Golf Swing Motion Analysis: Challenges And Solutions (P2)

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Events And Impact Conditions

A total of nine events were defined for the golf swing analysis: Start (ST), Address (BA), Top of Backswing (BT), Upright Club Position during Downswing (DV), Horizontal Club Position during Downswing (DH), Ball Impact (BI), Mid Follow-Through (MF), Start of Impact Zone (IZ1), and End of Impact Zone (IZ2). The event frames were identified by the automatic event detection function based on a set of standard and user-defined variables. To compute the impact conditions, the precise locations of the ball and clubhead, and orientation of the clubface at impact must be known and a pre-requisite for this is an accurate determination of the impact instant. When the sampling rate is not sufficiently high, interpolation of the position data is necessary to improve the time resolution. Auto detection of the BI, IZ1, and IZ2 events involved the following steps:

  1. The position data were re-sampled (interpolated) at 2,000 Hz. All standard and userdefined kinematic and kinetic variables were re-computed accordingly.
  2. The first frame in which the clubface was in contact with the ball was selected as the BI event. Fifteen centimeters before and after the impact position were set to IZ1 and IZ2, respectively. IZ1 and IZ2 define the ‘impact zone’ for the assessment of the error margins.

Impact Conditions And error MarginsImpact Conditions And error Margins

The impact conditions include the clubhead velocity, clubface orientation, and impact location on the clubface at the true impact instant, not BI. The following steps were taken in the computation of the impact conditions:

 

  1. The clubface reference frame was defined based on the clubface markers (Williams & Sih, 2002): X – away from the body; Y – normal to the clubface; Z – upward. The origin of the clubface reference frame was set at the clubhead center, the mean of the four clubface markers (Figure 1).
  2. The first frame the Y-coordinate of the ball described in the clubface reference frame became smaller than +2.134 cm (radius of the ball) after the DH event was automatically selected as the BI event. As a result, the true impact instant (when the Y-coordinate of the ball described in the clubface frame becomes equal to the radius of the ball) occurs sometime between the BI frame and one frame before. The true impact instant was determined by the sub-frame data reading function.
  3. The velocity of the clubhead center was used as the clubhead velocity (Figure 1). The angles formed by the Y-axis unit vector of the clubface frame with respect to the global horizontal and vertical planes toward the hole-cup were used as the clubface orientation parameters. The impact location on the clubface (X- and Z-coordinates) was determined by the ball position described in the clubface reference frame at the true impact instant.
  4. The ranges of the clubhead velocity, clubface orientation, and ball position described in the clubface frame within the impact zone (IZ1 to IZ2) were used as the error margins.

Functional Swing Plane

The functional swing plane (FSP) characterizes a golfer’s downswing motion (Shin, Casebolt, Lambert, Kim, & Kwon, 2008). The FSP is the plane formed by the clubhead trajectory within the delivery zone (DH to MF) (Figure 1). The FSP parameters (planarity, slope, and direction angle) were computed using the ‘Plane Analysis’ function. A special plane fitting algorithm based on the Newton-Raphson method is used in this function. The locations of the instantaneous rotation centers and radii of the rotation arms were computed (Figure 1).

The functional swing plane (FSP) characterizes a golfer’s downswing motion (Shin, Casebolt, Lambert, Kim, & Kwon, 2008). The FSP is the plane formed by the clubhead trajectory within the delivery zone (DH to MF) (Figure 1). The FSP parameters (planarity, slope, and direction angle) were computed using the ‘Plane Analysis’ function. A special plane fitting algorithm based on the Newton-Raphson method is used in this function. The locations of the instantaneous rotation centers and radii of the rotation arms were computed (Figure 1).

Segment Orientations And Joint Motions

Various body orientations and joint motions should be computed in the golf swing analysis based on the local reference frames fixed to the body segments. In a segment at least three fixed points are required to define a reference frame. The golf-specific marker set presented in Table 1 provides a complete array of local reference frames of the golfer’s body segments and club.

Reference frames were defined easily along with the relative orientation relationships and rotation sequences (e.g. mediolateral-anteroposterior-longitudinal sequence). In some cases, a segment (e.g. hand-club) was defined multiple times to assign different relative orientation relationships. The forearm segment was divided into ulna and radius and the pronation/supination of the forearm was computed separately from the wrist joint motion. The clubshaft and clubhead segments were also included in the model.

One way to expand the scope of analysis is to define and use various imaginary segments in addition to the real body segments. For instance, the FSP segment was included in the body model as an imaginary segment to utilize the FSP reference frame in the analysis. The shoulder girdle segment (and reference frame) was also defined to analyze the rotational motion of the shoulder about the hub.

Summary

To meet the unique challenges encountered in various analysis settings, it is important to develop/use a comprehensive motion analysis program characterized by strong mechanical foundation, flexibility, adaptability, and expandability. The program structure must allow the users to easily create/modify body models and the body models should define the mechanical characteristics of the body in detail. A vast array of processing capabilities and functions should be available and configurable by the user to streamline the analysis while eliminating the need for additional user programming. Golf swing analysis is an advanced application of motion analysis in which advanced analysis methods are required to resolve unique challenges encountered. It was demonstrated how the unique challenges could be resolved in golf swing analysis by using a comprehensive motion analysis program, such as Kwon3D.

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