Glossary

The analysis of the interaction between a person and the ground. Ground mechanics concerns the amount and location of force applied by a person to the ground, and how they change over the course of a movement.

Ground mechanics can relate to “footwork,” however footwork relates to the movement of the foot in space. Through force and pressure analysis, we know kinetic changes often occur without the feet physically changing their position.

Vertical GRF is the magnitude of a force on a surface, shown by how hard the surface is pushing back on an object (in this case the foot). GRF mirrors the force that a body applies to a surface. Any time a person is in contact with the ground they are applying a force into the ground, in turn the ground applies an equal but opposite reaction force back up onto the person.
The GRF is a vector, meaning it has both magnitude (i.e. how much force) and direction. The direction of force can be split into 3 components – vertical, anterior-posterior (front-to-back), and mediolateral (side-to-side). The BodiTrak sensors measure the vertical component, and display ‘dynamic vertical force.’ Dynamic Vertical Force is expressed relative to the force of a person’s body weight (BW) when standing statically. Over the course of a movement (e.g. golf swing, vertical jump, etc.), DVF will fluctuate above and below a value of 1x BW.

Pressure is the amount of vertical force acting over a surface area. Increasing the surface area over which a given force is applied will reduce the pressure on that surface. If the surface area remains fixed, the pressure will change proportionately with the applied force.

In the BodiTrak software, pressure is communicated as 1) a ‘pressure map,’ which visually displays areas of high and low pressure on the bottom surface of each foot; and 2) percentage of total pressure distribution (lead vs. trail foot, toes vs. heels).

In biomechanics, center of pressure (“CoP”) is the term given to the point of application of the GRF vector.

Consider it as the most average point of vertical force acting on a surface. The location of the CoP changes dynamically with changes in pressure during a movement.

A plotted line representing the location of the CoP on the ground at each instance in time during a particular movement.

The percentage of total force applied on one foot vs the other (lead vs. trail), or on the toes vs heel of a foot.

The distance the CoP moves – in both the lateral and heel-toe directions – compared to its initial position when the person is stationary. The slope of a CoP displacement curve at a given time represents the CoP velocity.

Lateral CoP Displacement (White Line)

• When the white line is below the baseline, the CoP is located on the trail foot side of its initial positon.
• When the white line is above the baseline, the CoP is located on the lead foot side of its initial position.

Heel-Toe CoP Displacement (Orange Line)

• When the orange line is below the baseline, the CoP is located toward the heels compared to its initial position.
• When the orange line is above the baseline, the CoP is located toward the toes compared to its initial position.

The distance between the two furthest points the CoP travels over the course of a movement (either in the lateral or the heel-toe direction).

The rate of movement of the CoP (in the lateral and heel-toe directions). The slope of a CoP displacement curve at a given time represents the COP velocity. The slope of a CoP velocity curve represents CoP acceleration

Lateral CoP Velocity (White Line)

• When the white line is below the baseline, the center of pressure is moving away from the target.
• When the white line is above the baseline, the center of pressure is moving toward the target.

Heel-Toe CoP Velocity (Orange Line)

• When the orange line is below the baseline, the CoP is moving toward the heels.
• When the orange line is above the baseline, the CoP is moving toward the toes.

The greater the magnitude of the velocity, the faster the CoP is moving in a given direction.

The balance point of a body, or the average location of a body’s mass. When standing upright, a person’s CoM is typically located around their belly button. However, as a person moves and changes the position of their body parts, the CoM also moves.

A plotted line representing the location of the CoM if projected down onto the 2D surface of the ground. During a golf swing, the CoM trace will always bestay “inside” the CoP trace.

A reversal in direction of the CoP movement in downswing – the CoP switches from moving towards the target to moving away from the target. This is caused by a sudden shift in the relative amount of pressure on the lead vs. the trail foot. In the downswing, the CoP will move towards the lead foot as it applies more pressure.

If, for instance, a golfer’s lead side lifts off the ground and their body tilts away from the target, then the CoP will reverse its direction since there will be relatively more pressure under the trail foot. It is important to note that there may not be a great amount of pressure on the trail foot, but that it has a greater percentage of the pressure distribution compared to the amount on the lead foot.

The fastest movement of the COP laterally during the movement. Peak Velocity generally occurs early in transition when pressure is transferred quickly to lead foot.

The foot that is located closest to the target during the golf swing (e.g. left foot for a right-handed golfer).

The foot that is located furthest from the target during a golf swing (e.g. right foot for a right-handed golfer).

The branch of biomechanics related to the internal and external forces that cause motion. The vertical GRF between the feet and the ground during a golf swing is an example of a kinetic variable.

The branch of biomechanics concerned with the description of motion. Position, displacement, velocity, and acceleration are all examples of kinematics.

1. SCATTERED
COP moves in a variable and inefficient pattern, marked with excessive perturbations.

2. ABBREVIATED
Little pressure shift onto trail foot in the backswing. Used for low trajectory shots.

3. LINEAR
Pressure shifts between trail and lead mid-feet, with minimal heel or toe displacement.

4. FISH HOOK
Pressure is transferred from trail heel to toes, then towards lead toes in downswing.

5. HEEL-TOE
Pressure transferred into the trail heel in the backswing, then directly into the lead toes in the downswing.

6. POWER DRIVING
CoP moves towards the lead foot in the downswing, then moves backwards toward the trail foot before impact. Associated with spine tilt away from the target and high launch conditions