Friday, September 21, 2012

The Basics 03: The Angular Motion Machine

So, what do I mean when I say the "Human Body is an Angular Motion Machine that creates Linear and/or Angular Motion Outcomes"?

The human body is comprised of joints that function as points of rotation for the bones that are connected at the joint.

  • Each wrist joint allows the carpals and the radius and ulna to rotate relative to each other.
  • Each elbow joint allows the humerus and the radius and ulna to rotate relative to each other.
  • Each shoulder joint allows the humerus and the scapula to rotate relative to each other.
  • The thoracic and lumbar spinal joints allow the shoulder girdle (the bones connecting the right and left shoulder joints - 2 scapulae and two clavicles) and the pelvis to rotate relative to each other.
  • Each hip joint allows the pelvis and the femur to rotate relative to each other.
  • Each knee joint allows the femur and the tibia to rotate relative to each other.
  • Each ankle joint allows the tibia and the tarsals to rotate relative to each other
Every joint can allow one to three joint rotations (i.e., angular motions).  These joint rotations are caused by joint torques (i.e., muscle forces pulling on bones on either side of a joint which cause the bones to rotate relative to each other).  Some joint rotations cannot occur because there are no muscles that cross the joint in the correct orientation to cause the joint rotation (e.g., there is no abduction at the elbow because there are no muscles that connect the lateral side of the humerus to the radius and there is no adduction at the elbow because there are no muscles that connect the medial side of the humerus to the ulna).  Even if a joint torque can be created, some joint rotations are not permitted because of bony structural limitations (e.g., the elbow has a maximum extension of approximately 180 degrees; this is because the olecranon process of the ulna collides with the olecranon fossa of the humerus).



Each joint rotation causes points on the connected bones at the joint to move linearly and angularly.  These  linear and/or angular motions of each bone are then combined to create the desired whole body movement outcomes.  With this information as our foundation, here is a description of how human motion is created.

  • Muscle forces create joint torques.
  • Joint torques create joint angular velocities.
    • The magnitude (i.e, size) of these joint angular velocity is determined by the magnitude of the joint torque and the time of application of the joint torque.
    • A larger magnitude and a greater time will result in a greater joint angular velocity.
  • Joint angular velocities cause the bones connected at the joint to rotate.
    • If the rotating bone can freely rotate (i.e., it is not in contact with the ground and it is not connected with another rotating bone at a different joint), then the bone will rotate around the joint.
    • If the rotating bone cannot freely rotate (i.e.,it is in contact with the ground or it is connected with another rotating bone at a different joint), then the bone will rotate around the fixed point (e.g., the ground or the other joint).
  • A rotating bone will create angular and linear velocities along the length of the rotating bone and along any bones attached to the rotating bone.
  • The proper sequencing of these angular and linear velocities will create the desired whole body angular and linear velocities outcomes.

Here's an example: The vertical jump for maximum height

A vertical jump is created by the summation of three torques in the lower extremity: an ankle plantarflexion torque, a knee extension torque, and a hip extension torque.  Some people will argue that a shoulder flexion torque also adds to the vertical jump, but I will offer an opposing opinion in an upcoming post.  So, here is how the three torques combine to create a vertical jump:
  • If the femur and the torso were free to rotate, a hip extension torque would cause the femur to rotate downward and the torso to rotate upward.
    • During the upward acceleration phase, the distal end of the upper leg cannot freely rotate downward because it is attached to the lower leg which is either stationary or rotating upward; it never rotates downward during the upward acceleration phase. Therefore, the hip extension torque will cause
      • an upward rotation of the the femur and
      • an upward rotation of the torso. 
    • The net result is the hip joint and the top of spine (the head) will move upward.
  • If the tibia and the femur were free to rotate, a knee extension torque would cause the tibia to rotate downward and the upper leg to rotate upward.
    • During the upward acceleration phase, the distal end of the tibia cannot freely rotate downward because it is attached to the foot which is either stationary or rotating upward; it never rotates downward during the upward acceleration phase. Therefore, a knee extension torque will cause 
      • an upward rotation of the tibia and
      • an upward rotation of the femur. 
    • The net result is the knee joint, the hip joint, and the top of the spine (the head) will all move upward .
  • Finally, if the foot and the tibia were free to rotate, a plantarflexion torque would cause the foot to rotate downward and the lower leg to rotate upward.
    • During the upward acceleration phase, the foot cannot freely rotate downward because it is in contact with the ground.  Therefore, a plantarflexion torque will cause 
      • an upward rotation of the foot and
      • an upward rotation of the tibia. 
    • The net result is the ankle joint, the knee joint, the hip joint, and the top of the spine (the head) will all move upward.
  • Maximum vertical jump height is achieved when the maximum upward of the velocity of the head due to each joint rotation occurs simultaneously at the instant when the feet leave the ground.
Human motion is created when muscles forces create joint torques which result in joint angular velocities and linear and angular velocities of rotating bones.  A performer's specific combination of joint torques, their technique, is the angular motion machine that creates unique linear and angular motion outcomes.  A teacher's, coach's, or instructor's job is to help each performer develop their own optimal combination of joint torques.



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