (0.9◦), while the lowest reproducibility is observed for hip internal/external rotation and pelvis rotation (2.6◦). The latter results from slight changes of the gait direction during the study. Inter-trial repeatability clearly depends on the phase of gait (Fig. 5). In particular, for knee flexion/extension and ankle inversion/eversion, inter-trial variability doubles during swing phase. Intra-protocol (Table 1) variability is highest for hip flexion/extension (14.2◦) and pelvic tilt (12.7◦) while for the other angles it does not exceed 10◦. Anatomical protocol for gait analysis: joint kinematics... 373 Fig. 4. Kinematic variables as calculated by the ISB 6-DOF of one representative subject (mean across three cycles – gray dashed line, +/- SD – gray dashed thin line) and ten subjects (averaged acrossmean curves of subjects – black solid line, +/− SD grey band) Table1.Average inter-trial and intra-protocol variability over thegait cycle across four subjects. Corresponding values fromManca et al. (2010) and Schwartz et al.(2004) Rotations [◦] Inter-trial Intra-protocol Present study Manca et al. Schwartz et al. Present study Pelvis tilt 1.2 0.9 0.8∗ 12.7 Pelvis obliquity 0.9 1.4 0.5∗ 4.6 Pelvis rotation 2.6 1.7 1.0∗ 10.0 Hip flex/ext 1.6 1.8 1.2∗ 14.2 Hip abd/add 1.4 1.7 0.5∗ 7.1 Hip intr/extr 2.6 2.9 1.2∗ 9.4 Knee flex/ext 1.9 2.2 1.6 6.3 Knee var/valg 1.0 1.6 0.5∗ 4.8 Knee intr/extr 1.2 4.3 1.2∗ 9.2 Ankle dor/pla 1.6 2.0 1.3∗ 4.5 Ankle inv/ev 1.8 2.3 – 6.3 Ankle abd/add 1.1 2.8 1.7 3.7 ∗ data estimated from figures provide 374 M. Żuk,M. Trzeciak Fig. 5. Patterns of standard deviation across all samples of the gait cycle, one representative subject (gray dashed line) and average for ten subjects (black solid line) 4. Discussion The proposed technical marker set enables full description of lower limb kinematics, including three-dimensional (3D) ankle-foot complex rotations according to the current biomechanical convention (Wu et al., 2002). Lower limb segments are tracked separatelywithout an assumption being made about joint constraints. Thus, this marker set can be applied to determination the joint centres and axes of rotation using functional methods, which was previously reported by Żuk et al. (2014). Besides, marker clusters in combination with an anatomical calibration allow definition of an unlimited number of virtual markers, freely placed within the segment, including those located beyond the “line of sight” of theposition sensor.Onlyoneposition sensor (consisting of at least two cameras) is needed to track a selected lower limb (clusters located laterally) as well as both limbs (clusters placed frontally). The application of an additional position sensor allows such an arrangement of the clusters, particularly location of the pelvis cluster on the sacrum (Borhani et al., 2013), which could reduce soft tissue artefacts (STA). Reference data for normal subjects have been collected. Although the obtained selected joint angle curves are in agreement with the literature (Leardini et al., 2007; Collins et al., 2009; Benedetti et al., 1998), caution is recommended when comparing the results among different protocols, especially in the case of non-sagittal planes (Ferrati et al., 2008). The obtained average inter-trial variability is acceptable in clinical application according to previous papers by Schwartz et al. (2004) and McGinley et al. (2009). A relatively low inter- trial variability indicates proper mounting of marker clusters, which eliminares sliding during examination. Further evaluation of the ISB 6-DOF protocol should include analysis of inter- Anatomical protocol for gait analysis: joint kinematics... 375 session and inter-assessor repeatability. However, inter-session and inter-assessor repeatability appear to be close to those achieved with other anatomically based protocols (Manca et al., 2010) due to a similar source of variability (palpation of external bony landmarks). An anatomically based protocol in which virtual markers are placed on bony landmarks without wands, increase reliability of musculoskeletal modelling by more accurate matching of marker trajectories to the scaled model. The ISB 6-DOF protocol provides a full 3D description of lower limb kinematics according to the current recommendation (Wu et al., 2002) with acceptable inter-trial variability. There are some limitations of the proposed method. The use of only one position sensor is associated with sub-optimal pelvis cluster location, which can affect pelvis and hip rotations.Moreover, at the present time, lack of relevant reference data for patients restricts the use of these methods in clinical practice. 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