Kinematic Sequence – Calculation Methods (P1)
Comparison of Kinematic Sequence and Kinetic Link
Firstly a little background; Phil Cheetham in collaboration with Greg Rose and the TPI Biomechanics Board of Advisors developed the version of the kinematic sequence method now used by TPI, AMM and KMI; Chris Welch developed the version of the kinetic link method used in the Zenolink analysis. Here we will look at these two different ways to calculate the angular velocities of the body segments during the golf swing. Both these methods display how a skilled athletic motion can efficiently make an end body segment or implement move very fast, as in hitting, kicking or throwing. This principle has been generically called “proximal to distal sequencing” and can be described in several ways; as joint angular velocities, as segmental angular velocities and as joint torques, to name just a few. If joint torques and forces are used it probably should be termed “kinetic”; if angular velocities are used it should be termed “kinematic”. Why? Well here’s a quick definition of these two terms:
- Kinematics is the study of motion without regard to the forces producing the motion; parameters such as position, angle, velocities and accelerations.
- Kinetics refers to the study of the forces and torques that create the motion.
Surprisingly, the term “kinetic link” is commonly used in biomechanics and by Zenolink, even when angular velocities are used. We chose the term kinematic sequence because we use a different set of angular velocities than Zenolink and wished to differentiate between the two methods. Let’s look at these two methods in detail; it will get a little complicated but I will summarize at the end.
Kinetic Link Calculation Method
The angular velocities for the kinetic link are calculated as follows: A pelvis origin is defined as a point mid way between the hip joints; an upper body origin is defined as a point mid way between the left and right shoulder joints; a fixed spine axis is defined as the line from the pelvis and upper body origins. A plane is produced perpendicular to the spine axis at the level of the pelvis origin; call it the “perpendicular to spine” plane (the PTS plane). A vertical plane parallel to the target line is defined to cut through the PTS plane; the line this produces on the PTS plane is used as the zero reference line. A line in the pelvis from hip joint to hip joint is projected into the PTS plane. A line in the upper body from shoulder joint to shoulder joint is projected into the PTS plane. Finally a line is created from the mid shoulder point to the mid hands on the club grip (called the “composite arm” line) and it is also projected into the PTS plane. These three lines allow a rotation angle to be calculated between each one and the zero reference line. Segmental rotation speeds are calculated as the time rate of change of these angles. This method so far allows us to calculate a rotation speed of pelvis, upper body and composite arm; backwards rotation speeds are negative and forward rotation speeds are positive. We still need and angular velocity for the club; strangely though the kinetic link method doesn’t use the club speed as the forth curve; it uses the wrist release speed instead. The wrist release speed is the relative speed of the club shaft with respect to the composite arm; that is; the speed that this “joint” opens during the downswing.