Three-dimensional motion and ground reaction force procedures and analysis

3.3.1 Marker placement

Data collection procedures were similar to previously published work (272, 273, 275) performed in the same laboratory (Human Performance Laboratory, G47, Mary Seacole building). Prior to the COD tasks, reflective markers (14 mm spheres) were placed bilaterally on the following body landmarks: iliac crest, anterior superior iliac spine, posterior superior iliac spine, greater trochanter, medial epicondyle, lateral epicondyle, lateral malleoli, medial malleoli, heel, fifth, second, and first metatarsal heads, acromion process, and a single marker for C7 and mid clavicle using double-sided adhesive tape (Figure 3.2). Participants also wore a 4-marker ‘‘cluster set’’ (4 retroreflective markers attached to a lightweight rigid plastic shell) on the right and left thigh and shin, which approximated the motion of these segments during the dynamic trials (Figure 3.2). This technique is suggested to reduce the influence of soft tissue artefacts and be more accurate and practical for tracking motion than individual skin markers (40, 71), with 4 markers suggested as optimal (339). The thigh and shin cluster sets were attached using Velcro-elasticated wraps. The thigh cluster sets were placed on the distal third of the thigh to reduce movement of the cluster set caused by muscle bulk and swinging of the hands. Female participants also wore an additional trunk 4-marker “cluster set” which was placed in between the scapulae (Chapter 6 only).3.3.

Imprecise marker placement opens up the prospect of errors and inaccuracies in joint centre or body segment identification, thus affecting anatomical coordinate systems and can therefore result in erroneous estimations of joint kinetic and kinematic data (40, 188, 251, 428, 523, 533, 550). Furthermore, inconsistent marker placement between testers, between sessions, and between laboratories is also problematic for repeatability (70, 78, 124, 201, 282, 523). As such, the lead researcher, who was experienced in anatomical identification and palpation, placed the markers for all participants and for each study. All participants wore Lycra shorts and standardised footwear (Balance W490, New Balance, Boston, MA, USA) to control for shoe-surface interface (152).

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Figure 3.2. Anterior and posterior views of marker placement 3.3.2 3D motion and GRF data analysis

A capture volume for the COD tasks was created to enable data collection of the approach prior to the PFC and exit following the FFC. The capture volume was dynamically calibrated by moving a 601.7 mm wand throughout the capture area (lower-floor level to head height) (Figure 3.3) for a duration of 70 seconds with residual errors < 0.5 mm. A rigid 566 mm × 365 mm L-frame with 4 markers was also positioned on the corner of the FFC force platform for static calibration to define the global coordinate system in three planes (x, y, and z) (Figure 3.3). 3D motions of these markers were collected during the COD trials using 10 Qualisys Oqus 7 (Gothenburg, Sweden) infrared cameras (240 Hz) mounted to the ceiling and wall operating through QTM software (Qualisys, version 2.16 (Build 3520), Gothenburg, Sweden). GRFs were collected from two 600 mm × 900 mm AMTI (Advanced Mechanical Technology, Inc, Watertown, MA, USA) force platforms (Model number: 600900) embedded into the running track sampling at 1200 Hz.

From a standing trial, a six degrees of freedom kinematic model of the lower-extremity and trunk was created for each participant (Figure 3.4), including pelvis, thigh, shank, and foot using Visual3D software (C-motion, version 6.01.12, Germantown, USA) which was scaled to height and BM. This kinematic model was used to quantify the motion at the hip, knee, and ankle joints using a Cardan angle sequence x-y-z (546). The local coordinate system was defined at the proximal joint centre for each segment. The static trial position was designated as the subject’s neutral (anatomical zero) alignment, and subsequent kinematic and kinetic measures were related back to this position.

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Segmental inertial characteristics were estimated for each participant (128). This model utilised a CODA pelvis orientation (30) to define the location of the hip joint centre. The knee and ankle joint centres were defined as the mid-point of the line between lateral and medial markers (114, 115).

Figure 3.3. Static L-frame and wand used for calibration

Figure 3.4. Six degrees of freedom kinematic model of the lower-extremity and trunk

Marker trajectories for all trials were labelled using QTM software before being exported as C3D files and analysed using Visual3D software. Using the pipeline function in Visual3D, joint coordinate (marker) and force data were smoothed with a fourth order Butterworth low-pass digital

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filter with COFs of 15 Hz and 25 Hz, based on a priori residual analysis (613), visual inspection of motion data, recommendations by Roewer et al. (493), and results of a pilot study presented in Appendix 2.1. Lower-limb joint moments were calculated using an inverse dynamics approach (612) through Visual3D software and were defined as external moments (i.e., external KAM will abduct the knee – distal end of tibia away from midline of body), which were normalised to BM because ACL ligament tensile strength also scales to mass (80, 415). Joint kinematics and GRFs were also calculated using Visual3D with Table 3.1 providing definitions and calculations.

The cutting trials were time normalised for each participant to 101 data points with each point representing 1% of the WA phase (0 to 100% of WA) or push-off phase (0 to 100% of push-off). Initial contact was defined as the instant of ground contact that the VGRF was higher than 20 N, and end of contact (toe-off) was defined as the point where the VGRF subsided past 20 N (272-274, 301, 304). The WA (braking) phase was defined as the instant of IC to the point of maximum knee flexion (228, 272- 274), and the push-off phase (propulsive) was defined as point of maximum knee flexion to toe-off.