Frequently Asked Questions
Reproduced from Golf BioDynamics' Professional Pages.
1. Explain the importance of biomechanics in transforming our understanding of the golf swing, golf instruction and physical conditioning.
Understanding various aspects of biomechanics is crucial to knowing why certain movements or patterns of movement are necessary features of any effective and efficient golf swing. For example, properties of our muscles such as their “force-velocity” characteristic and their ability to produce increased work when an eccentric muscle contraction is followed immediately by a concentric one determines, respectively, optimal “sequencing” or “timing” in the downswing as well as the types of movements that are most efficient and powerful. Once you understand these types of ideas, the flow on to instruction is (almost) obvious! Instruction needs to focus on the movements of the body that utilize the principles, allowing the body to “self-organize” and produce efficient movement sequences. Similarly, a person’s physical state will predicate the amplitudes of movements. That is, a person’s levels of strength, flexibility, power, control and coordination will dictate their optimal golf swing. It makes no sense, a coach asking their student to put their body into a certain position or to move it a certain way if they do not have the physical capability to do that. Flexibility is an obvious one (e.g., someone may just not be flexible enough to get a large X-Factor or coil at the top of the backswing) where the length of the muscles and tightness of the body control how much coil can be generated. Similarly, if a person does not have good single leg control (i.e., is not good at balancing on one leg), then they are unlikely to transfer weight from one leg to the other very well because they are unable to do so and at the same time maintain good balance and stability.
2. I first heard about practical methods of quantifying the biomechanical movements in the golf swing at the World Scientific Congress of Golf, at St Andrews in 1994, in a paper about the SportSense system. However, I understand that your thesis (originally using cameras) predates this. Please briefly discuss the background of your work and the development of the 3D system.
The first published papers investigating golf swing kinematics and kinetics appeared in the early 1980’s. My Master’s thesis and subsequent published work (Neal & Wilson, 1985) was pioneering work in 3D biomechanical analysis of the golf swing. In these early works, high speed film cameras were used to create images of the golfers from a variety of angles as they swung in a “calibrated” volume. Once the films were processed at a lab, every film frame from each camera view was projected and digitized (i.e., XY coordinates of key parts of the body were determined). These data were “combined” mathematically such that 3D coordinates of these landmarks were available for analysis. Standard mechanics principles were then used to generate the important variables required to describe the golf swing (angular and linear kinematics). This whole process of analysis took approximately 8 hours to complete (depending on the number of cameras that were used and the sophistication of the model). These days, depending on the type of system that is used, this whole process can be completed in close to real-time. Thus the efforts these days focus on making sense of the data and interpreting the information for the benefit of the coach and the player.
3. How has the concept moved on since those early days, and how does the magnetic field principle of the 3D system differ from the electronic and light/laser measuring principles of other biomechanical systems (SportSense, Biomeca, TaylorMade MATT system etc.)?
As stated above, various systems have evolved over the last 20 years for measuring movement kinematics. These measurement systems can be basically classified into two classes: active and passive marker systems. Passive systems (VICON™, Motion Analysis Corporation™ and TaylorMade’s MATT™ system) are based around the tracking of retro-reflective (passive) markers, attached to body landmarks, by multiple (4 - 10) high speed cameras. These cameras are usually permanently mounted around a laboratory. The major benefit of such systems is that the golfer is free to move anywhere (within a calibrated volume) without any wires attached. On the down side, ‘real-time’ 3D movement analysis is not possible (post-processing would typically take approximately 20 min per swing). Further, since only the 3 translational DOF are possible with spherical markers, substantial analytics need to be performed to get true 6DOF movement kinematics. Thus, real-time biofeedback is NOT possible with such systems. Importantly for golf, passive marker systems cannot be used when direct sunlight falls on the subject. Another weakness (overcome by using many cameras - at increased cost of course!!) is that a camera cannot see markers that are obscured by the body. For example, if a camera is placed down the target line, any marker on the left hand side of the body could not be “seen” by this camera when the golfer is at address (or a similar body orientation).
