2.1. Instrumentation and Data Analysis
In this study, rather than collecting new empirical data, we utilized an existing dataset from previous research that detailed the golf swings of two female golfers with different skill levels [
14] (
Table 1). This dataset was chosen for its comprehensive capture of biomechanical movements using high-precision motion capture technology and ground reaction force measurements, making it highly suitable for our analysis.
The original data were collected using a sophisticated array of twelve high-speed Qualisys cameras, which accurately recorded the three-dimensional positions of reflective markers attached to the golfers and their clubs. This setup was designed according to the standards of the International Society for the Advancement of Kinanthropometry, ensuring that the biomechanical analysis adhered to high measurement accuracy and reliability (
Figure 2). Ground reaction forces were measured using a Kistler force plate, providing essential data on the biomechanical forces exerted during the swings.
By reusing this existing dataset, we were able to directly apply our analytical models without the variability and resource constraints associated with new data collection. This approach also allowed us to focus on in-depth analysis using established data, ensuring that our study was both resource efficient and grounded in reliable biomechanical metrics. The dataset included detailed annotations of each golfer’s movements and the corresponding biomechanical outputs, which facilitated a nuanced analysis of the swing mechanics. A total of twenty-four reflective markers and four marker clusters were used to reconstruct twelve body segments, including the head, torso, upper arms, forearms, hands, pelvis, thighs, shanks, and feet, providing a detailed representation of the golfer’s body dynamics during the swing for a comprehensive biomechanical analysis. Additionally, specific markers were placed on the club and wrist joint to define their respective reference frames. Marker coordinates for the period between the beginning of the downswing of a golf swing and up to the instant before impact were then acquired.
Motion capture was undertaken using an optoelectronic system of twelve Qualisys cameras (type: Oqus-300, Qualisys AB, Göteborg, Sweden) operating at 300 Hz. The collected data were processed using Qualisys Track Manager (QTM) software version 2.6 to ensure accurate tracking and reconstruction of the marker trajectories. For the club, markers were placed at specific points along the shaft and clubhead to capture its motion accurately. The wrist joint was defined using markers placed on anatomical landmarks around the wrist to establish a local reference frame.
Using the predefined anatomical coordinate system from the original study, which was based on key landmarks identified on the golfer and the club, we analyzed the motion data to extract the Instantaneous Screw Axes (ISAs) and their evolution during the downswing phase. This previously recorded data provided a robust basis for exploring how biomechanical properties such as the ISAs correlate with golfer skill level and technique efficacy. Reusing data from a well-designed previous study can enhance the efficiency of the research process and contribute to the sustainability of research practices by utilizing existing resources. This method helps to ensure that findings are based on previously validated data, which can support the reliability of the conclusions drawn from the analysis.
Force Space Mapping via the Inertia Tensor: To connect the kinematic data with the force dynamics, we applied the inertia tensor to map the golfers’ spatial acceleration space in the Kinesphere (represented as an icosahedron) to the force space (Effort Cube). This mapping allowed us to analyze how golfers manage and adapt to the inertial forces of the club during different swing phases. The inertia tensor serves as a mathematical representation of how the mass and shape of the club influence the forces experienced by the golfer.
Inertia Tensor Calculation: The inertia tensor was computed for each swing, taking into account the mass, center of mass, and geometric distribution of the club relative to the wrist joint. This tensor was then decomposed into principal axes of inertia, allowing us to quantify how the club’s inertia influenced the golfer’s movements.
Principal Axes of Inertia: By comparing the principal axes of inertia to the calculated ISA, we were able to determine how well the golfer’s movements aligned with the club’s inertia. Skilled golfers were able to maintain alignment with these axes, resulting in more efficient and powerful swings, whereas novice golfers struggled to maintain this alignment.
Screw Theory and Volute Phrasing: The biomechanical analysis was further enhanced through the application of screw theory, which provides a unified framework for describing both the rotational and translational aspects of the golfer’s movement. This allowed us to quantify the three-dimensional spatial pulls that occur during the swing.
Volute Phrasing: Laban’s concept of volute phrasing, which describes three unequal spatial pulls that change in a graded, proportional manner, was applied to analyze how the golfers adapted to the club’s inertia. This phrasing allowed us to identify how golfers transitioned between phases of the swing (e.g., from backswing to downswing) and how they managed the changing forces.
