3.3 Measuring gait kinematics
3.3.2 Reconstructing a biomechanical model
3.3.2.1 Tracking segment motion
NDI ToolBench software was used for instantaneous tracking of thirty-three iRED markers attached to body segments to eventually obtain 6-DoF segment motion. Segments were tracked using an array of markers rigidly clustered on a body mounted plate (Manal, McClay, Richards, Galinat and Stanhope, 2002). These plates were attached to the feet, shank, thigh and pelvis. Table 3.3.2.1.1.a-b identifies the location of the tracking markers and this is also illustrated for body locations (Figures 3.3.2.1.1- 2).
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Figure 3.3.2.1.1.
Marker set up. Various views (A-F) describe the marker set up: A) frontal view; B) to minimize subject distraction the wires were passed behind the subject to a control box supported by the harness; C) sagittal view; D) cluster-technical-frame plate fastened to the shank; E) cluster-technical-frame attached to the foot; F) cluster-technical-frame fastened to the pelvis.
Neoprene rubber underlay with Velcro
loop Markers mounted on rigid
plastic plate Adhesive overlay wrap
Posterior view of trunk and pelvis marker set up.
Foot marker set up. Shank marker set up.
Back pack with NDI strober for active marker output. Head
IRED marker in white.
A B C D E F Sagittal view of marker set-up. Frontal view of marker set-up.
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Figure 3.3.2.1.2
Animation of cluster markers and virtual markers. Note: the pelvis cluster here is displayed to the side of the pelvis for illustration purposes. Left: The segment coordinate systems are positioned and oriented relative to anatomical landmarks (in red) at the distal and proximal endpoints of the segment by a combination of virtual markers and functional joint centre computation. The virtual points representing the part of the shoe, which gets closest to the ground during mid swing are indicated in green. Right: Tracking cluster markers are located on rigid plates, which are fastened onto body segments: posterior surface of trunk and pelvis; lateral surface of left and right lower limbs. These cluster technical frames track the segments. All abbreviations in are defined in Tables 3.3.2.1.1a and 3.3.2.1.1b. T1, T2, T3, T4 P1, P2, P3, P4 LT1, LT2, LT3, LT4 LS1, LS2, LS3, LS4 LF1, LF2, LF3, LF4 GCS LAC RAC LLR RLR LIC RIC LGT RGT RLK RMK RLA RMA R5MH R2MH LMTC4 LMTC2 LMTC3
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Table 3.3.2.1.1a Specific marker locations (head to hip)
Segment Description
Head
HD Head target marker (lateral to occipital process)
Thorax
Target Markers
T1
IRED target marker on Trunk Cluster Plate located between T11 and L2 T2
T3 T4
Virtual Markers
LTAC (RTAC) Left Acromion process ***
LTR (RTR) Left Lateral aspect of last rib ***
Pelvis
Target Markers
P1
IRED target marker on Pelvis Cluster Plate located between L5 and S2 P2
P3 P4
Virtual Markers
LPSIS (RPSIS) Posterior Superior Iliac Spine ***
LASIS (RASIS) Anterior Superior Iliac Spine ***
LIC (RIC) Iliac Crest ***
LGT (RGT) Greater Trochanter ***
RHIP _FUNC
(LHIP_FUNC) Hip joint centre *
LIC_lm (RIC_lm) Iliac Crest ***
* - virtual anatomical landmark (Schwartz et al. 2005) ** - projected virtual anatomical landmark
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Table 3.3.2.1.1b Specific marker locations (thigh to toe)
Segment Description
Thigh
Target Markers
LT1 (RT1)
IRED target markers on Thigh Cluster Plate located at distal part of segment LT2 (RT2)
LT3 (RT3) LT4 (RT4)
Virtual Markers
LLK (RLK) Lateral femoral epicondyle ***
LMK (RML) Medial femoral epicondyle ***
LKnee_FUNC
(RKnee_FUNC) Landmark #1 along knee joint axis *
LKnee_FUNC_X
(RKnee_FUNC_X) Landmark #2 along knee joint axis *
LLK_lm (RLK_lm) True lateral femoral epicondyle **
LMK_lm (RMK_lm) True medial femoral epicondyle **
Shank
Target Markers
LS1 (RS1)
IRED target marker on Shank Cluster Plate located at distal part of segment LS2 (RS2)
LS3 (or RS3) LS4 (or RS4) Virtual Markers
LLM (or RLM) Lateral malleolus ***
LMM (or RMM) Medial malleolus ***
Foot
Target Markers
LF1 (or RF1)
IRED target marker on Foot Cluster Plate located at lateral &medial part of segment
LF2 (or RF2) LF3 (or RF3) LF4 (or RF4)
Virtual Markers
LMTC1 (or RMTC1) Minimum toe clearance point1 ***
LMTC2 (or RMTC2) Minimum toe clearance point2 ***
LMTC3 (or RMTC3) Minimum toe clearance point3 ***
LMTC4 (or RMTC4) Minimum toe clearance point4 ***
L5MH (or R5MH) 5th metatarsal head ***
L1MH (or R1MH) 1st metatarsal head ***
LH1 (or RH1) Posterior aspect of calcaneus ***
LH2 (or RH2) Inferior and posterior aspect of shoe out-sole surface ***
* - virtual anatomical landmark (Schwartz et al. 2005) ** - projected virtual anatomical landmark
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The rigid plates were custom made from lightweight thermoplastic and moulded to the shape of the segments in which they were attached. The pelvis, thigh and shank cluster plates were positioned at areas to minimise soft tissue artefact. The body segments were circumference wrapped with anti-migration neoprene rubber (inner underlay with adhesive spray) with Velcro on the outer surface. The cluster technical frame plates were also fitted with Velcro material on the inner surface and were mated onto the neoprene bands. The plate-attached Velcro loop underlay was matted onto a body segment-attached Velcro loop. The neoprene underlay wrap served to also restrict soft tissue wobble (Manal, McClay, Stanhope, Richards and Galinat, 2000).
