In golf, the address position is a term broadly used to describe the position the body adopts to hit the ball (Zhang and Shan, 2014). The kinematics and kinetics of the address positions of elite golfers have been highlighted as important areas to study for improving swing skills (Okuda et al., 2010; Wrobel et al., 2012; Zheng et al., 2008a).
Ball position is among the various constituent elements of the address position that have been investigated in previous studies. They found that the ball position can lead to changes in the distance, direction, and trajectory of ball flight (Chen et al., 2007; Bradshaw et al., 2009; Zhang and Shan, 2014). Chen et al. (2007) found that the optimum ball position along the mediolateral direction increases club-head velocity at impact. Zhang and Shan (2014) found that variability in the ball position along the mediolateral direction directly influences the execution of a consistent golf swing, and that a variation of [+ or -] 0.6 cm in the ball position could translate into a variation of [+ or -] 3[degrees] in the vertical ball launch angle. Thus, minor changes in the ball position can lead to changes in the distance, direction, and trajectory of the ball flight.
However, these previous studies on ball position provide a limited biomechanical explanation regarding the relationship between ball position and ball flight. One possible interpretation is that the angle of the body joints in the address position changes to adapt to the different ball position; therefore, the change in the address position can influence the golf swing, which will ultimately affect the ball flight (Smith et al., 2012). However, it is unclear whether the address position is affected by a change in the ball position.
Many swing coaches also emphasize the importance of finding the optimal address position in their instructions for maximum performance, and focus on the ball position for a well-balanced address position (Cochran and Stobbs, 1968; Dusek, 2006; Hogan, 1957; Leadbetter, 1990; 1993; Murphy, 2011; Nicklaus, 2005; Peper and Frank, 1997; Watson, 2011; Wiren, 1990; Woods, 2001). If so, an in-depth evaluation of the address position in association with the ball position could provide valuable data for swing coaches.
There are relatively little published data on the address position. Most studies have included the address position as part of the golf swing, and therefore, few address position variables have been examined. Zheng et al. (2008b) quantified trunk orientation at the address position using a three-dimensional (3D) motion analysis system. They found that female professional golfers have 3[degrees] less trunk flexion than male professional golfers. Although previous studies have focused on the kinematics of trunk flexion, future work should explore the joint kinematics of the lower body, kinematics of the upper extremities, orientation of the golf club, vertical ground reaction force (VGRF), and center of pressure (COP), to have an in-depth understanding of the address position.
This study aimed to examine address position variables with respect to changes in the ball position along the mediolateral (M-L) and anteroposterior (A-P) directions. We hypothesized that there is a significant difference in address position variables in response to a change in the ball position along the M-L and A-P directions. Such variables include the kinematics of trunk flexion, joint kinematics of the lower body, kinematics of the upper extremities, orientation of the golf club, VGRF, and COP.
Eleven right-handed male professional golfers from the Korea Professional Golfers' Association (mean [+ or -] standard deviation: age 27.82 [+ or -] 3.87 years, height 1.78 [+ or -] 0.07 m, mass 75.23 [+ or -] 8.54 kg) volunteered to participate in this study. G*power was used to assess the number of participants required for this study ([beta] = 0.15 and [alpha] = 0.05). The participants had no history or complaints of chronic pain, major injuries, or had undergone surgery in the preceding 6 months. The study protocol was approved by the institutional review board, and all participants provided informed consent.
Testing was performed in an indoor facility, using a motion analysis system with eight infrared cameras (Vicon MXF20, Oxford, UK, 250 Hz) and two force platforms (OR67; AMTI, Watertown, MA, USA; 2000 Hz) to capture the address positions. Participants hit golf balls off an artificial turf surface into a net located 5 meters in front of the contact position. This experimental setting of the net position was intended to reduce bias in respect of the address position change due to the ball flight feedback. A target line with a diameter of 30 cm was attached to the center of the net; participants aimed at the target line.
Each participant wore a fitted indoor outfit and the same shoe type to ensure data accuracy. The necessary anthropometric information was obtained and entered into the motion analysis system. Anthropometric measurements of the lower extremities were taken, including body mass and height, leg length, knee width, and ankle width. Anthropometric measurements of the upper extremities included shoulder offset (the vertical distance from the center of the glenohumeral joint to the marker on the acromion-clavicular joint), elbow width, wrist width, and hand thickness. Thirty-five reflective markers (diameter 14 mm) were placed on anatomical landmarks based on the Vicon[R] Plug-in-Gait model: left front head, right front head, left back head, right back head, 7th cervical vertebrae, 10th thoracic vertebrae, jugular notch, xiphoid process, right scapula, acromion-clavicular joint (left/right), lateral epicondyle (elbow, left/right), radial side of the wrist bar (left/right), ulnar side of the wrist bar (left/right), hand (just below the head of the second metacarpal, left/right), left anterior superior iliac spine, right anterior superior iliac spine, left posterior superior iliac spine, right posterior superior iliac spine, lateral thigh (left/right), lateral epicondyle (knee, left/right), lateral malleolus (left/right), lateral tibia (left/right), second metatarsal head of the foot (left/right), and calcaneus (left/right). Additionally, three markers were attached to a five-iron club head to identify the club-face aim, loft angle, and the phase of the address. Two markers were attached to a club shaft to identify the lie angle. Reflective adhesive tape was attached to a golf ball (Figure 1).
The participants completed a self-selected warm-up for a minimum of 10 minutes that involved several golf shots. After the warm-up, the participants were asked to assume their preferred address position with the golf ball, and the positions of each foot were outlined by attaching tape to the force plate in the form of a cross over the toe and the heel (Figure 1). To find an accurate reference ball position, the participants were asked to assume their address position five times while the foot position was fixed to the outline, and participants were then able to move the ball position. We calculated the mean ball position using the positional data of the golf ball from the five trials. The positional data of the golf ball was calculated from the origin of the global coordinate system, which was located at the edge of the force plate that the right foot was located on.
The LPGA Teaching Manual (2000) states that the correct ball position is different for each golfer. Thus, in this study, the reference ball position was determined as the preferred ball position of each participant. Additionally, if the same reference ball position was used, different levels of discomfort bias could have occurred among the participants.
The M-L ball position testing conditions were the length of one golf ball...