2.2. A Brief Overview of the Framework of Laban Movement Analysis
Laban’s Eight Effort Actions, also known as Laban Effort Drives, are a key part of Rudolf Laban’s system for understanding and notating dance and movement, known as Laban Movement Analysis (LMA) [
15,
16]. These Effort Actions are used to describe the quality of movement and are organized according to Laban’s Effort-Shape theory, which considers both the inner intention and the outer shape of the movement. The eight Effort Actions are categorized into four pairs of opposite qualities, with each pair representing one of the factors of motion: Weight, Time, Space, and Flow (
Figure 1a).
The golf swing, a sophisticated biomechanical action, encapsulates these Effort elements in its execution. The swing begins with a preparatory phase where the golfer aligns with the target, transitioning to the backswing, culminating in the powerful downswing. Each phase can be mapped to Laban’s Effort Actions, offering a qualitative lens through which to analyze and refine movement (
Figure 1b). The icosahedrons in
Figure 1b are depicted from different perspectives to provide a comprehensive understanding of spatial orientations. The icosahedron on the left-side is viewed from the front, showing “A4” on the left and “A7” on the right. Conversely, the icosahedron below and to the right is viewed from behind the subject, with “A4” on the right and “A7” on the left. This distinction helps illustrate the spatial dynamics and orientations from multiple viewpoints, enhancing the analysis of movement.
Laban’s space harmony offers a unique perspective on the golf swing through the lens of the icosahedron and the A-scale: The A-scale, as shown in
Figure 1b, is a specific movement sequence that demonstrates the first half and second half of the A-scale, right arm leading, with volute phrasing [
17]. The A-scale, originally designed for analyzing fencing movements, can be applied to golf to understand the swing better. Here is a simplified explanation:
Remember, the A-scale does not represent the entire golf swing, but rather provides a framework for understanding and potentially improving the spatial aspects of the swing. It is a tool that can offer insights into the complex spatial relationships involved in this athletic movement.
The bottom-left panel of
Figure 1a illustrates true 3D directions, which are conceptually linked to the icosahedron depicted in
Figure 1b. The icosahedron represents spatial orientations and movements, known as “Ai”, that are critical in understanding the spatial dynamics within Laban Movement Analysis. This linkage helps in visualizing how movements transition through different spatial planes and directions.
It is not compulsory to always use motions with 6 + 6 “Ai” or to include all the “Ai” in every analysis. However, incorporating these motions can be a very useful tool for golfers to become more aware of their bodies and their movements through space in all directions. These motions, known as choreutic forms or forms of space harmony, represent shapes and designs of energy around and through the space of the mover. They are oriented on a grid of the 27 directions, divided into three groups: the 6 simple high, wide up and down depth directions; right and left; and forwards and backwards. Additionally, there are the 8 high and lower corner diagonals and the 12 in-between directions on the horizontal, frontal, and sagittal planes, such as deep right, forward high, and center.
The 6 + 6 motions are particularly useful as they combine on-balance stability in a natural way, while simultaneously mobilizing forces of tilting, twisting, and the shifting of weight off the vertical line. By practicing these motions, golfers can improve their spatial awareness, balance, and coordination, which are essential for executing effective and efficient swings.
Building on this foundation, we have designated the term “X-scale” to describe a series of key positions throughout the golf swing, as shown in
Figure 3. This designation is not a standard term in Laban Movement Analysis (LMA) but rather a label chosen for clarity and convenience within the context of this research. The X-scale is intended to illustrate the spatial and directional movements that a golfer performs during a swing, providing a framework for analyzing the golfer’s body mechanics and spatial orientation. By practicing and internalizing these movements, golfers can develop a heightened sense of spatial awareness and a smoother, more efficient swing.
Detailed Description of X-Scale Positions:
X1—Start Position (Downward in Front of
Figure 3): The golfer’s initial stance with the club positioned downward in front of the body. This position sets the foundation for balance and alignment.
X2—Right Sideward Middle (Door Plane, Vertical): The club moves to the right side, halfway up in the vertical plane. This marks the early phase of the backswing, initiating the rotational movement.
X3—Right Backward High (Table Plane, Horizontal): The club continues upward and backward in the horizontal plane, reaching a high point. This position represents the mid-backswing, where the shoulders and hips rotate, and the weight shifts to the right leg.