The cluster technical frames were defined by the four cluster markers of the respective segments (Table 3.3.2.1.2). The construction of the cluster technical frame plate design considered the recommendations proposed by Cappozzo et al. (1997) and the pelvis cluster technical frame plate attachment was an exception case. There are several methods which have been applied to describe the motion of the pelvis using a cluster technical frame (Benedetti et al., 1998; Whittle and Levine, 1999; Vogt,
Portscher, Brettmann, Pfeifer and Banzer, 2003). The design of the pelvis cluster technical frame in this study considered a pragmatic approach for female participants while considering cluster technical frame plate design recommendations by Cappozzo et al., (1997). Further steps were taken to ensure the pelvis cluster technical frame could mimic the pelvic motion by designing lycra shorts with rubber underlay to prevent material migration across the skin and a large Velcro patch sewed across the buttocks for the plate attachment site. The plate was secured onto the Velcro patch against the posterior superior iliac spine by wrapping a non-slip neoprene belt at the level of the anterior superior iliac spine. This is similar to the ‘Milwaukee Brace’ design (Benedetti et al., 1998) in the sense that the current study also used a band to attach a rigid plate on the pelvis from which a cluster technical frame was embedded.
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Table 3.3.2.1.2 Cluster technical frame definitions.
Segment Description
Thorax
Origin T1
Vertical (z) axis Line in direction of T2 to T1
Mediolateral (x) axis Perpendicular to vertical axis, in plane containing T1, T2 and T3 Anterior-posterior (y) axis Mutually perpendicular to other 2 axes
Pelvis
Origin P1
Vertical (z) axis Line in direction of P2 to P1
Mediolateral (x) axis Perpendicular to vertical axis, in plane containing P1, P2 and P3 Anterior-posterior (y) axis Mutually perpendicular to other 2 axes
Thigh
Origin RT1 (same for left side)
Vertical (z) axis In line with RT1 and RT2, direction away from RT2
Mediolateral (x) axis Perpendicular to vertical axis, in plane containing RT1, RT2 and RT3 Anterior-posterior (y) axis Mutually perpendicular to other 2 axes
Shank
Origin RS1 (same for left side)
Vertical (z) axis In line with RS1 and RS2, direction away from RS2
Mediolateral (x) axis Perpendicular to vertical axis, in plane containing RS1, RS2 and RS3 Anterior-posterior (y) axis Mutually perpendicular to other 2 axes
Foot
Origin RF1 (same for left side)
Vertical (z) axis In line with RF1 and RF2, direction away from RF2
Mediolateral (x) axis Perpendicular to vertical axis, in plane containing RF1, RF2 and RF3 Anterior-posterior (y) axis Mutually perpendicular to other 2 axes
3.3.2.1.1 Justification of cluster technical frame marker set up
The major advantage of using marker clusters on a plate is that it avoids the need to place markers at the joints which is where markers are most susceptible to artefacts from skin sliding (Cappozzo, Catani, Leardini, Benedetti and Croce, 1996). In practice, this relative motion can be up to 20mm in the lower limbs (Fuller, Liu, Murphy and Mann, 1997).
The attenuation and characterization of soft tissue artefact within individuals is an extremely difficult problem to solve because the frequency content of soft tissue
artefact is similar to bone (Fuller et al., 1997). This makes it difficult to use harmonic analysis or other filtering techniques to compensate for its unwanted effect. The most
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affected knee joint motion is associated with abduction/adduction and
internal/external rotation, while motion in the sagittal plane (flexion/extension) is less affected by soft tissue artefact (Reinschmidt, van Den Bogert, Murphy, Lundberg and Nigg, 1997). The effects of soft tissue artefact are also dependent upon the specific motor task undertaken (Cappozzo et al., 1996), such that muscle contraction, skin sliding and tissue deformation, gravity and inertial effects will all contribute
independently. Soft tissue artefact errors have shown to be larger in running compared to walking (Reinschmidt et al., 1997).
In consideration of the results and the limitations expressed in the literature for analysing three-dimensional segment kinematics, the marker set up approach adopted in this investigation has considered a balanced approach to model reconstruction. The application of cluster technical frame plates, like those adopted in this study, has been performed in many biomechanical studies (Seay, Haddad, van Emmerik and Hamill, 2006; Bruijn et al., 2007) and the advantage of this marker set up technique relative to other methods has been strongly supported in terms of relative validity and
pragmatism (Holden, Orsini, Siegel, Kepple, Gerber and Stanhope, 1997; Benedetti et al., 1998; Manal et al., 2000). The segment location of the cluster technical frame plates considered recent evidence which described the distal portion of the thigh and shank to contain relatively less soft tissue artefact in comparison to remaining sections of these segments (Stagni, Fantozzi, Cappello and Leardini, 2005).