Monday, October 8, 2012

Vertical Jump - Here is what I See (2)

So, when does the application time for the hip extension torque, the knee extension torque, and the ankle plantarflexion torque end?  This is the easy part, they all end when the foot leaves the ground.  At this point, any further plantarflexion, knee extension, or hip extension will merely push the feet downward.  There will be no additional upward movement of the joints.  Thus, any kicking of the legs or swinging of the arms will have no effect on jump height.

The next important event is landing.  Click on "read more" to read the Biomechanical explanation for how to land with the lowest magnitude (i.e., size) of internal forces that must be absorbed by the bones, cartilage, ligaments, and tendons.


Landing after a vertical jump for maximum height

After you reach your maximum jump height, your body will fall back to earth.  When the body makes contact with the earth, an external ground reaction force (GRF) is applied to stop the feet from continuing to move downward.  The magnitude (i.e., size) of the external GRF is determined by three factors: your speed when you hit the ground, your mass, and the amount of time each joint moves downward before actually stopping.  If the magnitude of the external GRF is reduced, then Newton's 3rd Law of Motion, the Action-Reaction Principle, tells us that the magnitude of the body's internal forces will also be reduced.

The Three Factors that Determine the Magnitude of the External GRF
  1. Speed: There is not much you can do about your speed when you hit the ground.  It is determined by how high you jump.  The higher you jump, the faster you will be travelling when you make contact with the ground and the greater the magnitude of the external ground reaction force.
  2. Mass: Your mass can be modified, but not during the jump itself.  If you decrease your fat mass over time by dieting and exercise, the magnitude of the external ground reaction force will decrease.
  3. Time: The only thing you have control over during the landing after a jump is the amount of time each joint moves downward before actually stopping.  If we increase this time by applying eccentric joint torques, then the magnitude of the external ground reaction force will decrease. 
The time the joints in the body continue to move downward during a jump landing is controlled by three torques in the lower extremity: an eccentric ankle plantarflexion torque, an eccentric knee extension torque, and an eccentric hip extension torque.  

Eccentric torques slow down a joint's rotation.  On the other hand, concentric torques speed up a joint's rotation.  During the acceleration phase, a stronger concentric plantarflexion torque causes a faster plantarflexion rotation.  But, during the deceleration phase, a stronger eccentric plantarflexion torques causes a slower dorsiflexion rotation.  The same is true for knee extension (concentric contraction during the acceleration phase; eccentric contraction during the deceleration phase) and hip extension (concentric contraction during the acceleration phase; eccentric contraction during the deceleration phase).

So, here is how the three eccentric torques combine to slow the body down when landing after vertical jump:
  • The first consideration is the position of the foot when the body makes contact with the ground.  If you land on the forefoot, both the foot and the tibia are free to rotate.  An ankle dorsiflexion rotation occurs because the heel and the ankle joint can move downward.  The speed of this dorsiflexion rotation is controlled by an eccentric plantarflexion torque.  The application time of the eccentric plantarflexion torque begins when the foot makes contact with the ground and ends when the heel makes contact with the ground.
    • If you land flatfooted, there is no application time of the eccentric plantar flexion torque.  This means that all joints in the body instantaneously stop moving down for a very brief moment; but this is significant event because the magnitude of the external GRF will be very large.
    • Flexing your knees and hips after the heels make contact has no effect on the magnitude of the external GRF.
  • The second torque is an eccentric knee extension torque that controls the speed of the knee flexion rotation.  The application time of the eccentric knee extension torque begins when the foot makes contact with the ground and ends when the heel makes contact with the ground or when the knee no longer moves in a downward direction.
  • The third torque is an eccentric hip extension torque that controls the speed of the hip flexion rotation.  The application time of the eccentric hip extension torque begins when the foot makes contact with the ground and ends when the heel makes contact with the ground or when the hip joint no longer moves in a downward direction.
As long as the ankle, knee, and hip jointscontinue to move downward and the heel does not make contact with the ground, then the amount of time that all joints in the body move downwards will increase.  This will result in a smaller external ground reaction force AND a smaller sum of internal joint forces.

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