X4—Backward High (Wheel Plane, Sagittal): At the top of the backswing, the club is positioned high and directly behind the golfer. This is a critical point where the potential energy is maximized, corresponding to A5 in the A-scale.
X5—Right Backward High (Table Plane, Horizontal): Similar to X3, this position reinforces the horizontal plane’s movement, emphasizing the rightward and backward trajectory during the backswing.
X6—Right Sideward Middle (Door Plane, Vertical): As the downswing begins, the club returns through the right side in the vertical plane. This transition involves uncoiling the body and shifting weight toward the left leg.
X7—Forward Low (Wheel Plane, Sagittal): The club moves forward and low, approaching the impact point. This position highlights the acceleration phase of the downswing, where speed and precision are crucial.
X8—Left Backward Low (Table Plane, Horizontal): Post-impact, the club follows through to the left and backward in the horizontal plane. This position indicates the initial phase of follow-through, where the golfer’s weight shifts fully to the left leg.
X9—Left Middle (Door Plane, Vertical): The club continues its path through the left side in the vertical plane. This phase ensures the continuation of the follow-through with maintained balance and coordination.
X10—Left Backward High (Table Plane, Horizontal): The club reaches a high point on the left side in the horizontal plane. This position represents the completion of the follow-through, with full body rotation.
X11—Backward High (Wheel Plane, Sagittal): The final position, where the club is high and behind the golfer, similar to X4. This position signifies the end of the swing, where the golfer’s body is fully extended and balanced, corresponding to A5 in the A-scale.
The X-scale is a practical application of the LMA principles, particularly the A-scale, in the context of golf. The A-scale provides a foundation for understanding movements in three directions, helping golfers develop a better spatial and body awareness. By practicing the X-scale, golfers can internalize the flow of movement, leading to a more natural and efficient swing.
The Effort Actions described in
Table 2 for each phase of the golf swing can be mapped onto Laban’s Effort Graph (
Figure 1a), providing a visual representation of the dynamic qualities of movement throughout the swing. This mapping allows us to see how the float, glide, slash, and free flow actions correspond to specific combinations of Effort qualities illustrated in the graph.
The correct understanding of
Figure 4 requires focusing on the arrows rather than the planes themselves. To aid in this interpretation, we will label the human figures expressing the three planes with the same a, b, and c labels. The colored red, blue, and green elements in the figure represent a two-dimensional projection of the Effort Cube, visualizing the “three unequal spatial pulls” throughout the swing. We distinguish the Effort Cube, representing the qualitative dynamics of Weight, Time, Space, and Flow, from the true 3D motions depicted by the icosahedron/Ai inclinations. This distinction clarifies how each analytical tool contributes to understanding the golf swing’s spatial and dynamic aspects.
The X-Scale provides a structured approach to analyzing the spatial and directional aspects of the golf swing, aiding in the development of spatial awareness and flow in the golfer’s movement.
2.3. Integrating Laban Movement Analysis with Golf Swing Dynamics: Mapping from the Effort Cube to the Kinesphere Through the Inertia Tensor
Laban’s theory of movement, rooted in the spatial dynamics of the human body, suggests that individuals can extend into and manipulate their surrounding space without changing their location, which we shall call the “stance” [
17]. This theoretical framework becomes particularly relevant in analyzing how a golfer modulates their movements in response to the golf club’s inertia during various swing phases. While
Section 2.2 provided an overview of spatial interactions, here we focus on how shaping influences these interactions by accommodating the characteristics of the golf club’s inertia surface.
Figure 3 is instrumental in illustrating this concept. It shows not just the directional orientations and areas within the Kinesphere, but more importantly, how these are modulated through the golfer’s adaptive responses to the inertia properties of the golf club. The “Xi” symbols in
Figure 3 transition from abstract indicators of Effort Actions to tangible demonstrations of how the golfer’s body shapes and conforms to the club’s inertia. The orientation of the 3D axes, clearly marked in the figure, helps delineate this process.
While Laban’s space harmonic research provided valuable insights into human movement, its application was limited by the use of Cartesian analysis, which does not fully capture the complex, three-dimensional nature of movement. To address this limitation and provide a more comprehensive understanding of the golf swing, we introduce screw theory as a unifying concept that combines rotation and translation into a single theoretical model.