Active marker systems (e.g., Polhemus™ & Ascension™) can be wireless or tethered and rely on actual transducers of some form inside the sensor that is attached to the body. The size of these sensors has, over the last 15 years, decreased while both their accuracy and sensitivity has improved; a result of developments in microchip technology. These systems are now capable of producing full 6-DOF 3D measurements in real-time. The greatest benefit of such systems is that they can then be used for real-time biofeedback. These systems rely on the well-known and documented physical principle that a metal object, when moved within a magnetic field, induces an electric current. Since magnetic fields are not blocked by the human body, markers can always be “seen” and the task of capturing the data is much simpler. On the down side, the best systems require that the sensors be hard-wired to the computer and large metal objects, close (<25 cm) to the magnetic field can distort the readings. Golf BioDynamics feels that these two limitations are minor tradeoff for the gains of real-time, 6DOF accurate 3D motion measurement!
4. Please briefly mention a few technical points about the hardware/software, including: the number of sensors; the difference between 6-degree and 3-degree standards; and the accuracy of the data, especially compared to wireless systems i.e. K-Vest, IClub.
To completely describe the movement of a rigid body in 3D space requires the measurement of both its linear movement, usually along the 3 axes of an XYZ coordinate system, as well as its orientation, often the rotations about these same axes. These six parameters (3 translations and 3 rotations) are known as the degrees of freedom (DOF) of movement. A measurement system, such as the one used by Golf BioDynamics (Polhemus) is capable of measuring all 6 DOF. Recently a number of “wireless” sensing systems have appeared on the market with their marketing materials making ambit claims about their capabilities. These types of sensors (based on the use of gyroscopes, magnetometers, accelerometers and inclinometers) do a very good job at measuring the three rotational DOF’s (i.e., the orientation of the sensor attached to the body part) but they do NOT measure the other 3DOF (the linear movement of the sensor!). What does this mean for their use in analyzing the golf swing? Simply put, you will not be able to measure any of the forward/backward, up/down and sideways movements of the body! Thus, based on the output of these types of sensors alone, you will have no insight into these VERY important features of the movement!
The accuracy of the 3DOF systems (e.g., K-vest or I-club) at measuring rotational movement is supposed to be quite good (at least according to one of the companies that makes the transducer, InterSense - www.intersense.com) with an accuracy of 1°. We have yet to test this. Another limitation of these sensors is that the maximum rotational speed that they are capable of measuring (accurately) is 1200°/s which means that these sensors are suitable for use on the large parts of the body (e.g., hips, torso, head, legs) but not for the arm, forearm, hand and golf club where rotational speeds are higher than the rated speed of the device!
There is only one company (to my knowledge) that has a portable, wireless 6DOF tracking system that does not require large setup times and is capable of working in virtually all environments, including full sunlight. This system is produced by Polhemus Inc. (www.polhemus.com), the same company that produces the tethered version that Golf BioDynamics uses at all its locations. We have not migrated to this system for a number of reasons including, primarily, it is not accurate enough (yet), particularly when more than two sensors are being used! Once Polhemus has overcome these limitations (and if the cost is not preclusive!) we would consider migrating to this technology.
5. How did you arrive at the data that defines the respective acceptable ranges/‘corridors’ for each parameter? How many golfers, and of what standard, were measured to arrive at the databank on which the figures are based?
GBD initially tested approximately 75 golfers from mostly the Australasian and European Tours measuring the 3D kinematics for each. Six of the best coaches/Teaching Professionals in Australia were asked to rank these golfers on their ball striking ability. The averages of the top ten ranked male golfers and female golfers were used as the starting point in an optimization program that I wrote. This optimization process had certain constraints (e.g., characteristic properties of muscles and the maximum torque a muscle can produce) and an objective function of maximum power, maximum consistency and minimum injury risk. Once an optimal solution was derived, a set of corridors was produced based on this solution plus and minus the average variability of the data of the ten best ball-strikers. Since then we have measured about 250 Golf Professionals from around the world and found that 75% of them fall within the corridors of our “ideal model”. In fact, having now measured over 5000 amateur golfers crossing the entire spectrum of skill level we are assured that the better the player the closer they are to our “ideal corridors”. These corridors are based on the fact that there must be certain fundamentals present in a quality golf swing; however, there is enough flexibility in the system to allow for individual idiosyncrasies. Recently (approximately three years ago), I modified the corridors so that they were age and sex dependent.
