Golfer Model Segment Definitions

The marker positions were used to define a whole body golfer model in order to calculate kinematic data. Visual 3D (C-Motion, USA) software was used to build the golfer model. To achieve this, a static trial was required along with at least three tracking markers attached to each segment in both the static and subsequent dynamic trials. Providing these requirements of Visual 3D were met, the position and orientation of every segment could be computed.

The marker set reported in § 4.2.5.3 was used to initially create a seventeen segment golfer model including; head, trunk, pelvis, left thigh, right thigh, left shank, right shank, left foot, right foot, left upper arm, right upper arm, left forearm, right forearm, left hand, right hand, golf club shaft and golf clubhead (Figure ‎4.8). Visual 3D assumes that segments are rigid objects (i.e. they do not deform when force is applied and inter marker distances are invariant), segments are implicitly linked (e.g. segments are not constrained) and that each segment is defined by a local co-ordinate system (LCS) based on a right handed Cartesian co-ordinate system (C-Motion, 2011).

101 Figure ‎4.8. Golfer model showing initial segments that were defined using the golfer marker set.

The method used to define a segment LCS in Visual 3D is illustrated for the right shank in Appendix D. The local co-ordinate system is based on a right hand Cartesian coordinate system. The initial stage of creating the LCS is defining the frontal plane, which is created by the markers placed at proximal and distal segment endpoints. Subsequently, the segment endpoints are defined based on the markers that were used. The origin of the LCS was positioned at a mid-point between the proximal endpoint markers. By default in Visual 3D, the z-axis (blue) was defined by the vector from the distal segment end point to the proximal segment end point. The y-axis (green) is defined as the vector which is perpendicular to the frontal plane and z-axis. Finally, the x-axis (red) was based on the right hand rule. In this thesis, the z-axis was directed from distal to proximal, the y-axis was anterior to posterior and the x-axis medial to lateral for the majority of LCS defined by the markers in (Table ‎4.7). The only exception was the

102 foot and clubhead segments. The variation between segment constructions was due to the difference in defining segment end points (Table ‎4.7).

In addition, the thigh and pelvis required additional calculations to form the segments. For the thigh, the distal segment end point was between the lateral and medial epicondyle of the knee and proximal end point was the hip static joint centre (SJC). Right and left hip SJC were estimated based on the following equation and ASIS distances (Bell et al., 1989):

Right hip SJC = (0.36*ASIS_distance, -0.19*ASIS_distance, -0.3*ASIS_distance) Left hip SJC = (-0.36*ASIS_distance, -0.19*ASIS_distance, -0.3*ASIS_distance) The ASIS distance was calculated in Visual 3D as the distance between RASIS and LASIS markers, therefore it was important to achieve correct positioning of these markers. The estimates of static hip joint centre positions was adapted from the work of Bell et al. (1989) who reported predicting hip joint centres in adults to within 2.6 cm of actual joint centre locations.

The pelvis segment was initially defined using the ASIS and PSIS marker positions. The origin of the pelvis LCS was defined as the mid-point between the ASIS markers. The x-axis was defined from the origin to the right ASIS, z-axis was vertical and y-axis was anterior-posterior (Figure ‎4.9). However, Visual 3D warn that using this segment definition will result in a pelvis segment that is tilted approximately 20º forward from the horizontal and advocate a segment with zero tilt should be created (i.e. x-axis parallel with the floor) (C-Motion, 2011). This is achieved by creating iliac crest landmarks to define the proximal joint end points and static hip joint centres to define distal joint end points. The static hip joint centres were offset in the z-direction of the laboratory co-ordinate system by 0.5*ASIS_distance to create iliac crest landmarks. Defining the pelvis in this way would result in a z-axis which is directed vertically upward and the pelvis has no anterior tilt in the static trial where the subject is standing in the anatomical position.

103 Figure ‎4.9. Pelvis segment created with Visual 3D based on ASIS and PSIS markers (C-Motion, 2011)

Each segment was considered to be a geometric shape based on the Hanavan model of the human body (Hanavan, 1964). Thereby, the mass, centre of mass and moment of inertia of each segment were defined. The segment mass was determined from the total golfer body mass and Dempster’s anthropometric data (Robertson et al., 2004). All other segment properties were computed based on the mathematical model of Hanavan (1964) and could be used in the calculation of whole body COG. Those segments which were custom built in later chapters (e.g. lumbar, thorax and upper thorax) were classified as kinematic only segments and did not affect COG calculations.

