2.2.1. Putting Stroke
The putting technique requires skills like green reading, control of ball distance, accurate aim, and excellent ball roll kinematics (Hurrion & Hurrion, 2002). Brooks (2002) described three types of putting strokes recommended for golf instruction, about the target line, which are, the straight back to straight through, inside to inside, and inside to straight through. After examining the strokes using mathematical models, Brooks (2002) did not arrive at a conclusion to which stroke is best. However, a putting stroke was during the backswing the putter head moves inside the aim line and where the putter face is square to the putter path, which means the putter face is open to the aim line at the end of the backswing was endorsed.
Pelz (2000) advocates a different type of putting stroke, where the path is linear, and the putter face is square to the path throughout the stroke. Pelz (2000) recommends this type of
stroke with the putter face square, as there may be timing limitations, resulting in the inability to square the clubface precisely at impact. The main argument against what was proposed by Pelz (2000) is that the straight stroke is more biomechanically complex 19 than it first seems, as it relies on a fully horizontal axis of rotation for the putter, and muscle activity that will compensate for the deviation from the horizontal axis (Karlsen et al. 2008a). Neal and Wilson (1985) modelled the golf putt as a double-pendulum system composed of two arms and the putter. It was described that the shoulder is meant to roll in an up-and-down fashion, and the two hands hold the putter, moving back and forth in a symmetrical pattern (Neal & Wilson, 1985).
2.2.2. Performance index putting stroke kinematics
Delay, Nougier, Orliaguet, and Coello (1997) observed novices showing the normal pendulum motion while expert players did not. Expert players demonstrated an asymmetrical pattern where a longer follow-through was observed (Delay et al. 1997; Sim & Kim, 2010).
Sim and Kim (2010) analyzed the differences between expert's and novice's accuracy in golf, putting in regards to impulse variability. Schmidt, Zelaznik, Hawkins, Frank, and Quinn (1979) developed an impulse variability model to send a ball close to the target, the magnitude of the impulse applied to the ball by the putter needs to be precise. Inputting, the moment of impact is extremely brief. Therefore, the putter's velocity at impact becomes extremely important in achieving accuracy (Sim & Kim, 2010). Research has demonstrated that movement at impact is not decided upon the moment of impact, but that movement is attuned and planned from initiation of the movement (Bootsma & van Wieringen, 1988; Coello, Delay, Nougier &
Orliaguet, 2000) and through the period of swinging up to impact referred to as downswing (Müller & Abernethy, 2006). Delay et al. (1997) suggest that movement control may not be complete at impact. Sim and Kim (2010) results showed that expert players had a lower level
of velocity than the novice group; the experts also achieved increased accuracy; Delay et al.
(1997) reported the same results. The question raised from both of these sets of results is how the expert group reached the target with a lower velocity. Delay et al. (1997) suggested that energy produced by novices might not entirely be transferred to the ball, with more energy loss at the moment of impact.
Sim and Kim (2010) support this claim as it was observed that expert players maximum velocity occurred after impact, which means the ball gained stronger impulse when it left the putter face rather than at the moment of impact. A second explanation was provided by Delay et al. (1997), which concerned the ball roll. During experimentation, it was observed that the novice players ball often bounced during rolling, whereas the experts' ball glided smoothly. Sim and Kim (2010) further this theory of different types of ball roll; they suggest expert players achieve greater energy efficiency by striking the ball with the putter during the rising phase of the stroke during increasing velocity so that it rolls rather than slides towards the target. This would explain how the 20 expert players reached the target with reduced impact velocity as a ball will lose less kinetic energy when it rolls in comparison to when it slides (Sim & Kim, 2010).
MacKenzie and Sprigings (2005) state that several elements are needed to hit a successful putt. The first being that the golfer must correctly read the green, to determine the correct target line and establish the optimal speed to impact on the golf ball to project it towards the target (hole). During the putting stroke, the putter should only demonstrate horizontal velocity in the direction of the decided target line, reducing elements of velocity in other directions, which would be undesirable. This will ensure the plane of the putter face with be perpendicular to the original putting line (MacKenzie & Sprignings, 2005). A technique that can be used to read the green is the plumb-bob method; this is where the golfer stands behind the ball, straddling an imaginary line that bisects the hole (MacKenzie & Sprignings, 2005). The golfer then suspends the putter at an arm's length in front of the face allowing gravity to pull the shaft into a correct
vertical alignment (Foston, 1992). Although it has been proven successful in individual professionals, Mackenzie and Sprigings (2005) deem it to be an unreliable method to determine the intended target line due to the high sensitivity of the plumb-bob method to confounding factors.
2.2.3. Relation of putting stroke kinematics parameters to ball kinematics
Karlsen et al. (2008a) examined the golf, putting stroke, and determined three main determinants of direction variability. These were putter face angle that was accountable for 80%
of the variability, putter path accounted for 17% variability, and the horizontal impact point on the putter face accounted for 3% (Karlsen et al., 2008a). Pelz (2000) only considered two factors that contribute to direction variability, firstly, putter face angle (83%) and putter path (17%).
Therefore, the putter face angle may be the most critical club head kinematic variable regarding golf ball direction variability, as highlighted by Karlsen et al. (2008a) and Pelz (2000). To date, no study has examined body movements' effect on the putter face and performance variability.