Biomechanical Model Landing.txt

Action - Reaction Principle

"the sum of joint forces that the body must absorb when slowing down is equal in magnitude and oppositely directed to the external forces applied to slow the body down"

External Forces Principle

"whenever the body is in contact with the ground, there are two ground reaction forces (one vertical and one horizontal) that slow the body down"

Friction Force Principle

"A decrease in friction force is caused by..."

a decrease in the coefficient of friction (μ)

and/or

a decrease in the vertical ground reaction force

EQUATON: ()

Linear Impulse - Momentum Principle

"a decrease in the vertical ground reaction force is caused by..."

a decrease in the mass (m) of the body

and/or

a decrease in the linear speed (s) of landing

and/or

an increase in the application time (t) of vertical ground reaction force

EQUATION: ()

A slower linear speed when the body collides with the ground will reduce the vertical ground reaction force and the friction force. This reduces the external forces that slow the body down and reduces the sum of joint forces that the body must absorb.

Your body will collide with the ground with the same speed it leaves the ground. If the objective of the jump is maximum vertical height or horizontal distance, then the linear speed when your body collides with the ground will be the largest linear speed you were able to create when you left the ground. Therefore, decreasing the linear speed when you collide with the ground is not a viable method for reducing the external forces applied to the body when it collides with the ground.

A longer application time of each external force that slows the body down will reduce the vertical ground reaction force and the friction force. This reduces the external forces that slow the body down and reduces the sum of joint forces.

When landing after a vertical jump, four (4) real-world actions must be performed in order to increase the application time of the external forces slowing the body down.

First, initial contact must be on the forefoot (i.e., the heels should not be in contact with the ground).

Second, the ankle joints should be maximally plantar flexed prior to contact with the ground and then upon contact with the ground, the ankle plantar flexor muscles should be contracted to create eccentric ankle dorsiflexion torques. These torques will slowly lower the heels towards the ground. 

Third, the knee joints should be slightly flexed prior to contact with the ground. If these joints are fully extended when contact is made with the ground, large external forces will be created to slow the body down because the application time of these external forces will be very small. Upon contact with the ground, the knee extensor muscles should be contracted to create eccentric knee flexion torqes. These torques will slowly lower the knees towards the ground.

Fourth, the hip joints should be slightly flexed prior to contact with the ground. If these joints are fully extended when contact is made with the ground, large external forces will be created to slow the body down because the application time of these external forces will be very small. Upon contact with the ground, the hip extensor muscles should be contracted to create eccentric hip flexion torques. These torques will slowly lower the hips towards the ground. 

When landing after a horizontal jump, initial contact is made with the heels. The length of the time of application of the external forces is thus dependent on the surface that you land on and the time of application of the eccentric knee and hip flexion torques. Three (3) real-world actions must be performed in order to increase the application time of the external forces slowing the body down.

First, the ground that you land on should be soft. This will allow the heels to continue to move while you perform real-world actions 2 and 3. If the heels stop moving, the application time of the torques to slow the body down will stop.

Second, the knee joints should be slightly flexed prior to contact with the ground. If these joints are fully extended when contact is made with the ground, large external forces will created to slow the body down because the application time of these external forces will be very small. Upon contact with the ground, the knee extensor muscles should be contracted to create eccentric knee flexion torques. These torques will slowly lower the knees towards the ground as well as slow the forward motion of the knees, hips and torso.

Third, the hip joints should be slightly flexed prior to contact with the ground. If these joints are fully extended when contact is made with the ground, large external forces will be created to slow the body down because the application time of these external forces will be very small. Upon contact with the ground, the hip extensor muscles should be contracted to create eccentric hip flexion torques. These torques will slowly lower the hips towards the ground as well as slow the forward motion of the hips and torso.

A smaller body mass when the body collides with the ground will reduce the vertical ground reaction force and the friction force. This reduces the external forces that slow the body down and reduces the sum of joint forces that the body must absorb.

To create a smaller body component mass, you must consider short-term and long-term techniques. In the short-term, two things can be done to reduce body component mass:

• (1) wear the lightest clothing possible
• (2) wear the lightest shoes possible

You can also lose fat mass

A smaller coefficient of friction when the body collides with the ground will reduce the friction force. This reduces an external force that slows the body down and reduces the sum of joint forces that the body must absorb.

Theoretically, we could make the material of the shoe harder and the surface of the material smoother. This would decrease the magnitude of the friction force that slows you down. Unfortunately, the real-world application is something very different. That's because the amount of friction required when your foot collides with the ground is the amount necessary to stop the foot from sliding. If you don't have this necessary friction, your foot will slip and your likelihood of falling will increase. So decreasing the coefficient of friction will have no affect on the magnitude of the friction force slowing you down until you reduce it to a level that would allow the show to slip. This is an unacceptable method for reducing the sum of joint forces that the body must absorb.