104 Table ‎4.7. Visual3D golfer model segment definitions

Segment Name Tracking Markers Origin Proximal Endpoint Distal Endpoint

Head RFHD, LFHD,

RBHD, LBHD

Mid-point between RFHD and LFHD

RFHD - LFHD RBHD - LBHD

Left Forearm LFA, LLELB,

LRAD, LULN

Mid-point between LLELB and LMELB

LLELB - LMELB LRAD - LULN

Right Forearm RFA, RLELB,

RRAD, RULN

Mid-point between RLELB and RMELB

RLELB - RMELB LRAD - LULN

Left Shank LSK1, LSK2, LSK3, LSK4 Mid-point between LLK and LMK LLK - LMK LLA – LMA Right Shank RSK1, RSK2, RSK3, RSK4 Mid-point between RLK and RMK RLK – RMK RLA - RMA

Left Upper Arm LSHO, LUP1, LUP2 Left static shoulder joint

centre

Left static shoulder joint centre. Negative offset from LAC by measured shoulder width.

LLELB - LMELB

Right Upper Arm RSHO, RUP1,

RUP2

Rights static shoulder joint centre

Right static shoulder joint centre. Negative offset from RAC by measured shoulder width.

RLELB - RMELB

Left Thigh LTH1, LTH2, LTH3 Left static hip joint centre Left static hip joint centre defined using equation by

Bell et al., (1989)

LLK - LMK

Right Thigh RTH1, RTH2, RTH3 Right static hip joint

centre

Right static hip joint centre defined using equation by Bell et al., (1989)

RLK - RMK

Pelvis (without tilt) RASIS, LASIS,

RPSIS, LPSIS

Mid-point between RASIS and LASIS

RT_ILLIAC - LT_ILLIAC Right static hip joint centre to left

static hip joint centre Trunk (Thorax &

Abdomen)

CLAV, STRN, C7, T10, RBAK

Mid-point of iliac crest RT_ILLIAC - LT_ILLIAC RAC - LAC

Right Hand RRAD, RULN,

RHA

Mid-point of RRAD and RULN

RRAD – RULN RHA and radius of 0.05 m

Left Hand LRAD, LULN, LHA Mid-point of LRAD and

LULN

LRAD - LULN LHA and radius of 0.05 m

Right Foot RLA, RMA, RTOE,

RHEEL

Mid-point of RLA and RMA

RLA - RMA RTOE and radius of 0.05 m

Left Foot LLA, LMA, LTOE,

LHEEL

Mid-point of LLA and LMA

LLA - LMA LTOE and radius of 0.05 m

Golf Club Shaft OBJ1, OBJ2, RHA RHA RHA and radius of 0.02 m OBJ3 and radius of 0.005 m

105

4.3.2.1 Functional Joint Centres

An additional feature of Visual 3D is the ability to determine functional joint centres (FJC) as opposed to SJC which rely on predictive methods. The limitations of determining joint centres with predictive methods (e.g. SJC) are the errors associated with estimating joint centre co-ordinates through palpation techniques and errors due to the regression equations used. Functional joint centres allow the determination of subject-specific joint centres based on marker displacement data and can overcome the limitations associated with the predictive methods. The algorithm used to determine functional joint centres requires movement of one segment relative to another segment and then finds a position that is stationary relative to the two segments (C-Motion, 2011). The algorithm used by Visual 3D is based on Schwartz and Rozumalski (2005) method for estimating joint parameters. For the hip and shoulder joints, with three degrees of freedom (3 DOF) a movement trial is required where the joint moves about all three axes of rotation individually (i.e. flexion-extension, abduction-adduction and circumduction).

In this thesis, to determine shoulder FJC the golfers stood in the anatomical position and were asked to perform shoulder flexion-extension, abduction-adduction and shoulder circumduction movements. For the hip FJC golfers were asked to perform right thigh flexion, abduction and circumduction movements. Previous studies have examined the effect of the number of cycles of movements, velocities and range of movement (Begon et al. 2007). Based on these recommendations and those in the Visual 3D documentation, the golfers were asked to perform five cycles of the movements, at a moderate speed and to limit movement to approximately 20º in each direction. A detailed description of the process can be found in Schwartz and Rozumalski (2005). Calculated FJC were then used to determine segments relative to these subject specific anatomically determined locations (Figure ‎4.10).

106 Figure ‎4.10. Example locations of functional (FJC) and static (SJC) joint centres for the right hip. Example difference in x, y and z positions for a single golfer are 0.03 m, 0.02 m and 0.04 m respectively