The golf swing, particularly the downswing phase, exemplifies what Laban referred to as volute phrasing [
17]. This involves three unequal spatial pulls that constantly change their relationship to each other through transverse movement. This movement can be likened to a three-dimensional spiral, where vertical, sagittal, and horizontal components change in a graded, proportionate way, cutting or sweeping through space.
This integration of screw theory with Laban’s concepts provides a powerful tool for understanding the complex spatial relationships and dynamic qualities of the golf swing, offering insights that can be applied to both skilled and novice golfers. It allows us to quantify and visualize the “three unequal spatial pulls” [
17] and their constant changes throughout the swing, providing a more accurate representation of the movement’s space harmonic qualities.
This study employs screw theory [
18,
19] as a foundational mathematical framework to describe the spatial vector quantities relevant to the golf swing. Screw theory facilitates the representation of the six degrees of freedom of a rigid body (three translational and three rotational) in a unified manner. By defining the Instantaneous Screw Axis (ISA) of the golf club, we capture both the rotational and translational dynamics essential for analyzing the complex movements in golf.
Consider the situation in
Figure 5: A club has a mass,
m; its center of mass,
CM, is given by the position vector,
C; and the inertia tensor about its center of mass is
J. The club is at rest, and experiences a force,
f, acting along a line passing through the center of mass, and a couple,
τC. The resulting acceleration is determined by an angular acceleration, α, along an axis passing through the center of mass (
CM), the linear acceleration,
aC, on
CM.
To clarify our approach, we did not directly measure the inertia tensor. Instead, we used data obtained from a club manufactured by the same company as the one used in our experiment. We then applied a geometric scaling method to these data to estimate the mass moment of inertia for our specific club. This scaling was based on the principle, as detailed by Zatsiorsky (1998) in Section 4.4.2 of his book [
20], that the moments of inertia of each segment are proportional to the mass times the square of a linear dimension.
Furthermore, we fine-tuned the segment parameters (including mass, center of mass location, and inertia) using the method described by MacKenzie and Sprigings (2009) [
4]. This tuning process allowed us to adjust the parameters to match the specific club used in our experiment. The resulting inertia tensor, presented in
Figure 5, is therefore a product of this scaled and tuned approach, derived from established data and methodologies, rather than direct measurement. This method allowed us to obtain a reliable estimate of the inertial properties while accounting for the specific characteristics of our test club.
Therefore, the inertia matrix included in
Figure 5 is a result of our own calculations based on these established methods and datasets (units in
). As such, the matrix is our original work, derived from the referenced data.
The equation of motion is considered as a mapping from the twist-like screw acceleration to a wrench space [
14].
where
== M, and 1 is the 3×3 identity matrix.
Since all of the spatial quantities are referred to with respect to the center of mass, the linear and angular components of motion are decoupled—the linear acceleration being entirely due to the force, and the angular acceleration being a result of the couple. To transform Equation (1) into the origin of the joint axis (
Figure 5), we obtain the following:
where
H =
C ×
M,
I =
J +
C ×
MC ×
T, and
C× is the anti-symmetric skew matrix corresponding to C.
Due to its special 6×6 form, the spatial inertial tensor is as follows:
is expected to have special eigen structures.
The spatial inertia tensor
M0 represented at the origin is a symmetric, positive definite tensor and transforms to any point ‘A’ by the spatial Jacobian,
Φ, according to the following:
where
and
.
The eigenvalue problem provides a unique decomposition of
M0 as follows:
where
mf and
mγ are representing the eigen values of mass and mass moment of inertia, respectively (following the common notational tactics for the principal axes of inertia, we use
for the corresponding eigenvectors to
mγ and, to the corresponding principal moment of inertia,
for
γ). We state that
I3 is the largest principal moment of inertia, representing the mass distribution along the longitudinal axis of the golf club. The
e3 eigenvector, known as the principal axis of inertia, is aligned perpendicular to the longitudinal axis and passes through the mass center of the golf club. This alignment contributes to the perceptual simplifications obtained by using it as the axis of reference and is critical for understanding the dynamics of the golf swing.
One might wonder whether the decomposition based on the solution for the free-vector eigenvalue problem would be different at another point A. We apply the transformation rule (Equation (5)) to the above decomposition (Equation (6)) as follows:
which shows that
MA is decomposed by the same eigenscrews in the same manner, just represented at B. That is, no matter where the problem is posed, the same eigenscrews form the basis of the decomposition.